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perlfaq4 (1)


perlfaq4 - Data Manipulation


Please see following description for synopsis


Perl Programmers Reference Guide                      PERLFAQ4(1)

     perlfaq4 - Data Manipulation

     This section of the FAQ answers questions related to
     manipulating numbers, dates, strings, arrays, hashes, and
     miscellaneous data issues.

Data: Numbers
  Why am I getting long decimals (eg, 19.9499999999999) instead
     of the numbers I should be getting (eg, 19.95)?
     For the long explanation, see David Goldberg's "What Every
     Computer Scientist Should Know About Floating-Point

     Internally, your computer represents floating-point numbers
     in binary.  Digital (as in powers of two) computers cannot
     store all numbers exactly.  Some real numbers lose precision
     in the process.  This is a problem with how computers store
     numbers and affects all computer languages, not just Perl.

     perlnumber shows the gory details of number representations
     and conversions.

     To limit the number of decimal places in your numbers, you
     can use the "printf" or "sprintf" function.  See the
     "Floating Point Arithmetic" for more details.

             printf "%.2f", 10/3;

             my $number = sprintf "%.2f", 10/3;

  Why is int() broken?
     Your "int()" is most probably working just fine.  It's the
     numbers that aren't quite what you think.

     First, see the answer to "Why am I getting long decimals
     (eg, 19.9499999999999) instead of the numbers I should be
     getting (eg, 19.95)?".

     For example, this

             print int(0.6/0.2-2), "\n";

     will in most computers print 0, not 1, because even such
     simple numbers as 0.6 and 0.2 cannot be presented exactly by
     floating-point numbers.  What you think in the above as
     'three' is really more like 2.9999999999999995559.

  Why isn't my octal data interpreted correctly?
     (contributed by brian d foy)

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     You're probably trying to convert a string to a number,
     which Perl only converts as a decimal number. When Perl
     converts a string to a number, it ignores leading spaces and
     zeroes, then assumes the rest of the digits are in base 10:

             my $string = '0644';

             print $string + 0;  # prints 644

             print $string + 44; # prints 688, certainly not octal!

     This problem usually involves one of the Perl built-ins that
     has the same name a Unix command that uses octal numbers as
     arguments on the command line. In this example, "chmod" on
     the command line knows that its first argument is octal
     because that's what it does:

             %prompt> chmod 644 file

     If you want to use the same literal digits (644) in Perl,
     you have to tell Perl to treat them as octal numbers either
     by prefixing the digits with a 0 or using "oct":

             chmod(     0644, $file);   # right, has leading zero
             chmod( oct(644), $file );  # also correct

     The problem comes in when you take your numbers from
     something that Perl thinks is a string, such as a command
     line argument in @ARGV:

             chmod( $ARGV[0],      $file);   # wrong, even if "0644"

             chmod( oct($ARGV[0]), $file );  # correct, treat string as octal

     You can always check the value you're using by printing it
     in octal notation to ensure it matches what you think it
     should be. Print it in octal  and decimal format:

             printf "0%o %d", $number, $number;

  Does Perl have a round() function?  What about ceil() and
     floor()?  Trig functions?
     Remember that "int()" merely truncates toward 0.  For
     rounding to a certain number of digits, "sprintf()" or
     "printf()" is usually the easiest route.

             printf("%.3f", 3.1415926535);   # prints 3.142

     The "POSIX" module (part of the standard Perl distribution)
     implements "ceil()", "floor()", and a number of other
     mathematical and trigonometric functions.

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             use POSIX;
             $ceil   = ceil(3.5);   # 4
             $floor  = floor(3.5);  # 3

     In 5.000 to 5.003 perls, trigonometry was done in the
     "Math::Complex" module.  With 5.004, the "Math::Trig" module
     (part of the standard Perl distribution) implements the
     trigonometric functions. Internally it uses the
     "Math::Complex" module and some functions can break out from
     the real axis into the complex plane, for example the
     inverse sine of 2.

     Rounding in financial applications can have serious
     implications, and the rounding method used should be
     specified precisely.  In these cases, it probably pays not
     to trust whichever system rounding is being used by Perl,
     but to instead implement the rounding function you need

     To see why, notice how you'll still have an issue on half-
     way-point alternation:

             for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

             0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
             0.8 0.8 0.9 0.9 1.0 1.0

     Don't blame Perl.  It's the same as in C.  IEEE says we have
     to do this. Perl numbers whose absolute values are integers
     under 2**31 (on 32 bit machines) will work pretty much like
     mathematical integers.  Other numbers are not guaranteed.

  How do I convert between numeric representations/bases/radixes?

     As always with Perl there is more than one way to do it.
     Below are a few examples of approaches to making common
     conversions between number representations.  This is
     intended to be representational rather than exhaustive.

     Some of the examples later in perlfaq4 use the "Bit::Vector"
     module from CPAN. The reason you might choose "Bit::Vector"
     over the perl built in functions is that it works with
     numbers of ANY size, that it is optimized for speed on some
     operations, and for at least some programmers the notation
     might be familiar.

     How do I convert hexadecimal into decimal
         Using perl's built in conversion of "0x" notation:

                 $dec = 0xDEADBEEF;

         Using the "hex" function:

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                 $dec = hex("DEADBEEF");

         Using "pack":

                 $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));

         Using the CPAN module "Bit::Vector":

                 use Bit::Vector;
                 $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
                 $dec = $vec->to_Dec();

     How do I convert from decimal to hexadecimal
         Using "sprintf":

                 $hex = sprintf("%X", 3735928559); # upper case A-F
                 $hex = sprintf("%x", 3735928559); # lower case a-f

         Using "unpack":

                 $hex = unpack("H*", pack("N", 3735928559));

         Using "Bit::Vector":

                 use Bit::Vector;
                 $vec = Bit::Vector->new_Dec(32, -559038737);
                 $hex = $vec->to_Hex();

         And "Bit::Vector" supports odd bit counts:

                 use Bit::Vector;
                 $vec = Bit::Vector->new_Dec(33, 3735928559);
                 $vec->Resize(32); # suppress leading 0 if unwanted
                 $hex = $vec->to_Hex();

     How do I convert from octal to decimal
         Using Perl's built in conversion of numbers with leading

                 $dec = 033653337357; # note the leading 0!

         Using the "oct" function:

                 $dec = oct("33653337357");

         Using "Bit::Vector":

                 use Bit::Vector;
                 $vec = Bit::Vector->new(32);
                 $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
                 $dec = $vec->to_Dec();

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     How do I convert from decimal to octal
         Using "sprintf":

                 $oct = sprintf("%o", 3735928559);

         Using "Bit::Vector":

                 use Bit::Vector;
                 $vec = Bit::Vector->new_Dec(32, -559038737);
                 $oct = reverse join('', $vec->Chunk_List_Read(3));

     How do I convert from binary to decimal
         Perl 5.6 lets you write binary numbers directly with the
         "0b" notation:

                 $number = 0b10110110;

         Using "oct":

                 my $input = "10110110";
                 $decimal = oct( "0b$input" );

         Using "pack" and "ord":

                 $decimal = ord(pack('B8', '10110110'));

         Using "pack" and "unpack" for larger strings:

                 $int = unpack("N", pack("B32",
                 substr("0" x 32 . "11110101011011011111011101111", -32)));
                 $dec = sprintf("%d", $int);

                 # substr() is used to left pad a 32 character string with zeros.

         Using "Bit::Vector":

                 $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
                 $dec = $vec->to_Dec();

     How do I convert from decimal to binary
         Using "sprintf" (perl 5.6+):

                 $bin = sprintf("%b", 3735928559);

         Using "unpack":

                 $bin = unpack("B*", pack("N", 3735928559));

         Using "Bit::Vector":

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                 use Bit::Vector;
                 $vec = Bit::Vector->new_Dec(32, -559038737);
                 $bin = $vec->to_Bin();

         The remaining transformations (e.g. hex -> oct, bin ->
         hex, etc.)  are left as an exercise to the inclined

  Why doesn't & work the way I want it to?
     The behavior of binary arithmetic operators depends on
     whether they're used on numbers or strings.  The operators
     treat a string as a series of bits and work with that (the
     string "3" is the bit pattern 00110011).  The operators work
     with the binary form of a number (the number 3 is treated as
     the bit pattern 00000011).

     So, saying "11 & 3" performs the "and" operation on numbers
     (yielding 3).  Saying "11" & "3" performs the "and"
     operation on strings (yielding "1").

     Most problems with "&" and "|" arise because the programmer
     thinks they have a number but really it's a string.  The
     rest arise because the programmer says:

             if ("\020\020" & "\101\101") {
                     # ...

     but a string consisting of two null bytes (the result of
     "\020\020" & "\101\101") is not a false value in Perl.  You

             if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
                     # ...

  How do I multiply matrices?
     Use the "Math::Matrix" or "Math::MatrixReal" modules
     (available from CPAN) or the "PDL" extension (also available
     from CPAN).

  How do I perform an operation on a series of integers?
     To call a function on each element in an array, and collect
     the results, use:

             @results = map { my_func($_) } @array;

     For example:

             @triple = map { 3 * $_ } @single;

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     To call a function on each element of an array, but ignore
     the results:

             foreach $iterator (@array) {

     To call a function on each integer in a (small) range, you
     can use:

             @results = map { some_func($_) } (5 .. 25);

     but you should be aware that the ".." operator creates an
     array of all integers in the range.  This can take a lot of
     memory for large ranges.  Instead use:

             @results = ();
             for ($i=5; $i < 500_005; $i++) {
                     push(@results, some_func($i));

     This situation has been fixed in Perl5.005. Use of ".." in a
     "for" loop will iterate over the range, without creating the
     entire range.

             for my $i (5 .. 500_005) {
                     push(@results, some_func($i));

     will not create a list of 500,000 integers.

  How can I output Roman numerals?
     Get the
     <> module.

  Why aren't my random numbers random?
     If you're using a version of Perl before 5.004, you must
     call "srand" once at the start of your program to seed the
     random number generator.

              BEGIN { srand() if $] < 5.004 }

     5.004 and later automatically call "srand" at the beginning.
     Don't call "srand" more than once--you make your numbers
     less random, rather than more.

     Computers are good at being predictable and bad at being
     random (despite appearances caused by bugs in your programs
     :-).  see the random article in the "Far More Than You Ever
     Wanted To Know" collection in
     <>, courtesy of
     Tom Phoenix, talks more about this.  John von Neumann said,

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     "Anyone who attempts to generate random numbers by
     deterministic means is, of course, living in a state of

     If you want numbers that are more random than "rand" with
     "srand" provides, you should also check out the
     "Math::TrulyRandom" module from CPAN.  It uses the
     imperfections in your system's timer to generate random
     numbers, but this takes quite a while.  If you want a better
     pseudorandom generator than comes with your operating
     system, look at "Numerical Recipes in C" at

  How do I get a random number between X and Y?
     To get a random number between two values, you can use the
     "rand()" built-in to get a random number between 0 and 1.
     From there, you shift that into the range that you want.

     "rand($x)" returns a number such that "0 <= rand($x) < $x".
     Thus what you want to have perl figure out is a random
     number in the range from 0 to the difference between your X
     and Y.

     That is, to get a number between 10 and 15, inclusive, you
     want a random number between 0 and 5 that you can then add
     to 10.

             my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )

     Hence you derive the following simple function to abstract
     that. It selects a random integer between the two given
     integers (inclusive), For example:

             sub random_int_between {
                     my($min, $max) = @_;
                     # Assumes that the two arguments are integers themselves!
                     return $min if $min == $max;
                     ($min, $max) = ($max, $min)  if  $min > $max;
                     return $min + int rand(1 + $max - $min);

Data: Dates
  How do I find the day or week of the year?
     The "localtime" function returns the day of the year.
     Without an argument "localtime" uses the current time.

             $day_of_year = (localtime)[7];

     The "POSIX" module can also format a date as the day of the
     year or week of the year.

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             use POSIX qw/strftime/;
             my $day_of_year  = strftime "%j", localtime;
             my $week_of_year = strftime "%W", localtime;

     To get the day of year for any date, use "POSIX"'s "mktime"
     to get a time in epoch seconds for the argument to

             use POSIX qw/mktime strftime/;
             my $week_of_year = strftime "%W",
                     localtime( mktime( 0, 0, 0, 18, 11, 87 ) );

     The "Date::Calc" module provides two functions to calculate

             use Date::Calc;
             my $day_of_year  = Day_of_Year(  1987, 12, 18 );
             my $week_of_year = Week_of_Year( 1987, 12, 18 );

  How do I find the current century or millennium?
     Use the following simple functions:

             sub get_century    {
                     return int((((localtime(shift || time))[5] + 1999))/100);

             sub get_millennium {
                     return 1+int((((localtime(shift || time))[5] + 1899))/1000);

     On some systems, the "POSIX" module's "strftime()" function
     has been extended in a non-standard way to use a %C format,
     which they sometimes claim is the "century". It isn't,
     because on most such systems, this is only the first two
     digits of the four-digit year, and thus cannot be used to
     reliably determine the current century or millennium.

  How can I compare two dates and find the difference?
     (contributed by brian d foy)

     You could just store all your dates as a number and then
     subtract.  Life isn't always that simple though. If you want
     to work with formatted dates, the "Date::Manip",
     "Date::Calc", or "DateTime" modules can help you.

  How can I take a string and turn it into epoch seconds?
     If it's a regular enough string that it always has the same
     format, you can split it up and pass the parts to
     "timelocal" in the standard "Time::Local" module.
     Otherwise, you should look into the "Date::Calc" and
     "Date::Manip" modules from CPAN.

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  How can I find the Julian Day?
     (contributed by brian d foy and Dave Cross)

     You can use the "Time::JulianDay" module available on CPAN.
     Ensure that you really want to find a Julian day, though, as
     many people have different ideas about Julian days.  See for instance.

     You can also try the "DateTime" module, which can convert a
     date/time to a Julian Day.

             $ perl -MDateTime -le'print DateTime->today->jd'

     Or the modified Julian Day

             $ perl -MDateTime -le'print DateTime->today->mjd'

     Or even the day of the year (which is what some people think
     of as a Julian day)

             $ perl -MDateTime -le'print DateTime->today->doy'

  How do I find yesterday's date?
     (contributed by brian d foy)

     Use one of the Date modules. The "DateTime" module makes it
     simple, and give you the same time of day, only the day

             use DateTime;

             my $yesterday = DateTime->now->subtract( days => 1 );

             print "Yesterday was $yesterday\n";

     You can also use the "Date::Calc" module using its
     "Today_and_Now" function.

             use Date::Calc qw( Today_and_Now Add_Delta_DHMS );

             my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );

             print "@date_time\n";

     Most people try to use the time rather than the calendar to
     figure out dates, but that assumes that days are twenty-four
     hours each.  For most people, there are two days a year when
     they aren't: the switch to and from summer time throws this
     off. Let the modules do the work.

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     If you absolutely must do it yourself (or can't use one of
     the modules), here's a solution using "Time::Local", which
     comes with Perl:

             # contributed by Gunnar Hjalmarsson
              use Time::Local;
              my $today = timelocal 0, 0, 12, ( localtime )[3..5];
              my ($d, $m, $y) = ( localtime $today-86400 )[3..5];
              printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;

     In this case, you measure the day starting at noon, and
     subtract 24 hours. Even if the length of the calendar day is
     23 or 25 hours, you'll still end up on the previous calendar
     day, although not at noon. Since you don't care about the
     time, the one hour difference doesn't matter and you end up
     with the previous date.

  Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K
     (contributed by brian d foy)

     Perl itself never had a Y2K problem, although that never
     stopped people from creating Y2K problems on their own. See
     the documentation for "localtime" for its proper use.

     Starting with Perl 5.11, "localtime" and "gmtime" can handle
     dates past 03:14:08 January 19, 2038, when a 32-bit based
     time would overflow. You still might get a warning on a
     32-bit "perl":

             % perl5.11.2 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
             Integer overflow in hexadecimal number at -e line 1.
             Wed Nov  1 19:42:39 5576711

     On a 64-bit "perl", you can get even larger dates for those
     really long running projects:

             % perl5.11.2 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
             Thu Nov  2 00:42:39 5576711

     You're still out of luck if you need to keep tracking of
     decaying protons though.

Data: Strings
  How do I validate input?
     (contributed by brian d foy)

     There are many ways to ensure that values are what you
     expect or want to accept. Besides the specific examples that
     we cover in the perlfaq, you can also look at the modules
     with "Assert" and "Validate" in their names, along with
     other modules such as "Regexp::Common".

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     Some modules have validation for particular types of input,
     such as "Business::ISBN", "Business::CreditCard",
     "Email::Valid", and "Data::Validate::IP".

  How do I unescape a string?
     It depends just what you mean by "escape".  URL escapes are
     dealt with in perlfaq9.  Shell escapes with the backslash
     ("\") character are removed with


     This won't expand "\n" or "\t" or any other special escapes.

  How do I remove consecutive pairs of characters?
     (contributed by brian d foy)

     You can use the substitution operator to find pairs of
     characters (or runs of characters) and replace them with a
     single instance. In this substitution, we find a character
     in "(.)". The memory parentheses store the matched character
     in the back-reference "\1" and we use that to require that
     the same thing immediately follow it. We replace that part
     of the string with the character in $1.


     We can also use the transliteration operator, "tr///". In
     this example, the search list side of our "tr///" contains
     nothing, but the "c" option complements that so it contains
     everything. The replacement list also contains nothing, so
     the transliteration is almost a no-op since it won't do any
     replacements (or more exactly, replace the character with
     itself). However, the "s" option squashes duplicated and
     consecutive characters in the string so a character does not
     show up next to itself

             my $str = 'Haarlem';   # in the Netherlands
             $str =~ tr///cs;       # Now Harlem, like in New York

  How do I expand function calls in a string?
     (contributed by brian d foy)

     This is documented in perlref, and although it's not the
     easiest thing to read, it does work. In each of these
     examples, we call the function inside the braces used to
     dereference a reference. If we have more than one return
     value, we can construct and dereference an anonymous array.
     In this case, we call the function in list context.

             print "The time values are @{ [localtime] }.\n";

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     If we want to call the function in scalar context, we have
     to do a bit more work. We can really have any code we like
     inside the braces, so we simply have to end with the scalar
     reference, although how you do that is up to you, and you
     can use code inside the braces. Note that the use of parens
     creates a list context, so we need "scalar" to force the
     scalar context on the function:

             print "The time is ${\(scalar localtime)}.\n"

             print "The time is ${ my $x = localtime; \$x }.\n";

     If your function already returns a reference, you don't need
     to create the reference yourself.

             sub timestamp { my $t = localtime; \$t }

             print "The time is ${ timestamp() }.\n";

     The "Interpolation" module can also do a lot of magic for
     you. You can specify a variable name, in this case "E", to
     set up a tied hash that does the interpolation for you. It
     has several other methods to do this as well.

             use Interpolation E => 'eval';
             print "The time values are $E{localtime()}.\n";

     In most cases, it is probably easier to simply use string
     concatenation, which also forces scalar context.

             print "The time is " . localtime() . ".\n";

  How do I find matching/nesting anything?
     This isn't something that can be done in one regular
     expression, no matter how complicated.  To find something
     between two single characters, a pattern like "/x([^x]*)x/"
     will get the intervening bits in $1. For multiple ones, then
     something more like "/alpha(.*?)omega/" would be needed. But
     none of these deals with nested patterns.  For balanced
     expressions using "(", "{", "[" or "<" as delimiters, use
     the CPAN module Regexp::Common, or see "(??{ code })" in
     perlre.  For other cases, you'll have to write a parser.

     If you are serious about writing a parser, there are a
     number of modules or oddities that will make your life a lot
     easier.  There are the CPAN modules "Parse::RecDescent",
     "Parse::Yapp", and "Text::Balanced"; and the "byacc"
     program. Starting from perl 5.8 the "Text::Balanced" is part
     of the standard distribution.

     One simple destructive, inside-out approach that you might
     try is to pull out the smallest nesting parts one at a time:

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             while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
                     # do something with $1

     A more complicated and sneaky approach is to make Perl's
     regular expression engine do it for you.  This is courtesy
     Dean Inada, and rather has the nature of an Obfuscated Perl
     Contest entry, but it really does work:

             # $_ contains the string to parse
             # BEGIN and END are the opening and closing markers for the
             # nested text.

             @( = ('(','');
             @) = (')','');
             @$ = (eval{/$re/},$@!~/unmatched/i);
             print join("\n",@$[0..$#$]) if( $$[-1] );

  How do I reverse a string?
     Use "reverse()" in scalar context, as documented in
     "reverse" in perlfunc.

             $reversed = reverse $string;

  How do I expand tabs in a string?
     You can do it yourself:

             1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;

     Or you can just use the "Text::Tabs" module (part of the
     standard Perl distribution).

             use Text::Tabs;
             @expanded_lines = expand(@lines_with_tabs);

  How do I reformat a paragraph?
     Use "Text::Wrap" (part of the standard Perl distribution):

             use Text::Wrap;
             print wrap("\t", '  ', @paragraphs);

     The paragraphs you give to "Text::Wrap" should not contain
     embedded newlines.  "Text::Wrap" doesn't justify the lines

     Or use the CPAN module "Text::Autoformat".  Formatting files
     can be easily done by making a shell alias, like so:

             alias fmt="perl -i -MText::Autoformat -n0777 \
                     -e 'print autoformat $_, {all=>1}' $*"

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     See the documentation for "Text::Autoformat" to appreciate
     its many capabilities.

  How can I access or change N characters of a string?
     You can access the first characters of a string with
     substr().  To get the first character, for example, start at
     position 0 and grab the string of length 1.

             $string = "Just another Perl Hacker";
             $first_char = substr( $string, 0, 1 );  #  'J'

     To change part of a string, you can use the optional fourth
     argument which is the replacement string.

             substr( $string, 13, 4, "Perl 5.8.0" );

     You can also use substr() as an lvalue.

             substr( $string, 13, 4 ) =  "Perl 5.8.0";

  How do I change the Nth occurrence of something?
     You have to keep track of N yourself.  For example, let's
     say you want to change the fifth occurrence of "whoever" or
     "whomever" into "whosoever" or "whomsoever", case
     insensitively.  These all assume that $_ contains the string
     to be altered.

             $count = 0;
             ++$count == 5       # is it the 5th?
                 ? "${2}soever"  # yes, swap
                 : $1            # renege and leave it there

     In the more general case, you can use the "/g" modifier in a
     "while" loop, keeping count of matches.

             $WANT = 3;
             $count = 0;
             $_ = "One fish two fish red fish blue fish";
             while (/(\w+)\s+fish\b/gi) {
                     if (++$count == $WANT) {
                             print "The third fish is a $1 one.\n";

     That prints out: "The third fish is a red one."  You can
     also use a repetition count and repeated pattern like this:


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  How can I count the number of occurrences of a substring within
     a string?
     There are a number of ways, with varying efficiency.  If you
     want a count of a certain single character (X) within a
     string, you can use the "tr///" function like so:

             $string = "ThisXlineXhasXsomeXx'sXinXit";
             $count = ($string =~ tr/X//);
             print "There are $count X characters in the string";

     This is fine if you are just looking for a single character.
     However, if you are trying to count multiple character
     substrings within a larger string, "tr///" won't work.  What
     you can do is wrap a while() loop around a global pattern
     match.  For example, let's count negative integers:

             $string = "-9 55 48 -2 23 -76 4 14 -44";
             while ($string =~ /-\d+/g) { $count++ }
             print "There are $count negative numbers in the string";

     Another version uses a global match in list context, then
     assigns the result to a scalar, producing a count of the
     number of matches.

             $count = () = $string =~ /-\d+/g;

  How do I capitalize all the words on one line?
     (contributed by brian d foy)

     Damian Conway's Text::Autoformat handles all of the thinking
     for you.

             use Text::Autoformat;
             my $x = "Dr. Strangelove or: How I Learned to Stop ".
               "Worrying and Love the Bomb";

             print $x, "\n";
             for my $style (qw( sentence title highlight )) {
                     print autoformat($x, { case => $style }), "\n";

     How do you want to capitalize those words?

             FRED AND BARNEY'S LODGE        # all uppercase
             Fred And Barney's Lodge        # title case
             Fred and Barney's Lodge        # highlight case

     It's not as easy a problem as it looks. How many words do
     you think are in there? Wait for it... wait for it.... If
     you answered 5 you're right. Perl words are groups of "\w+",
     but that's not what you want to capitalize. How is Perl
     supposed to know not to capitalize that "s" after the

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     apostrophe? You could try a regular expression:

             $string =~ s/ (
                                      (^\w)    #at the beginning of the line
                                        |      # or
                                      (\s\w)   #preceded by whitespace

             $string =~ s/([\w']+)/\u\L$1/g;

     Now, what if you don't want to capitalize that "and"? Just
     use Text::Autoformat and get on with the next problem. :)

  How can I split a [character] delimited string except when
     inside [character]?
     Several modules can handle this sort of
     parsing--"Text::Balanced", "Text::CSV", "Text::CSV_XS", and
     "Text::ParseWords", among others.

     Take the example case of trying to split a string that is
     comma-separated into its different fields. You can't use
     "split(/,/)" because you shouldn't split if the comma is
     inside quotes.  For example, take a data line like this:

             SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

     Due to the restriction of the quotes, this is a fairly
     complex problem.  Thankfully, we have Jeffrey Friedl, author
     of Mastering Regular Expressions, to handle these for us.
     He suggests (assuming your string is contained in $text):

              @new = ();
              push(@new, $+) while $text =~ m{
                      "([^\"\\]*(?:\\.[^\"\\]*)*)",?  # groups the phrase inside the quotes
                     | ([^,]+),?
                     | ,
              push(@new, undef) if substr($text,-1,1) eq ',';

     If you want to represent quotation marks inside a quotation-
     mark-delimited field, escape them with backslashes (eg,
     "like \"this\"".

     Alternatively, the "Text::ParseWords" module (part of the
     standard Perl distribution) lets you say:

             use Text::ParseWords;
             @new = quotewords(",", 0, $text);

  How do I strip blank space from the beginning/end of a string?

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     (contributed by brian d foy)

     A substitution can do this for you. For a single line, you
     want to replace all the leading or trailing whitespace with
     nothing. You can do that with a pair of substitutions.


     You can also write that as a single substitution, although
     it turns out the combined statement is slower than the
     separate ones. That might not matter to you, though.


     In this regular expression, the alternation matches either
     at the beginning or the end of the string since the anchors
     have a lower precedence than the alternation. With the "/g"
     flag, the substitution makes all possible matches, so it
     gets both. Remember, the trailing newline matches the "\s+",
     and  the "$" anchor can match to the physical end of the
     string, so the newline disappears too. Just add the newline
     to the output, which has the added benefit of preserving
     "blank" (consisting entirely of whitespace) lines which the
     "^\s+" would remove all by itself.

             while( <> )
                     print "$_\n";

     For a multi-line string, you can apply the regular
     expression to each logical line in the string by adding the
     "/m" flag (for "multi-line"). With the "/m" flag, the "$"
     matches before an embedded newline, so it doesn't remove it.
     It still removes the newline at the end of the string.

             $string =~ s/^\s+|\s+$//gm;

     Remember that lines consisting entirely of whitespace will
     disappear, since the first part of the alternation can match
     the entire string and replace it with nothing. If need to
     keep embedded blank lines, you have to do a little more
     work. Instead of matching any whitespace (since that
     includes a newline), just match the other whitespace.

             $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

  How do I pad a string with blanks or pad a number with zeroes?

     In the following examples, $pad_len is the length to which

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     you wish to pad the string, $text or $num contains the
     string to be padded, and $pad_char contains the padding
     character. You can use a single character string constant
     instead of the $pad_char variable if you know what it is in
     advance. And in the same way you can use an integer in place
     of $pad_len if you know the pad length in advance.

     The simplest method uses the "sprintf" function. It can pad
     on the left or right with blanks and on the left with zeroes
     and it will not truncate the result. The "pack" function can
     only pad strings on the right with blanks and it will
     truncate the result to a maximum length of $pad_len.

             # Left padding a string with blanks (no truncation):
             $padded = sprintf("%${pad_len}s", $text);
             $padded = sprintf("%*s", $pad_len, $text);  # same thing

             # Right padding a string with blanks (no truncation):
             $padded = sprintf("%-${pad_len}s", $text);
             $padded = sprintf("%-*s", $pad_len, $text); # same thing

             # Left padding a number with 0 (no truncation):
             $padded = sprintf("%0${pad_len}d", $num);
             $padded = sprintf("%0*d", $pad_len, $num); # same thing

             # Right padding a string with blanks using pack (will truncate):
             $padded = pack("A$pad_len",$text);

     If you need to pad with a character other than blank or zero
     you can use one of the following methods.  They all generate
     a pad string with the "x" operator and combine that with
     $text. These methods do not truncate $text.

     Left and right padding with any character, creating a new

             $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
             $padded = $text . $pad_char x ( $pad_len - length( $text ) );

     Left and right padding with any character, modifying $text

             substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
             $text .= $pad_char x ( $pad_len - length( $text ) );

  How do I extract selected columns from a string?
     (contributed by brian d foy)

     If you know the columns that contain the data, you can use
     "substr" to extract a single column.

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             my $column = substr( $line, $start_column, $length );

     You can use "split" if the columns are separated by
     whitespace or some other delimiter, as long as whitespace or
     the delimiter cannot appear as part of the data.

             my $line    = ' fred barney   betty   ';
             my @columns = split /\s+/, $line;
                     # ( '', 'fred', 'barney', 'betty' );

             my $line    = 'fred||barney||betty';
             my @columns = split /\|/, $line;
                     # ( 'fred', '', 'barney', '', 'betty' );

     If you want to work with comma-separated values, don't do
     this since that format is a bit more complicated. Use one of
     the modules that handle that format, such as "Text::CSV",
     "Text::CSV_XS", or "Text::CSV_PP".

     If you want to break apart an entire line of fixed columns,
     you can use "unpack" with the A (ASCII) format. By using a
     number after the format specifier, you can denote the column
     width. See the "pack" and "unpack" entries in perlfunc for
     more details.

             my @fields = unpack( $line, "A8 A8 A8 A16 A4" );

     Note that spaces in the format argument to "unpack" do not
     denote literal spaces. If you have space separated data, you
     may want "split" instead.

  How do I find the soundex value of a string?
     (contributed by brian d foy)

     You can use the Text::Soundex module. If you want to do
     fuzzy or close matching, you might also try the
     "String::Approx", and "Text::Metaphone", and
     "Text::DoubleMetaphone" modules.

  How can I expand variables in text strings?
     (contributed by brian d foy)

     If you can avoid it, don't, or if you can use a templating
     system, such as "Text::Template" or "Template" Toolkit, do
     that instead. You might even be able to get the job done
     with "sprintf" or "printf":

             my $string = sprintf 'Say hello to %s and %s', $foo, $bar;

     However, for the one-off simple case where I don't want to
     pull out a full templating system, I'll use a string that
     has two Perl scalar variables in it. In this example, I want

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     to expand $foo and $bar to their variable's values:

             my $foo = 'Fred';
             my $bar = 'Barney';
             $string = 'Say hello to $foo and $bar';

     One way I can do this involves the substitution operator and
     a double "/e" flag.  The first "/e" evaluates $1 on the
     replacement side and turns it into $foo. The second /e
     starts with $foo and replaces it with its value. $foo, then,
     turns into 'Fred', and that's finally what's left in the

             $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'

     The "/e" will also silently ignore violations of strict,
     replacing undefined variable names with the empty string.
     Since I'm using the "/e" flag (twice even!), I have all of
     the same security problems I have with "eval" in its string
     form. If there's something odd in $foo, perhaps something
     like "@{[ system "rm -rf /" ]}", then I could get myself in

     To get around the security problem, I could also pull the
     values from a hash instead of evaluating variable names.
     Using a single "/e", I can check the hash to ensure the
     value exists, and if it doesn't, I can replace the missing
     value with a marker, in this case "???" to signal that I
     missed something:

             my $string = 'This has $foo and $bar';

             my %Replacements = (
                     foo  => 'Fred',

             # $string =~ s/\$(\w+)/$Replacements{$1}/g;
             $string =~ s/\$(\w+)/
                     exists $Replacements{$1} ? $Replacements{$1} : '???'

             print $string;

  What's wrong with always quoting "$vars"?
     The problem is that those double-quotes force
     stringification--coercing numbers and references into
     strings--even when you don't want them to be strings.  Think
     of it this way: double-quote expansion is used to produce
     new strings.  If you already have a string, why do you need

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     If you get used to writing odd things like these:

             print "$var";           # BAD
             $new = "$old";          # BAD
             somefunc("$var");       # BAD

     You'll be in trouble.  Those should (in 99.8% of the cases)
     be the simpler and more direct:

             print $var;
             $new = $old;

     Otherwise, besides slowing you down, you're going to break
     code when the thing in the scalar is actually neither a
     string nor a number, but a reference:

             sub func {
                     my $aref = shift;
                     my $oref = "$aref";  # WRONG

     You can also get into subtle problems on those few
     operations in Perl that actually do care about the
     difference between a string and a number, such as the
     magical "++" autoincrement operator or the syscall()

     Stringification also destroys arrays.

             @lines = `command`;
             print "@lines";     # WRONG - extra blanks
             print @lines;       # right

  Why don't my <<HERE documents work?
     Check for these three things:

     There must be no space after the << part.
     There (probably) should be a semicolon at the end.
     You can't (easily) have any space in front of the tag.

     If you want to indent the text in the here document, you can
     do this:

         # all in one
         ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
             your text
             goes here

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     But the HERE_TARGET must still be flush against the margin.
     If you want that indented also, you'll have to quote in the

         ($quote = <<'    FINIS') =~ s/^\s+//gm;
                 ...we will have peace, when you and all your works have
                 perished--and the works of your dark master to whom you
                 would deliver us. You are a liar, Saruman, and a corrupter
                 of men's hearts.  --Theoden in /usr/src/perl/taint.c
         $quote =~ s/\s+--/\n--/;

     A nice general-purpose fixer-upper function for indented
     here documents follows.  It expects to be called with a here
     document as its argument.  It looks to see whether each line
     begins with a common substring, and if so, strips that
     substring off.  Otherwise, it takes the amount of leading
     whitespace found on the first line and removes that much off
     each subsequent line.

         sub fix {
             local $_ = shift;
             my ($white, $leader);  # common whitespace and common leading string
             if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
                 ($white, $leader) = ($2, quotemeta($1));
             } else {
                 ($white, $leader) = (/^(\s+)/, '');
             return $_;

     This works with leading special strings, dynamically

             $remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
             @@@ int
             @@@ runops() {
             @@@     SAVEI32(runlevel);
             @@@     runlevel++;
             @@@     while ( op = (*op->op_ppaddr)() );
             @@@     TAINT_NOT;
             @@@     return 0;
             @@@ }

     Or with a fixed amount of leading whitespace, with remaining
     indentation correctly preserved:

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             $poem = fix<<EVER_ON_AND_ON;
            Now far ahead the Road has gone,
               And I must follow, if I can,
            Pursuing it with eager feet,
               Until it joins some larger way
            Where many paths and errands meet.
               And whither then? I cannot say.
                     --Bilbo in /usr/src/perl/pp_ctl.c

Data: Arrays
  What is the difference between a list and an array?
     (contributed by brian d foy)

     A list is a fixed collection of scalars. An array is a
     variable that holds a variable collection of scalars. An
     array can supply its collection for list operations, so list
     operations also work on arrays:

             # slices
             ( 'dog', 'cat', 'bird' )[2,3];

             # iteration
             foreach ( qw( dog cat bird ) ) { ... }
             foreach ( @animals ) { ... }

             my @three = grep { length == 3 } qw( dog cat bird );
             my @three = grep { length == 3 } @animals;

             # supply an argument list
             wash_animals( qw( dog cat bird ) );
             wash_animals( @animals );

     Array operations, which change the scalars, reaaranges them,
     or adds or subtracts some scalars, only work on arrays.
     These can't work on a list, which is fixed. Array operations
     include "shift", "unshift", "push", "pop", and "splice".

     An array can also change its length:

             $#animals = 1;  # truncate to two elements
             $#animals = 10000; # pre-extend to 10,001 elements

     You can change an array element, but you can't change a list

             $animals[0] = 'Rottweiler';
             qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!

             foreach ( @animals ) {
                     s/^d/fr/;  # works fine

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             foreach ( qw( dog cat bird ) ) {
                     s/^d/fr/;  # Error! Modification of read only value!

     However, if the list element is itself a variable, it
     appears that you can change a list element. However, the
     list element is the variable, not the data. You're not
     changing the list element, but something the list element
     refers to. The list element itself doesn't change: it's
     still the same variable.

     You also have to be careful about context. You can assign an
     array to a scalar to get the number of elements in the
     array. This only works for arrays, though:

             my $count = @animals;  # only works with arrays

     If you try to do the same thing with what you think is a
     list, you get a quite different result. Although it looks
     like you have a list on the righthand side, Perl actually
     sees a bunch of scalars separated by a comma:

             my $scalar = ( 'dog', 'cat', 'bird' );  # $scalar gets bird

     Since you're assigning to a scalar, the righthand side is in
     scalar context. The comma operator (yes, it's an operator!)
     in scalar context evaluates its lefthand side, throws away
     the result, and evaluates it's righthand side and returns
     the result. In effect, that list-lookalike assigns to
     $scalar it's rightmost value. Many people mess this up
     becuase they choose a list-lookalike whose last element is
     also the count they expect:

             my $scalar = ( 1, 2, 3 );  # $scalar gets 3, accidentally

  What is the difference between $array[1] and @array[1]?
     (contributed by brian d foy)

     The difference is the sigil, that special character in front
     of the array name. The "$" sigil means "exactly one item",
     while the "@" sigil means "zero or more items". The "$" gets
     you a single scalar, while the "@" gets you a list.

     The confusion arises because people incorrectly assume that
     the sigil denotes the variable type.

     The $array[1] is a single-element access to the array. It's
     going to return the item in index 1 (or undef if there is no
     item there).  If you intend to get exactly one element from
     the array, this is the form you should use.

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     The @array[1] is an array slice, although it has only one
     index.  You can pull out multiple elements simultaneously by
     specifying additional indices as a list, like

     Using a slice on the lefthand side of the assignment
     supplies list context to the righthand side. This can lead
     to unexpected results.  For instance, if you want to read a
     single line from a filehandle, assigning to a scalar value
     is fine:

             $array[1] = <STDIN>;

     However, in list context, the line input operator returns
     all of the lines as a list. The first line goes into
     @array[1] and the rest of the lines mysteriously disappear:

             @array[1] = <STDIN>;  # most likely not what you want

     Either the "use warnings" pragma or the -w flag will warn
     you when you use an array slice with a single index.

  How can I remove duplicate elements from a list or array?
     (contributed by brian d foy)

     Use a hash. When you think the words "unique" or
     "duplicated", think "hash keys".

     If you don't care about the order of the elements, you could
     just create the hash then extract the keys. It's not
     important how you create that hash: just that you use "keys"
     to get the unique elements.

             my %hash   = map { $_, 1 } @array;
             # or a hash slice: @hash{ @array } = ();
             # or a foreach: $hash{$_} = 1 foreach ( @array );

             my @unique = keys %hash;

     If you want to use a module, try the "uniq" function from
     "List::MoreUtils". In list context it returns the unique
     elements, preserving their order in the list. In scalar
     context, it returns the number of unique elements.

             use List::MoreUtils qw(uniq);

             my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
             my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7

     You can also go through each element and skip the ones
     you've seen before. Use a hash to keep track. The first time
     the loop sees an element, that element has no key in %Seen.

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     The "next" statement creates the key and immediately uses
     its value, which is "undef", so the loop continues to the
     "push" and increments the value for that key. The next time
     the loop sees that same element, its key exists in the hash
     and the value for that key is true (since it's not 0 or
     "undef"), so the next skips that iteration and the loop goes
     to the next element.

             my @unique = ();
             my %seen   = ();

             foreach my $elem ( @array )
                     next if $seen{ $elem }++;
                     push @unique, $elem;

     You can write this more briefly using a grep, which does the
     same thing.

             my %seen = ();
             my @unique = grep { ! $seen{ $_ }++ } @array;

  How can I tell whether a certain element is contained in a list
     or array?
     (portions of this answer contributed by Anno Siegel and
     brian d foy)

     Hearing the word "in" is an indication that you probably
     should have used a hash, not a list or array, to store your
     data.  Hashes are designed to answer this question quickly
     and efficiently.  Arrays aren't.

     That being said, there are several ways to approach this.
     In Perl 5.10 and later, you can use the smart match operator
     to check that an item is contained in an array or a hash:

             use 5.010;

             if( $item ~~ @array )
                     say "The array contains $item"

             if( $item ~~ %hash )
                     say "The hash contains $item"

     With earlier versions of Perl, you have to do a bit more
     work. If you are going to make this query many times over
     arbitrary string values, the fastest way is probably to

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     invert the original array and maintain a hash whose keys are
     the first array's values:

             @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
             %is_blue = ();
             for (@blues) { $is_blue{$_} = 1 }

     Now you can check whether $is_blue{$some_color}.  It might
     have been a good idea to keep the blues all in a hash in the
     first place.

     If the values are all small integers, you could use a simple
     indexed array.  This kind of an array will take up less

             @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
             @is_tiny_prime = ();
             for (@primes) { $is_tiny_prime[$_] = 1 }
             # or simply  @istiny_prime[@primes] = (1) x @primes;

     Now you check whether $is_tiny_prime[$some_number].

     If the values in question are integers instead of strings,
     you can save quite a lot of space by using bit strings

             @articles = ( 1..10, 150..2000, 2017 );
             undef $read;
             for (@articles) { vec($read,$_,1) = 1 }

     Now check whether "vec($read,$n,1)" is true for some $n.

     These methods guarantee fast individual tests but require a
     re-organization of the original list or array.  They only
     pay off if you have to test multiple values against the same

     If you are testing only once, the standard module
     "List::Util" exports the function "first" for this purpose.
     It works by stopping once it finds the element. It's written
     in C for speed, and its Perl equivalent looks like this

             sub first (&@) {
                     my $code = shift;
                     foreach (@_) {
                             return $_ if &{$code}();

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     If speed is of little concern, the common idiom uses grep in
     scalar context (which returns the number of items that
     passed its condition) to traverse the entire list. This does
     have the benefit of telling you how many matches it found,

             my $is_there = grep $_ eq $whatever, @array;

     If you want to actually extract the matching elements,
     simply use grep in list context.

             my @matches = grep $_ eq $whatever, @array;

  How do I compute the difference of two arrays?  How do I
     compute the intersection of two arrays?
     Use a hash.  Here's code to do both and more.  It assumes
     that each element is unique in a given array:

             @union = @intersection = @difference = ();
             %count = ();
             foreach $element (@array1, @array2) { $count{$element}++ }
             foreach $element (keys %count) {
                     push @union, $element;
                     push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;

     Note that this is the symmetric difference, that is, all
     elements in either A or in B but not in both.  Think of it
     as an xor operation.

  How do I test whether two arrays or hashes are equal?
     With Perl 5.10 and later, the smart match operator can give
     you the answer with the least amount of work:

             use 5.010;

             if( @array1 ~~ @array2 )
                     say "The arrays are the same";

             if( %hash1 ~~ %hash2 ) # doesn't check values!
                     say "The hash keys are the same";

     The following code works for single-level arrays.  It uses a
     stringwise comparison, and does not distinguish defined
     versus undefined empty strings.  Modify if you have other

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             $are_equal = compare_arrays(\@frogs, \@toads);

             sub compare_arrays {
                     my ($first, $second) = @_;
                     no warnings;  # silence spurious -w undef complaints
                     return 0 unless @$first == @$second;
                     for (my $i = 0; $i < @$first; $i++) {
                             return 0 if $first->[$i] ne $second->[$i];
                     return 1;

     For multilevel structures, you may wish to use an approach
     more like this one.  It uses the CPAN module "FreezeThaw":

             use FreezeThaw qw(cmpStr);
             @a = @b = ( "this", "that", [ "more", "stuff" ] );

             printf "a and b contain %s arrays\n",
                     cmpStr(\@a, \@b) == 0
                     ? "the same"
                     : "different";

     This approach also works for comparing hashes.  Here we'll
     demonstrate two different answers:

             use FreezeThaw qw(cmpStr cmpStrHard);

             %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
             $a{EXTRA} = \%b;
             $b{EXTRA} = \%a;

             printf "a and b contain %s hashes\n",
             cmpStr(\%a, \%b) == 0 ? "the same" : "different";

             printf "a and b contain %s hashes\n",
             cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";

     The first reports that both those the hashes contain the
     same data, while the second reports that they do not.  Which
     you prefer is left as an exercise to the reader.

  How do I find the first array element for which a condition is
     To find the first array element which satisfies a condition,
     you can use the "first()" function in the "List::Util"
     module, which comes with Perl 5.8. This example finds the
     first element that contains "Perl".

             use List::Util qw(first);

             my $element = first { /Perl/ } @array;

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     If you cannot use "List::Util", you can make your own loop
     to do the same thing.  Once you find the element, you stop
     the loop with last.

             my $found;
             foreach ( @array ) {
                     if( /Perl/ ) { $found = $_; last }

     If you want the array index, you can iterate through the
     indices and check the array element at each index until you
     find one that satisfies the condition.

             my( $found, $index ) = ( undef, -1 );
             for( $i = 0; $i < @array; $i++ ) {
                     if( $array[$i] =~ /Perl/ ) {
                             $found = $array[$i];
                             $index = $i;

  How do I handle linked lists?
     In general, you usually don't need a linked list in Perl,
     since with regular arrays, you can push and pop or shift and
     unshift at either end, or you can use splice to add and/or
     remove arbitrary number of elements at arbitrary points.
     Both pop and shift are O(1) operations on Perl's dynamic
     arrays.  In the absence of shifts and pops, push in general
     needs to reallocate on the order every log(N) times, and
     unshift will need to copy pointers each time.

     If you really, really wanted, you could use structures as
     described in perldsc or perltoot and do just what the
     algorithm book tells you to do.  For example, imagine a list
     node like this:

             $node = {
                     VALUE => 42,
                     LINK  => undef,

     You could walk the list this way:

             print "List: ";
             for ($node = $head;  $node; $node = $node->{LINK}) {
                     print $node->{VALUE}, " ";
             print "\n";

     You could add to the list this way:

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             my ($head, $tail);
             $tail = append($head, 1);       # grow a new head
             for $value ( 2 .. 10 ) {
                     $tail = append($tail, $value);

             sub append {
                     my($list, $value) = @_;
                     my $node = { VALUE => $value };
                     if ($list) {
                             $node->{LINK} = $list->{LINK};
                             $list->{LINK} = $node;
                     else {
                             $_[0] = $node;      # replace caller's version
                     return $node;

     But again, Perl's built-in are virtually always good enough.

  How do I handle circular lists?
     (contributed by brian d foy)

     If you want to cycle through an array endlessly, you can
     increment the index modulo the number of elements in the

             my @array = qw( a b c );
             my $i = 0;

             while( 1 ) {
                     print $array[ $i++ % @array ], "\n";
                     last if $i > 20;

     You can also use "Tie::Cycle" to use a scalar that always
     has the next element of the circular array:

             use Tie::Cycle;

             tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];

             print $cycle; # FFFFFF
             print $cycle; # 000000
             print $cycle; # FFFF00

     The "Array::Iterator::Circular" creates an iterator object
     for circular arrays:

             use Array::Iterator::Circular;

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             my $color_iterator = Array::Iterator::Circular->new(
                     qw(red green blue orange)

             foreach ( 1 .. 20 ) {
                     print $color_iterator->next, "\n";

  How do I shuffle an array randomly?
     If you either have Perl 5.8.0 or later installed, or if you
     have Scalar-List-Utils 1.03 or later installed, you can say:

             use List::Util 'shuffle';

             @shuffled = shuffle(@list);

     If not, you can use a Fisher-Yates shuffle.

             sub fisher_yates_shuffle {
                     my $deck = shift;  # $deck is a reference to an array
                     return unless @$deck; # must not be empty!

                     my $i = @$deck;
                     while (--$i) {
                             my $j = int rand ($i+1);
                             @$deck[$i,$j] = @$deck[$j,$i];

             # shuffle my mpeg collection
             my @mpeg = <audio/*/*.mp3>;
             fisher_yates_shuffle( \@mpeg );    # randomize @mpeg in place
             print @mpeg;

     Note that the above implementation shuffles an array in
     place, unlike the "List::Util::shuffle()" which takes a list
     and returns a new shuffled list.

     You've probably seen shuffling algorithms that work using
     splice, randomly picking another element to swap the current
     element with

             @new = ();
             @old = 1 .. 10;  # just a demo
             while (@old) {
                     push(@new, splice(@old, rand @old, 1));

     This is bad because splice is already O(N), and since you do
     it N times, you just invented a quadratic algorithm; that

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     is, O(N**2).  This does not scale, although Perl is so
     efficient that you probably won't notice this until you have
     rather largish arrays.

  How do I process/modify each element of an array?
     Use "for"/"foreach":

             for (@lines) {
                     s/foo/bar/;     # change that word
                     tr/XZ/ZX/;      # swap those letters

     Here's another; let's compute spherical volumes:

             for (@volumes = @radii) {   # @volumes has changed parts
                     $_ **= 3;
                     $_ *= (4/3) * 3.14159;  # this will be constant folded

     which can also be done with "map()" which is made to
     transform one list into another:

             @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;

     If you want to do the same thing to modify the values of the
     hash, you can use the "values" function.  As of Perl 5.6 the
     values are not copied, so if you modify $orbit (in this
     case), you modify the value.

             for $orbit ( values %orbits ) {
                     ($orbit **= 3) *= (4/3) * 3.14159;

     Prior to perl 5.6 "values" returned copies of the values, so
     older perl code often contains constructions such as
     @orbits{keys %orbits} instead of "values %orbits" where the
     hash is to be modified.

  How do I select a random element from an array?
     Use the "rand()" function (see "rand" in perlfunc):

             $index   = rand @array;
             $element = $array[$index];

     Or, simply:

             my $element = $array[ rand @array ];

  How do I permute N elements of a list?
     Use the "List::Permutor" module on CPAN. If the list is
     actually an array, try the "Algorithm::Permute" module (also
     on CPAN). It's written in XS code and is very efficient:

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             use Algorithm::Permute;

             my @array = 'a'..'d';
             my $p_iterator = Algorithm::Permute->new ( \@array );

             while (my @perm = $p_iterator->next) {
                print "next permutation: (@perm)\n";

     For even faster execution, you could do:

             use Algorithm::Permute;

             my @array = 'a'..'d';

             Algorithm::Permute::permute {
                     print "next permutation: (@array)\n";
                     } @array;

     Here's a little program that generates all permutations of
     all the words on each line of input. The algorithm embodied
     in the "permute()" function is discussed in Volume 4 (still
     unpublished) of Knuth's The Art of Computer Programming and
     will work on any list:

             #!/usr/bin/perl -n
             # Fischer-Krause ordered permutation generator

             sub permute (&@) {
                     my $code = shift;
                     my @idx = 0..$#_;
                     while ( $code->(@_[@idx]) ) {
                             my $p = $#idx;
                             --$p while $idx[$p-1] > $idx[$p];
                             my $q = $p or return;
                             push @idx, reverse splice @idx, $p;
                             ++$q while $idx[$p-1] > $idx[$q];

             permute { print "@_\n" } split;

     The "Algorithm::Loops" module also provides the
     "NextPermute" and "NextPermuteNum" functions which
     efficiently find all unique permutations of an array, even
     if it contains duplicate values, modifying it in-place: if
     its elements are in reverse-sorted order then the array is
     reversed, making it sorted, and it returns false; otherwise
     the next permutation is returned.

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     "NextPermute" uses string order and "NextPermuteNum" numeric
     order, so you can enumerate all the permutations of 0..9
     like this:

             use Algorithm::Loops qw(NextPermuteNum);

         my @list= 0..9;
         do { print "@list\n" } while NextPermuteNum @list;

  How do I sort an array by (anything)?
     Supply a comparison function to sort() (described in "sort"
     in perlfunc):

             @list = sort { $a <=> $b } @list;

     The default sort function is cmp, string comparison, which
     would sort "(1, 2, 10)" into "(1, 10, 2)".  "<=>", used
     above, is the numerical comparison operator.

     If you have a complicated function needed to pull out the
     part you want to sort on, then don't do it inside the sort
     function.  Pull it out first, because the sort BLOCK can be
     called many times for the same element.  Here's an example
     of how to pull out the first word after the first number on
     each item, and then sort those words case-insensitively.

             @idx = ();
             for (@data) {
                     ($item) = /\d+\s*(\S+)/;
                     push @idx, uc($item);
             @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];

     which could also be written this way, using a trick that's
     come to be known as the Schwartzian Transform:

             @sorted = map  { $_->[0] }
                     sort { $a->[1] cmp $b->[1] }
                     map  { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;

     If you need to sort on several fields, the following
     paradigm is useful.

             @sorted = sort {
                     field1($a) <=> field1($b) ||
                     field2($a) cmp field2($b) ||
                     field3($a) cmp field3($b)
                     } @data;

     This can be conveniently combined with precalculation of
     keys as given above.

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     See the sort article in the "Far More Than You Ever Wanted
     To Know" collection in for more about
     this approach.

     See also the question later in perlfaq4 on sorting hashes.

  How do I manipulate arrays of bits?
     Use "pack()" and "unpack()", or else "vec()" and the bitwise

     For example, you don't have to store individual bits in an
     array (which would mean that you're wasting a lot of space).
     To convert an array of bits to a string, use "vec()" to set
     the right bits. This sets $vec to have bit N set only if
     $ints[N] was set:

             @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
             $vec = '';
             foreach( 0 .. $#ints ) {
                     vec($vec,$_,1) = 1 if $ints[$_];

     The string $vec only takes up as many bits as it needs. For
     instance, if you had 16 entries in @ints, $vec only needs
     two bytes to store them (not counting the scalar variable

     Here's how, given a vector in $vec, you can get those bits
     into your @ints array:

             sub bitvec_to_list {
                     my $vec = shift;
                     my @ints;
                     # Find null-byte density then select best algorithm
                     if ($vec =~ tr/\0// / length $vec > 0.95) {
                             use integer;
                             my $i;

                             # This method is faster with mostly null-bytes
                             while($vec =~ /[^\0]/g ) {
                                     $i = -9 + 8 * pos $vec;
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);
                                     push @ints, $i if vec($vec, ++$i, 1);

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                     else {
                             # This method is a fast general algorithm
                             use integer;
                             my $bits = unpack "b*", $vec;
                             push @ints, 0 if $bits =~ s/^(\d)// && $1;
                             push @ints, pos $bits while($bits =~ /1/g);

                     return \@ints;

     This method gets faster the more sparse the bit vector is.
     (Courtesy of Tim Bunce and Winfried Koenig.)

     You can make the while loop a lot shorter with this
     suggestion from Benjamin Goldberg:

             while($vec =~ /[^\0]+/g ) {
                     push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;

     Or use the CPAN module "Bit::Vector":

             $vector = Bit::Vector->new($num_of_bits);
             @ints = $vector->Index_List_Read();

     "Bit::Vector" provides efficient methods for bit vector,
     sets of small integers and "big int" math.

     Here's a more extensive illustration using vec():

             # vec demo
             $vector = "\xff\x0f\xef\xfe";
             print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
             unpack("N", $vector), "\n";
             $is_set = vec($vector, 23, 1);
             print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";




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             sub set_vec {
                     my ($offset, $width, $value) = @_;
                     my $vector = '';
                     vec($vector, $offset, $width) = $value;
                     print "offset=$offset width=$width value=$value\n";

             sub pvec {
                     my $vector = shift;
                     my $bits = unpack("b*", $vector);
                     my $i = 0;
                     my $BASE = 8;

                     print "vector length in bytes: ", length($vector), "\n";
                     @bytes = unpack("A8" x length($vector), $bits);
                     print "bits are: @bytes\n\n";

  Why does defined() return true on empty arrays and hashes?
     The short story is that you should probably only use defined
     on scalars or functions, not on aggregates (arrays and
     hashes).  See "defined" in perlfunc in the 5.004 release or
     later of Perl for more detail.

Data: Hashes (Associative Arrays)
  How do I process an entire hash?
     (contributed by brian d foy)

     There are a couple of ways that you can process an entire
     hash. You can get a list of keys, then go through each key,
     or grab a one key-value pair at a time.

     To go through all of the keys, use the "keys" function. This
     extracts all of the keys of the hash and gives them back to
     you as a list. You can then get the value through the
     particular key you're processing:

             foreach my $key ( keys %hash ) {
                     my $value = $hash{$key}

     Once you have the list of keys, you can process that list
     before you process the hash elements. For instance, you can
     sort the keys so you can process them in lexical order:

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             foreach my $key ( sort keys %hash ) {
                     my $value = $hash{$key}

     Or, you might want to only process some of the items. If you
     only want to deal with the keys that start with "text:", you
     can select just those using "grep":

             foreach my $key ( grep /^text:/, keys %hash ) {
                     my $value = $hash{$key}

     If the hash is very large, you might not want to create a
     long list of keys. To save some memory, you can grab one
     key-value pair at a time using "each()", which returns a
     pair you haven't seen yet:

             while( my( $key, $value ) = each( %hash ) ) {

     The "each" operator returns the pairs in apparently random
     order, so if ordering matters to you, you'll have to stick
     with the "keys" method.

     The "each()" operator can be a bit tricky though. You can't
     add or delete keys of the hash while you're using it without
     possibly skipping or re-processing some pairs after Perl
     internally rehashes all of the elements. Additionally, a
     hash has only one iterator, so if you use "keys", "values",
     or "each" on the same hash, you can reset the iterator and
     mess up your processing. See the "each" entry in perlfunc
     for more details.

  How do I merge two hashes?
     (contributed by brian d foy)

     Before you decide to merge two hashes, you have to decide
     what to do if both hashes contain keys that are the same and
     if you want to leave the original hashes as they were.

     If you want to preserve the original hashes, copy one hash
     (%hash1) to a new hash (%new_hash), then add the keys from
     the other hash (%hash2 to the new hash. Checking that the
     key already exists in %new_hash gives you a chance to decide
     what to do with the duplicates:

             my %new_hash = %hash1; # make a copy; leave %hash1 alone

             foreach my $key2 ( keys %hash2 )

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                     if( exists $new_hash{$key2} )
                             warn "Key [$key2] is in both hashes!";
                             # handle the duplicate (perhaps only warning)
                             $new_hash{$key2} = $hash2{$key2};

     If you don't want to create a new hash, you can still use
     this looping technique; just change the %new_hash to %hash1.

             foreach my $key2 ( keys %hash2 )
                     if( exists $hash1{$key2} )
                             warn "Key [$key2] is in both hashes!";
                             # handle the duplicate (perhaps only warning)
                             $hash1{$key2} = $hash2{$key2};

     If you don't care that one hash overwrites keys and values
     from the other, you could just use a hash slice to add one
     hash to another. In this case, values from %hash2 replace
     values from %hash1 when they have keys in common:

             @hash1{ keys %hash2 } = values %hash2;

  What happens if I add or remove keys from a hash while
     iterating over it?
     (contributed by brian d foy)

     The easy answer is "Don't do that!"

     If you iterate through the hash with each(), you can delete
     the key most recently returned without worrying about it.
     If you delete or add other keys, the iterator may skip or
     double up on them since perl may rearrange the hash table.
     See the entry for "each()" in perlfunc.

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  How do I look up a hash element by value?
     Create a reverse hash:

             %by_value = reverse %by_key;
             $key = $by_value{$value};

     That's not particularly efficient.  It would be more space-
     efficient to use:

             while (($key, $value) = each %by_key) {
                     $by_value{$value} = $key;

     If your hash could have repeated values, the methods above
     will only find one of the associated keys.   This may or may
     not worry you.  If it does worry you, you can always reverse
     the hash into a hash of arrays instead:

             while (($key, $value) = each %by_key) {
                      push @{$key_list_by_value{$value}}, $key;

  How can I know how many entries are in a hash?
     (contributed by brian d foy)

     This is very similar to "How do I process an entire hash?",
     also in perlfaq4, but a bit simpler in the common cases.

     You can use the "keys()" built-in function in scalar context
     to find out have many entries you have in a hash:

             my $key_count = keys %hash; # must be scalar context!

     If you want to find out how many entries have a defined
     value, that's a bit different. You have to check each value.
     A "grep" is handy:

             my $defined_value_count = grep { defined } values %hash;

     You can use that same structure to count the entries any way
     that you like. If you want the count of the keys with vowels
     in them, you just test for that instead:

             my $vowel_count = grep { /[aeiou]/ } keys %hash;

     The "grep" in scalar context returns the count. If you want
     the list of matching items, just use it in list context

             my @defined_values = grep { defined } values %hash;

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     The "keys()" function also resets the iterator, which means
     that you may see strange results if you use this between
     uses of other hash operators such as "each()".

  How do I sort a hash (optionally by value instead of key)?
     (contributed by brian d foy)

     To sort a hash, start with the keys. In this example, we
     give the list of keys to the sort function which then
     compares them ASCIIbetically (which might be affected by
     your locale settings). The output list has the keys in
     ASCIIbetical order. Once we have the keys, we can go through
     them to create a report which lists the keys in ASCIIbetical

             my @keys = sort { $a cmp $b } keys %hash;

             foreach my $key ( @keys )
                     printf "%-20s %6d\n", $key, $hash{$key};

     We could get more fancy in the "sort()" block though.
     Instead of comparing the keys, we can compute a value with
     them and use that value as the comparison.

     For instance, to make our report order case-insensitive, we
     use the "\L" sequence in a double-quoted string to make
     everything lowercase. The "sort()" block then compares the
     lowercased values to determine in which order to put the

             my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;

     Note: if the computation is expensive or the hash has many
     elements, you may want to look at the Schwartzian Transform
     to cache the computation results.

     If we want to sort by the hash value instead, we use the
     hash key to look it up. We still get out a list of keys, but
     this time they are ordered by their value.

             my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;

     From there we can get more complex. If the hash values are
     the same, we can provide a secondary sort on the hash key.

             my @keys = sort {
                     $hash{$a} <=> $hash{$b}
                     "\L$a" cmp "\L$b"
                     } keys %hash;

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  How can I always keep my hash sorted?
     You can look into using the "DB_File" module and "tie()"
     using the $DB_BTREE hash bindings as documented in "In
     Memory Databases" in DB_File. The "Tie::IxHash" module from
     CPAN might also be instructive. Although this does keep your
     hash sorted, you might not like the slow down you suffer
     from the tie interface. Are you sure you need to do this? :)

  What's the difference between "delete" and "undef" with hashes?

     Hashes contain pairs of scalars: the first is the key, the
     second is the value.  The key will be coerced to a string,
     although the value can be any kind of scalar: string,
     number, or reference.  If a key $key is present in %hash,
     "exists($hash{$key})" will return true.  The value for a
     given key can be "undef", in which case $hash{$key} will be
     "undef" while "exists $hash{$key}" will return true.  This
     corresponds to ($key, "undef") being in the hash.

     Pictures help...  Here's the %hash table:

               keys  values
             |  a   |  3   |
             |  x   |  7   |
             |  d   |  0   |
             |  e   |  2   |

     And these conditions hold

             $hash{'a'}                       is true
             $hash{'d'}                       is false
             defined $hash{'d'}               is true
             defined $hash{'a'}               is true
             exists $hash{'a'}                is true (Perl 5 only)
             grep ($_ eq 'a', keys %hash)     is true

     If you now say

             undef $hash{'a'}

     your table now reads:

               keys  values
             |  a   | undef|
             |  x   |  7   |
             |  d   |  0   |
             |  e   |  2   |

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     and these conditions now hold; changes in caps:

             $hash{'a'}                       is FALSE
             $hash{'d'}                       is false
             defined $hash{'d'}               is true
             defined $hash{'a'}               is FALSE
             exists $hash{'a'}                is true (Perl 5 only)
             grep ($_ eq 'a', keys %hash)     is true

     Notice the last two: you have an undef value, but a defined

     Now, consider this:

             delete $hash{'a'}

     your table now reads:

               keys  values
             |  x   |  7   |
             |  d   |  0   |
             |  e   |  2   |

     and these conditions now hold; changes in caps:

             $hash{'a'}                       is false
             $hash{'d'}                       is false
             defined $hash{'d'}               is true
             defined $hash{'a'}               is false
             exists $hash{'a'}                is FALSE (Perl 5 only)
             grep ($_ eq 'a', keys %hash)     is FALSE

     See, the whole entry is gone!

  Why don't my tied hashes make the defined/exists distinction?

     This depends on the tied hash's implementation of EXISTS().
     For example, there isn't the concept of undef with hashes
     that are tied to DBM* files. It also means that exists() and
     defined() do the same thing with a DBM* file, and what they
     end up doing is not what they do with ordinary hashes.

  How do I reset an each() operation part-way through?
     (contributed by brian d foy)

     You can use the "keys" or "values" functions to reset
     "each". To simply reset the iterator used by "each" without
     doing anything else, use one of them in void context:

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             keys %hash; # resets iterator, nothing else.
             values %hash; # resets iterator, nothing else.

     See the documentation for "each" in perlfunc.

  How can I get the unique keys from two hashes?
     First you extract the keys from the hashes into lists, then
     solve the "removing duplicates" problem described above.
     For example:

             %seen = ();
             for $element (keys(%foo), keys(%bar)) {
             @uniq = keys %seen;

     Or more succinctly:

             @uniq = keys %{{%foo,%bar}};

     Or if you really want to save space:

             %seen = ();
             while (defined ($key = each %foo)) {
             while (defined ($key = each %bar)) {
             @uniq = keys %seen;

  How can I store a multidimensional array in a DBM file?
     Either stringify the structure yourself (no fun), or else
     get the MLDBM (which uses Data::Dumper) module from CPAN and
     layer it on top of either DB_File or GDBM_File.

  How can I make my hash remember the order I put elements into
     Use the "Tie::IxHash" from CPAN.

             use Tie::IxHash;

             tie my %myhash, 'Tie::IxHash';

             for (my $i=0; $i<20; $i++) {
                     $myhash{$i} = 2*$i;

             my @keys = keys %myhash;
             # @keys = (0,1,2,3,...)

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  Why does passing a subroutine an undefined element in a hash
     create it?
     (contributed by brian d foy)

     Are you using a really old version of Perl?

     Normally, accessing a hash key's value for a nonexistent key
     will not create the key.

             my %hash  = ();
             my $value = $hash{ 'foo' };
             print "This won't print\n" if exists $hash{ 'foo' };

     Passing $hash{ 'foo' } to a subroutine used to be a special
     case, though.  Since you could assign directly to $_[0],
     Perl had to be ready to make that assignment so it created
     the hash key ahead of time:

         my_sub( $hash{ 'foo' } );
             print "This will print before 5.004\n" if exists $hash{ 'foo' };

             sub my_sub {
                     # $_[0] = 'bar'; # create hash key in case you do this

     Since Perl 5.004, however, this situation is a special case
     and Perl creates the hash key only when you make the

         my_sub( $hash{ 'foo' } );
             print "This will print, even after 5.004\n" if exists $hash{ 'foo' };

             sub my_sub {
                     $_[0] = 'bar';

     However, if you want the old behavior (and think carefully
     about that because it's a weird side effect), you can pass a
     hash slice instead.  Perl 5.004 didn't make this a special

             my_sub( @hash{ qw/foo/ } );

  How can I make the Perl equivalent of a C structure/C++
     class/hash or array of hashes or arrays?
     Usually a hash ref, perhaps like this:

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             $record = {
                     NAME   => "Jason",
                     EMPNO  => 132,
                     TITLE  => "deputy peon",
                     AGE    => 23,
                     SALARY => 37_000,
                     PALS   => [ "Norbert", "Rhys", "Phineas"],

     References are documented in perlref and the upcoming
     perlreftut.  Examples of complex data structures are given
     in perldsc and perllol.  Examples of structures and object-
     oriented classes are in perltoot.

  How can I use a reference as a hash key?
     (contributed by brian d foy and Ben Morrow)

     Hash keys are strings, so you can't really use a reference
     as the key.  When you try to do that, perl turns the
     reference into its stringified form (for instance,
     "HASH(0xDEADBEEF)"). From there you can't get back the
     reference from the stringified form, at least without doing
     some extra work on your own.

     Remember that the entry in the hash will still be there even
     if the referenced variable  goes out of scope, and that it
     is entirely possible for Perl to subsequently allocate a
     different variable at the same address. This will mean a new
     variable might accidentally be associated with the value for
     an old.

     If you have Perl 5.10 or later, and you just want to store a
     value against the reference for lookup later, you can use
     the core Hash::Util::Fieldhash module. This will also handle
     renaming the keys if you use multiple threads (which causes
     all variables to be reallocated at new addresses, changing
     their stringification), and garbage-collecting the entries
     when the referenced variable goes out of scope.

     If you actually need to be able to get a real reference back
     from each hash entry, you can use the Tie::RefHash module,
     which does the required work for you.

Data: Misc
  How do I handle binary data correctly?
     Perl is binary clean, so it can handle binary data just
     fine.  On Windows or DOS, however, you have to use "binmode"
     for binary files to avoid conversions for line endings. In
     general, you should use "binmode" any time you want to work
     with binary data.

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     Also see "binmode" in perlfunc or perlopentut.

     If you're concerned about 8-bit textual data then see
     perllocale.  If you want to deal with multibyte characters,
     however, there are some gotchas.  See the section on Regular

  How do I determine whether a scalar is a
     Assuming that you don't care about IEEE notations like "NaN"
     or "Infinity", you probably just want to use a regular

             if (/\D/)            { print "has nondigits\n" }
             if (/^\d+$/)         { print "is a whole number\n" }
             if (/^-?\d+$/)       { print "is an integer\n" }
             if (/^[+-]?\d+$/)    { print "is a +/- integer\n" }
             if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
             if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
             if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                             { print "a C float\n" }

     There are also some commonly used modules for the task.
     Scalar::Util (distributed with 5.8) provides access to
     perl's internal function "looks_like_number" for determining
     whether a variable looks like a number.  Data::Types exports
     functions that validate data types using both the above and
     other regular expressions. Thirdly, there is
     "Regexp::Common" which has regular expressions to match
     various types of numbers. Those three modules are available
     from the CPAN.

     If you're on a POSIX system, Perl supports the
     "POSIX::strtod" function.  Its semantics are somewhat
     cumbersome, so here's a "getnum" wrapper function for more
     convenient access.  This function takes a string and returns
     the number it found, or "undef" for input that isn't a C
     float.  The "is_numeric" function is a front end to "getnum"
     if you just want to say, "Is this a float?"

             sub getnum {
                     use POSIX qw(strtod);
                     my $str = shift;
                     $str =~ s/^\s+//;
                     $str =~ s/\s+$//;
                     $! = 0;
                     my($num, $unparsed) = strtod($str);
                     if (($str eq '') || ($unparsed != 0) || $!) {
                                     return undef;
                     else {
                             return $num;

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             sub is_numeric { defined getnum($_[0]) }

     Or you could check out the String::Scanf module on the CPAN
     instead. The "POSIX" module (part of the standard Perl
     distribution) provides the "strtod" and "strtol" for
     converting strings to double and longs, respectively.

  How do I keep persistent data across program calls?
     For some specific applications, you can use one of the DBM
     modules.  See AnyDBM_File.  More generically, you should
     consult the "FreezeThaw" or "Storable" modules from CPAN.
     Starting from Perl 5.8 "Storable" is part of the standard
     distribution.  Here's one example using "Storable"'s "store"
     and "retrieve" functions:

             use Storable;
             store(\%hash, "filename");

             # later on...
             $href = retrieve("filename");        # by ref
             %hash = %{ retrieve("filename") };   # direct to hash

  How do I print out or copy a recursive data structure?
     The "Data::Dumper" module on CPAN (or the 5.005 release of
     Perl) is great for printing out data structures.  The
     "Storable" module on CPAN (or the 5.8 release of Perl),
     provides a function called "dclone" that recursively copies
     its argument.

             use Storable qw(dclone);
             $r2 = dclone($r1);

     Where $r1 can be a reference to any kind of data structure
     you'd like.  It will be deeply copied.  Because "dclone"
     takes and returns references, you'd have to add extra
     punctuation if you had a hash of arrays that you wanted to

             %newhash = %{ dclone(\%oldhash) };

  How do I define methods for every class/object?
     (contributed by Ben Morrow)

     You can use the "UNIVERSAL" class (see UNIVERSAL). However,
     please be very careful to consider the consequences of doing
     this: adding methods to every object is very likely to have
     unintended consequences. If possible, it would be better to
     have all your object inherit from some common base class, or
     to use an object system like Moose that supports roles.

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  How do I verify a credit card checksum?
     Get the "Business::CreditCard" module from CPAN.

  How do I pack arrays of doubles or floats for XS code?
     The arrays.h/arrays.c code in the "PGPLOT" module on CPAN
     does just this.  If you're doing a lot of float or double
     processing, consider using the "PDL" module from CPAN
     instead--it makes number-crunching easy.

     See <> for the code.

     Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington,
     and other authors as noted. All rights reserved.

     This documentation is free; you can redistribute it and/or
     modify it under the same terms as Perl itself.

     Irrespective of its distribution, all code examples in this
     file are hereby placed into the public domain.  You are
     permitted and encouraged to use this code in your own
     programs for fun or for profit as you see fit.  A simple
     comment in the code giving credit would be courteous but is
     not required.

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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