perlfaq4
(1)
Name
perlfaq4 - Data Manipulation
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLFAQ4(1)
NAME
perlfaq4 - Data Manipulation
DESCRIPTION
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
Arithmetic"
(http://docs.sun.com/source/806-3568/ncg_goldberg.html).
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
$floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the
"Math::Complex" module. With 5.004, the "Math::Trig" module
(part of the standard Perl distribution) implements the
trigonometric functions. Internally it uses the
"Math::Complex" module and some functions can break out from
the real axis into the complex plane, for example the
inverse sine of 2.
Rounding in financial applications can have serious
implications, and the rounding method used should be
specified precisely. In these cases, it probably pays not
to trust whichever system rounding is being used by Perl,
but to instead implement the rounding function you need
yourself.
To see why, notice how you'll still have an issue on half-
way-point alternation:
for ($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
zeros:
$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 = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin ->
hex, etc.) are left as an exercise to the inclined
reader.
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on
whether they're used on numbers or strings. The operators
treat a string as a series of bits and work with that (the
string "3" is the bit pattern 00110011). The operators work
with the binary form of a number (the number 3 is treated as
the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers
(yielding 3). Saying "11" & "3" performs the "and"
operation on strings (yielding "1").
Most problems with "&" and "|" arise because the programmer
thinks they have a number but really it's a string. The
rest arise because the programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of
"\020\020" & "\101\101") is not a false value in Perl. You
need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
How do I multiply matrices?
Use the "Math::Matrix" or "Math::MatrixReal" modules
(available from CPAN) or the "PDL" extension (also available
from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect
the results, use:
@results = map { my_func($_) } @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 $iterator (@array) {
some_func($iterator);
}
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 http://www.cpan.org/modules/by-module/Roman
<http://www.cpan.org/modules/by-module/Roman> 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 $] < 5.004 }
5.004 and later automatically call "srand" at the beginning.
Don't call "srand" more than once--you make your numbers
less random, rather than more.
Computers are good at being predictable and bad at being
random (despite appearances caused by bugs in your programs
:-). see the random article in the "Far More Than You Ever
Wanted To Know" collection in
<http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy of
Tom Phoenix, talks more about this. John von Neumann said,
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"Anyone who attempts to generate random numbers by
deterministic means is, of course, living in a state of
sin."
If you want numbers that are more random than "rand" with
"srand" provides, you should also check out the
"Math::TrulyRandom" module from CPAN. It uses the
imperfections in your system's timer to generate random
numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating
system, look at "Numerical Recipes in C" at
<http://www.nr.com/>.
How do I get a random number between X and Y?
To get a random number between two values, you can use the
"rand()" built-in to get a random number between 0 and 1.
From there, you shift that into the range that you want.
"rand($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 $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )
Hence you derive the following simple function to abstract
that. It selects a random integer between the two given
integers (inclusive), For example:
"random_int_between(50,120)".
sub random_int_between {
my($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
"localtime".
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
these.
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
http://www.hermetic.ch/cal_stud/jdn.htm 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'
2453401.5
Or the modified Julian Day
$ perl -MDateTime -le'print DateTime->today->mjd'
53401
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'
31
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
before.
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
compliant?
(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
s/\\(.)/$1/g;
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.
s/(.)\1/$1/g;
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:
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
@$ = (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
(flush-right).
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.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;
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:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
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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
)
/\U$1/xg;
$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 $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($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, $text);
How do I strip blank space from the beginning/end of a string?
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(contributed by brian d foy)
A substitution can do this for you. For a single line, you
want to replace all the leading or trailing whitespace with
nothing. You can do that with a pair of substitutions.
s/^\s+//;
s/\s+$//;
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.
s/^\s+|\s+$//g;
In this regular expression, the alternation matches either
at the beginning or the end of the string since the anchors
have a lower precedence than the alternation. With the "/g"
flag, the substitution makes all possible matches, so it
gets both. Remember, the trailing newline matches the "\s+",
and the "$" 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( <> )
{
s/^\s+|\s+$//g;
print "$_\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
string:
$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
directly:
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:
$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
trouble.
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} : '???'
/eg;
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
more?
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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 = $old;
somefunc($var);
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:
func(\@array);
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()
function.
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
($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGET
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But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote in the
indentation.
($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
FINIS
$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, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}
This works with leading special strings, dynamically
determined:
$remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
@@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:
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$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
EVER_ON_AND_ON
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];
@animals[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
element:
$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
@array[1,4,3,0].
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
space:
@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
instead:
@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
array.
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
subroutine:
sub first (&@) {
my $code = shift;
foreach (@_) {
return $_ if &{$code}();
}
undef;
}
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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,
though.
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 }, $element;
}
Note that this is the symmetric difference, that is, all
elements in either A or in B but not in both. Think of it
as an xor operation.
How do I test whether two arrays or hashes are equal?
With Perl 5.10 and later, the smart match operator can give
you the answer with the least amount of work:
use 5.010;
if( @array1 ~~ @array2 )
{
say "The arrays are the same";
}
if( %hash1 ~~ %hash2 ) # doesn't check values!
{
say "The hash keys are the same";
}
The following code works for single-level arrays. It uses a
stringwise comparison, and does not distinguish defined
versus undefined empty strings. Modify if you have other
needs.
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$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
true?
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;
last;
}
}
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
array:
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
srand;
@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];
@idx[$p-1,$q]=@idx[$q,$p-1];
}
}
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
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz 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
operations.
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
overhead).
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);
$vector->Index_List_Store(@ints);
@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 ", $is_set ? "set" : "clear", ".\n";
pvec($vector);
set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);
set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);
set_vec(0,32,17);
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set_vec(1,32,17);
sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print "offset=$offset width=$width value=$value\n";
pvec($vector);
}
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)
...
next;
}
else
{
$new_hash{$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)
...
next;
}
else
{
$hash1{$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
instead:
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
order.
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
keys.
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}
or
"\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
key!
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)) {
$seen{$element}++;
}
@uniq = keys %seen;
Or more succinctly:
@uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
%seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
@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
it?
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
1;
}
Since Perl 5.004, however, this situation is a special case
and Perl creates the hash key only when you make the
assignment:
my_sub( $hash{ 'foo' } );
print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
$_[0] = 'bar';
}
However, if you want the old behavior (and think carefully
about that because it's a weird side effect), you can pass a
hash slice instead. Perl 5.004 didn't make this a special
case:
my_sub( @hash{ qw/foo/ } );
How can I make the Perl equivalent of a C structure/C++
class/hash or array of hashes or arrays?
Usually a hash ref, perhaps like this:
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$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
Expressions.
How do I determine whether a scalar is a
number/whole/integer/float?
Assuming that you don't care about IEEE notations like "NaN"
or "Infinity", you probably just want to use a regular
expression.
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
copy.
%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 <http://search.cpan.org/dist/PGPLOT> for the code.
AUTHOR AND COPYRIGHT
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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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