perldata
(1)
Name
perldata - Perl data types
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLDATA(1)
NAME
perldata - Perl data types
DESCRIPTION
Variable names
Perl has three built-in data types: scalars, arrays of
scalars, and associative arrays of scalars, known as
"hashes". A scalar is a single string (of any size, limited
only by the available memory), number, or a reference to
something (which will be discussed in perlref). Normal
arrays are ordered lists of scalars indexed by number,
starting with 0. Hashes are unordered collections of scalar
values indexed by their associated string key.
Values are usually referred to by name, or through a named
reference. The first character of the name tells you to
what sort of data structure it refers. The rest of the name
tells you the particular value to which it refers. Usually
this name is a single identifier, that is, a string
beginning with a letter or underscore, and containing
letters, underscores, and digits. In some cases, it may be
a chain of identifiers, separated by "::" (or by the
slightly archaic "'"); all but the last are interpreted as
names of packages, to locate the namespace in which to look
up the final identifier (see "Packages" in perlmod for
details). It's possible to substitute for a simple
identifier, an expression that produces a reference to the
value at runtime. This is described in more detail below
and in perlref.
Perl also has its own built-in variables whose names don't
follow these rules. They have strange names so they don't
accidentally collide with one of your normal variables.
Strings that match parenthesized parts of a regular
expression are saved under names containing only digits
after the "$" (see perlop and perlre). In addition, several
special variables that provide windows into the inner
working of Perl have names containing punctuation characters
and control characters. These are documented in perlvar.
Scalar values are always named with '$', even when referring
to a scalar that is part of an array or a hash. The '$'
symbol works semantically like the English word "the" in
that it indicates a single value is expected.
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted
by '@', which works much like the word "these" or "those"
perl v5.12.5 Last change: 2012-11-03 1
Perl Programmers Reference Guide PERLDATA(1)
does in English, in that it indicates multiple values are
expected.
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
Entire hashes are denoted by '%':
%days # (key1, val1, key2, val2 ...)
In addition, subroutines are named with an initial '&',
though this is optional when unambiguous, just as the word
"do" is often redundant in English. Symbol table entries
can be named with an initial '*', but you don't really care
about that yet (if ever :-).
Every variable type has its own namespace, as do several
non-variable identifiers. This means that you can, without
fear of conflict, use the same name for a scalar variable,
an array, or a hash--or, for that matter, for a filehandle,
a directory handle, a subroutine name, a format name, or a
label. This means that $foo and @foo are two different
variables. It also means that $foo[1] is a part of @foo,
not a part of $foo. This may seem a bit weird, but that's
okay, because it is weird.
Because variable references always start with '$', '@', or
'%', the "reserved" words aren't in fact reserved with
respect to variable names. They are reserved with respect
to labels and filehandles, however, which don't have an
initial special character. You can't have a filehandle
named "log", for instance. Hint: you could say
"open(LOG,'logfile')" rather than "open(log,'logfile')".
Using uppercase filehandles also improves readability and
protects you from conflict with future reserved words. Case
is significant--"FOO", "Foo", and "foo" are all different
names. Names that start with a letter or underscore may
also contain digits and underscores.
It is possible to replace such an alphanumeric name with an
expression that returns a reference to the appropriate type.
For a description of this, see perlref.
Names that start with a digit may contain only more digits.
Names that do not start with a letter, underscore, digit or
a caret (i.e. a control character) are limited to one
character, e.g., $% or $$. (Most of these one character
names have a predefined significance to Perl. For instance,
$$ is the current process id.)
perl v5.12.5 Last change: 2012-11-03 2
Perl Programmers Reference Guide PERLDATA(1)
Context
The interpretation of operations and values in Perl
sometimes depends on the requirements of the context around
the operation or value. There are two major contexts: list
and scalar. Certain operations return list values in
contexts wanting a list, and scalar values otherwise. If
this is true of an operation it will be mentioned in the
documentation for that operation. In other words, Perl
overloads certain operations based on whether the expected
return value is singular or plural. Some words in English
work this way, like "fish" and "sheep".
In a reciprocal fashion, an operation provides either a
scalar or a list context to each of its arguments. For
example, if you say
int( <STDIN> )
the integer operation provides scalar context for the <>
operator, which responds by reading one line from STDIN and
passing it back to the integer operation, which will then
find the integer value of that line and return that. If, on
the other hand, you say
sort( <STDIN> )
then the sort operation provides list context for <>, which
will proceed to read every line available up to the end of
file, and pass that list of lines back to the sort routine,
which will then sort those lines and return them as a list
to whatever the context of the sort was.
Assignment is a little bit special in that it uses its left
argument to determine the context for the right argument.
Assignment to a scalar evaluates the right-hand side in
scalar context, while assignment to an array or hash
evaluates the righthand side in list context. Assignment to
a list (or slice, which is just a list anyway) also
evaluates the righthand side in list context.
When you use the "use warnings" pragma or Perl's -w command-
line option, you may see warnings about useless uses of
constants or functions in "void context". Void context just
means the value has been discarded, such as a statement
containing only ""fred";" or "getpwuid(0);". It still
counts as scalar context for functions that care whether or
not they're being called in list context.
User-defined subroutines may choose to care whether they are
being called in a void, scalar, or list context. Most
subroutines do not need to bother, though. That's because
both scalars and lists are automatically interpolated into
perl v5.12.5 Last change: 2012-11-03 3
Perl Programmers Reference Guide PERLDATA(1)
lists. See "wantarray" in perlfunc for how you would
dynamically discern your function's calling context.
Scalar values
All data in Perl is a scalar, an array of scalars, or a hash
of scalars. A scalar may contain one single value in any of
three different flavors: a number, a string, or a reference.
In general, conversion from one form to another is
transparent. Although a scalar may not directly hold
multiple values, it may contain a reference to an array or
hash which in turn contains multiple values.
Scalars aren't necessarily one thing or another. There's no
place to declare a scalar variable to be of type "string",
type "number", type "reference", or anything else. Because
of the automatic conversion of scalars, operations that
return scalars don't need to care (and in fact, cannot care)
whether their caller is looking for a string, a number, or a
reference. Perl is a contextually polymorphic language
whose scalars can be strings, numbers, or references (which
includes objects). Although strings and numbers are
considered pretty much the same thing for nearly all
purposes, references are strongly-typed, uncastable pointers
with builtin reference-counting and destructor invocation.
A scalar value is interpreted as TRUE in the Boolean sense
if it is not the null string or the number 0 (or its string
equivalent, "0"). The Boolean context is just a special
kind of scalar context where no conversion to a string or a
number is ever performed.
There are actually two varieties of null strings (sometimes
referred to as "empty" strings), a defined one and an
undefined one. The defined version is just a string of
length zero, such as "". The undefined version is the value
that indicates that there is no real value for something,
such as when there was an error, or at end of file, or when
you refer to an uninitialized variable or element of an
array or hash. Although in early versions of Perl, an
undefined scalar could become defined when first used in a
place expecting a defined value, this no longer happens
except for rare cases of autovivification as explained in
perlref. You can use the defined() operator to determine
whether a scalar value is defined (this has no meaning on
arrays or hashes), and the undef() operator to produce an
undefined value.
To find out whether a given string is a valid non-zero
number, it's sometimes enough to test it against both
numeric 0 and also lexical "0" (although this will cause
noises if warnings are on). That's because strings that
aren't numbers count as 0, just as they do in awk:
perl v5.12.5 Last change: 2012-11-03 4
Perl Programmers Reference Guide PERLDATA(1)
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
That method may be best because otherwise you won't treat
IEEE notations like "NaN" or "Infinity" properly. At other
times, you might prefer to determine whether string data can
be used numerically by calling the POSIX::strtod() function
or by inspecting your string with a regular expression (as
documented in perlre).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # rejects -3
warn "not an integer" unless /^-?\d+$/; # rejects +3
warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find the
length of array @days by evaluating $#days, as in csh.
However, this isn't the length of the array; it's the
subscript of the last element, which is a different value
since there is ordinarily a 0th element. Assigning to
$#days actually changes the length of the array. Shortening
an array this way destroys intervening values. Lengthening
an array that was previously shortened does not recover
values that were in those elements. (It used to do so in
Perl 4, but we had to break this to make sure destructors
were called when expected.)
You can also gain some minuscule measure of efficiency by
pre-extending an array that is going to get big. You can
also extend an array by assigning to an element that is off
the end of the array. You can truncate an array down to
nothing by assigning the null list () to it. The following
are equivalent:
@whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the
length of the array. (Note that this is not true of lists,
which return the last value, like the C comma operator, nor
of built-in functions, which return whatever they feel like
returning.) The following is always true:
scalar(@whatever) == $#whatever - $[ + 1;
Version 5 of Perl changed the semantics of $[: files that
don't set the value of $[ no longer need to worry about
perl v5.12.5 Last change: 2012-11-03 5
Perl Programmers Reference Guide PERLDATA(1)
whether another file changed its value. (In other words,
use of $[ is deprecated.) So in general you can assume that
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as
to leave nothing to doubt:
$element_count = scalar(@whatever);
If you evaluate a hash in scalar context, it returns false
if the hash is empty. If there are any key/value pairs, it
returns true; more precisely, the value returned is a string
consisting of the number of used buckets and the number of
allocated buckets, separated by a slash. This is pretty
much useful only to find out whether Perl's internal hashing
algorithm is performing poorly on your data set. For
example, you stick 10,000 things in a hash, but evaluating
%HASH in scalar context reveals "1/16", which means only one
out of sixteen buckets has been touched, and presumably
contains all 10,000 of your items. This isn't supposed to
happen. If a tied hash is evaluated in scalar context, a
fatal error will result, since this bucket usage information
is currently not available for tied hashes.
You can preallocate space for a hash by assigning to the
keys() function. This rounds up the allocated buckets to
the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
Scalar value constructors
Numeric literals are specified in any of the following
floating point or integer formats:
12345
12345.67
.23E-10 # a very small number
3.14_15_92 # a very important number
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
0377 # octal (only numbers, begins with 0)
0b011011 # binary
You are allowed to use underscores (underbars) in numeric
literals between digits for legibility. You could, for
example, group binary digits by threes (as for a Unix-style
mode argument such as 0b110_100_100) or by fours (to
represent nibbles, as in 0b1010_0110) or in other groups.
perl v5.12.5 Last change: 2012-11-03 6
Perl Programmers Reference Guide PERLDATA(1)
String literals are usually delimited by either single or
double quotes. They work much like quotes in the standard
Unix shells: double-quoted string literals are subject to
backslash and variable substitution; single-quoted strings
are not (except for "\'" and "\\"). The usual C-style
backslash rules apply for making characters such as newline,
tab, etc., as well as some more exotic forms. See "Quote
and Quote-like Operators" in perlop for a list.
Hexadecimal, octal, or binary, representations in string
literals (e.g. '0xff') are not automatically converted to
their integer representation. The hex() and oct() functions
make these conversions for you. See "hex" in perlfunc and
"oct" in perlfunc for more details.
You can also embed newlines directly in your strings, i.e.,
they can end on a different line than they begin. This is
nice, but if you forget your trailing quote, the error will
not be reported until Perl finds another line containing the
quote character, which may be much further on in the script.
Variable substitution inside strings is limited to scalar
variables, arrays, and array or hash slices. (In other
words, names beginning with $ or @, followed by an optional
bracketed expression as a subscript.) The following code
segment prints out "The price is $100."
$Price = '$100'; # not interpolated
print "The price is $Price.\n"; # interpolated
There is no double interpolation in Perl, so the $100 is
left as is.
By default floating point numbers substituted inside strings
use the dot (".") as the decimal separator. If "use
locale" is in effect, and POSIX::setlocale() has been
called, the character used for the decimal separator is
affected by the LC_NUMERIC locale. See perllocale and
POSIX.
As in some shells, you can enclose the variable name in
braces to disambiguate it from following alphanumerics (and
underscores). You must also do this when interpolating a
variable into a string to separate the variable name from a
following double-colon or an apostrophe, since these would
be otherwise treated as a package separator:
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
Without the braces, Perl would have looked for a $whospeak,
a $who::0, and a "$who's" variable. The last two would be
perl v5.12.5 Last change: 2012-11-03 7
Perl Programmers Reference Guide PERLDATA(1)
the $0 and the $s variables in the (presumably) non-existent
package "who".
In fact, an identifier within such curlies is forced to be a
string, as is any simple identifier within a hash subscript.
Neither need quoting. Our earlier example, $days{'Feb'} can
be written as $days{Feb} and the quotes will be assumed
automatically. But anything more complicated in the
subscript will be interpreted as an expression. This means
for example that "$version{2.0}++" is equivalent to
"$version{2}++", not to "$version{'2.0'}++".
Version Strings
A literal of the form "v1.20.300.4000" is parsed as a string
composed of characters with the specified ordinals. This
form, known as v-strings, provides an alternative, more
readable way to construct strings, rather than use the
somewhat less readable interpolation form
"\x{1}\x{14}\x{12c}\x{fa0}". This is useful for
representing Unicode strings, and for comparing version
"numbers" using the string comparison operators, "cmp",
"gt", "lt" etc. If there are two or more dots in the
literal, the leading "v" may be omitted.
print v9786; # prints SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both "require" and "use" for
doing a version check. Note that using the v-strings for
IPv4 addresses is not portable unless you also use the
inet_aton()/inet_ntoa() routines of the Socket package.
Note that since Perl 5.8.1 the single-number v-strings (like
"v65") are not v-strings before the "=>" operator (which is
usually used to separate a hash key from a hash value),
instead they are interpreted as literal strings ('v65').
They were v-strings from Perl 5.6.0 to Perl 5.8.0, but that
caused more confusion and breakage than good. Multi-number
v-strings like "v65.66" and 65.66.67 continue to be
v-strings always.
Special Literals
The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package
name at that point in your program. They may be used only
as separate tokens; they will not be interpolated into
strings. If there is no current package (due to an empty
"package;" directive), __PACKAGE__ is the undefined value.
perl v5.12.5 Last change: 2012-11-03 8
Perl Programmers Reference Guide PERLDATA(1)
The two control characters ^D and ^Z, and the tokens __END__
and __DATA__ may be used to indicate the logical end of the
script before the actual end of file. Any following text is
ignored.
Text after __DATA__ may be read via the filehandle
"PACKNAME::DATA", where "PACKNAME" is the package that was
current when the __DATA__ token was encountered. The
filehandle is left open pointing to the contents after
__DATA__. It is the program's responsibility to "close
DATA" when it is done reading from it. For compatibility
with older scripts written before __DATA__ was introduced,
__END__ behaves like __DATA__ in the top level script (but
not in files loaded with "require" or "do") and leaves the
remaining contents of the file accessible via "main::DATA".
See SelfLoader for more description of __DATA__, and an
example of its use. Note that you cannot read from the DATA
filehandle in a BEGIN block: the BEGIN block is executed as
soon as it is seen (during compilation), at which point the
corresponding __DATA__ (or __END__) token has not yet been
seen.
Barewords
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known
as "barewords". As with filehandles and labels, a bareword
that consists entirely of lowercase letters risks conflict
with future reserved words, and if you use the "use
warnings" pragma or the -w switch, Perl will warn you about
any such words. Perl limits barewords (like identifiers) to
about 250 characters. Future versions of Perl are likely to
eliminate these arbitrary limitations.
Some people may wish to outlaw barewords entirely. If you
say
use strict 'subs';
then any bareword that would NOT be interpreted as a
subroutine call produces a compile-time error instead. The
restriction lasts to the end of the enclosing block. An
inner block may countermand this by saying "no strict
'subs'".
Array Joining Delimiter
Arrays and slices are interpolated into double-quoted
strings by joining the elements with the delimiter specified
in the $" variable ($LIST_SEPARATOR if "use English;" is
specified), space by default. The following are equivalent:
perl v5.12.5 Last change: 2012-11-03 9
Perl Programmers Reference Guide PERLDATA(1)
$temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish
substitution) there is an unfortunate ambiguity: Is
"/$foo[bar]/" to be interpreted as "/${foo}[bar]/" (where
"[bar]" is a character class for the regular expression) or
as "/${foo[bar]}/" (where "[bar]" is the subscript to array
@foo)? If @foo doesn't otherwise exist, then it's obviously
a character class. If @foo exists, Perl takes a good guess
about "[bar]", and is almost always right. If it does guess
wrong, or if you're just plain paranoid, you can force the
correct interpretation with curly braces as above.
If you're looking for the information on how to use here-
documents, which used to be here, that's been moved to
"Quote and Quote-like Operators" in perlop.
List value constructors
List values are denoted by separating individual values by
commas (and enclosing the list in parentheses where
precedence requires it):
(LIST)
In a context not requiring a list value, the value of what
appears to be a list literal is simply the value of the
final element, as with the C comma operator. For example,
@foo = ('cc', '-E', $bar);
assigns the entire list value to array @foo, but
$foo = ('cc', '-E', $bar);
assigns the value of variable $bar to the scalar variable
$foo. Note that the value of an actual array in scalar
context is the length of the array; the following assigns
the value 3 to $foo:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing
parenthesis of a list literal, so that you can say:
perl v5.12.5 Last change: 2012-11-03 10
Perl Programmers Reference Guide PERLDATA(1)
@foo = (
1,
2,
3,
);
To use a here-document to assign an array, one line per
element, you might use an approach like this:
@sauces = <<End_Lines =~ m/(\S.*\S)/g;
normal tomato
spicy tomato
green chile
pesto
white wine
End_Lines
LISTs do automatic interpolation of sublists. That is, when
a LIST is evaluated, each element of the list is evaluated
in list context, and the resulting list value is
interpolated into LIST just as if each individual element
were a member of LIST. Thus arrays and hashes lose their
identity in a LIST--the list
(@foo,@bar,&SomeSub,%glarch)
contains all the elements of @foo followed by all the
elements of @bar, followed by all the elements returned by
the subroutine named SomeSub called in list context,
followed by the key/value pairs of %glarch. To make a list
reference that does NOT interpolate, see perlref.
The null list is represented by (). Interpolating it in a
list has no effect. Thus ((),(),()) is equivalent to ().
Similarly, interpolating an array with no elements is the
same as if no array had been interpolated at that point.
This interpolation combines with the facts that the opening
and closing parentheses are optional (except when necessary
for precedence) and lists may end with an optional comma to
mean that multiple commas within lists are legal syntax. The
list "1,,3" is a concatenation of two lists, "1," and 3, the
first of which ends with that optional comma. "1,,3" is
"(1,),(3)" is "1,3" (And similarly for "1,,,3" is
"(1,),(,),3" is "1,3" and so on.) Not that we'd advise you
to use this obfuscation.
A list value may also be subscripted like a normal array.
You must put the list in parentheses to avoid ambiguity.
For example:
perl v5.12.5 Last change: 2012-11-03 11
Perl Programmers Reference Guide PERLDATA(1)
# Stat returns list value.
$time = (stat($file))[8];
# SYNTAX ERROR HERE.
$time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
# Find a hex digit.
$hexdigit = ('a','b','c','d','e','f')[$digit-10];
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
Lists may be assigned to only when each element of the list
is itself legal to assign to:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
An exception to this is that you may assign to "undef" in a
list. This is useful for throwing away some of the return
values of a function:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
List assignment in scalar context returns the number of
elements produced by the expression on the right side of the
assignment:
$x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
$x = (($foo,$bar) = f()); # set $x to f()'s return count
This is handy when you want to do a list assignment in a
Boolean context, because most list functions return a null
list when finished, which when assigned produces a 0, which
is interpreted as FALSE.
It's also the source of a useful idiom for executing a
function or performing an operation in list context and then
counting the number of return values, by assigning to an
empty list and then using that assignment in scalar context.
For example, this code:
$count = () = $string =~ /\d+/g;
will place into $count the number of digit groups found in
$string. This happens because the pattern match is in list
context (since it is being assigned to the empty list), and
will therefore return a list of all matching parts of the
string. The list assignment in scalar context will translate
that into the number of elements (here, the number of times
the pattern matched) and assign that to $count. Note that
perl v5.12.5 Last change: 2012-11-03 12
Perl Programmers Reference Guide PERLDATA(1)
simply using
$count = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar
context will only return true or false, rather than a count
of matches.
The final element of a list assignment may be an array or a
hash:
($a, $b, @rest) = split;
my($a, $b, %rest) = @_;
You can actually put an array or hash anywhere in the list,
but the first one in the list will soak up all the values,
and anything after it will become undefined. This may be
useful in a my() or local().
A hash can be initialized using a literal list holding pairs
of items to be interpreted as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
While literal lists and named arrays are often
interchangeable, that's not the case for hashes. Just
because you can subscript a list value like a normal array
does not mean that you can subscript a list value as a hash.
Likewise, hashes included as parts of other lists (including
parameters lists and return lists from functions) always
flatten out into key/value pairs. That's why it's good to
use references sometimes.
It is often more readable to use the "=>" operator between
key/value pairs. The "=>" operator is mostly just a more
visually distinctive synonym for a comma, but it also
arranges for its left-hand operand to be interpreted as a
string if it's a bareword that would be a legal simple
identifier. "=>" doesn't quote compound identifiers, that
contain double colons. This makes it nice for initializing
hashes:
%map = (
red => 0x00f,
blue => 0x0f0,
green => 0xf00,
);
or for initializing hash references to be used as records:
perl v5.12.5 Last change: 2012-11-03 13
Perl Programmers Reference Guide PERLDATA(1)
$rec = {
witch => 'Mable the Merciless',
cat => 'Fluffy the Ferocious',
date => '10/31/1776',
};
or for using call-by-named-parameter to complicated
functions:
$field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order
doesn't mean that it comes out in that order. See "sort" in
perlfunc for examples of how to arrange for an output
ordering.
Subscripts
An array is subscripted by specifying a dollar sign ("$"),
then the name of the array (without the leading "@"), then
the subscript inside square brackets. For example:
@myarray = (5, 50, 500, 5000);
print "The Third Element is", $myarray[2], "\n";
The array indices start with 0. A negative subscript
retrieves its value from the end. In our example,
$myarray[-1] would have been 5000, and $myarray[-2] would
have been 500.
Hash subscripts are similar, only instead of square brackets
curly brackets are used. For example:
%scientists =
(
"Newton" => "Isaac",
"Einstein" => "Albert",
"Darwin" => "Charles",
"Feynman" => "Richard",
);
print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
Slices
A common way to access an array or a hash is one scalar
element at a time. You can also subscript a list to get a
single element from it.
perl v5.12.5 Last change: 2012-11-03 14
Perl Programmers Reference Guide PERLDATA(1)
$whoami = $ENV{"USER"}; # one element from the hash
$parent = $ISA[0]; # one element from the array
$dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a
hash simultaneously using a list of subscripts. It's more
convenient than writing out the individual elements as a
list of separate scalar values.
($him, $her) = @folks[0,-1]; # array slice
@them = @folks[0 .. 3]; # array slice
($who, $home) = @ENV{"USER", "HOME"}; # hash slice
($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also
assign to an array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
= (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
= (0xff0000, 0x0000ff, 0x00ff00);
($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
Since changing a slice changes the original array or hash
that it's slicing, a "foreach" construct will alter some--or
even all--of the values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{qw[key1 key2]}) {
s/^\s+//; # trim leading whitespace
s/\s+$//; # trim trailing whitespace
s/(\w+)/\u\L$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (0,1)[2,3]; # @c has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
perl v5.12.5 Last change: 2012-11-03 15
Perl Programmers Reference Guide PERLDATA(1)
This makes it easy to write loops that terminate when a null
list is returned:
while ( ($home, $user) = (getpwent)[7,0]) {
printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list
assignment is the number of elements on the right-hand side
of the assignment. The null list contains no elements, so
when the password file is exhausted, the result is 0, not 2.
If you're confused about why you use an '@' there on a hash
slice instead of a '%', think of it like this. The type of
bracket (square or curly) governs whether it's an array or a
hash being looked at. On the other hand, the leading symbol
('$' or '@') on the array or hash indicates whether you are
getting back a singular value (a scalar) or a plural one (a
list).
Typeglobs and Filehandles
Perl uses an internal type called a typeglob to hold an
entire symbol table entry. The type prefix of a typeglob is
a "*", because it represents all types. This used to be the
preferred way to pass arrays and hashes by reference into a
function, but now that we have real references, this is
seldom needed.
The main use of typeglobs in modern Perl is create symbol
table aliases. This assignment:
*this = *that;
makes $this an alias for $that, @this an alias for @that,
%this an alias for %that, &this an alias for &that, etc.
Much safer is to use a reference. This:
local *Here::blue = \$There::green;
temporarily makes $Here::blue an alias for $There::green,
but doesn't make @Here::blue an alias for @There::green, or
%Here::blue an alias for %There::green, etc. See "Symbol
Tables" in perlmod for more examples of this. Strange
though this may seem, this is the basis for the whole module
import/export system.
Another use for typeglobs is to pass filehandles into a
function or to create new filehandles. If you need to use a
typeglob to save away a filehandle, do it this way:
$fh = *STDOUT;
perl v5.12.5 Last change: 2012-11-03 16
Perl Programmers Reference Guide PERLDATA(1)
or perhaps as a real reference, like this:
$fh = \*STDOUT;
See perlsub for examples of using these as indirect
filehandles in functions.
Typeglobs are also a way to create a local filehandle using
the local() operator. These last until their block is
exited, but may be passed back. For example:
sub newopen {
my $path = shift;
local *FH; # not my!
open (FH, $path) or return undef;
return *FH;
}
$fh = newopen('/etc/passwd');
Now that we have the *foo{THING} notation, typeglobs aren't
used as much for filehandle manipulations, although they're
still needed to pass brand new file and directory handles
into or out of functions. That's because *HANDLE{IO} only
works if HANDLE has already been used as a handle. In other
words, *FH must be used to create new symbol table entries;
*foo{THING} cannot. When in doubt, use *FH.
All functions that are capable of creating filehandles
(open(), opendir(), pipe(), socketpair(), sysopen(),
socket(), and accept()) automatically create an anonymous
filehandle if the handle passed to them is an uninitialized
scalar variable. This allows the constructs such as "open(my
$fh, ...)" and "open(local $fh,...)" to be used to create
filehandles that will conveniently be closed automatically
when the scope ends, provided there are no other references
to them. This largely eliminates the need for typeglobs when
opening filehandles that must be passed around, as in the
following example:
sub myopen {
open my $fh, "@_"
or die "Can't open '@_': $!";
return $fh;
}
{
my $f = myopen("</etc/motd");
print <$f>;
# $f implicitly closed here
}
perl v5.12.5 Last change: 2012-11-03 17
Perl Programmers Reference Guide PERLDATA(1)
Note that if an initialized scalar variable is used instead
the result is different: "my $fh='zzz'; open($fh, ...)" is
equivalent to "open( *{'zzz'}, ...)". "use strict 'refs'"
forbids such practice.
Another way to create anonymous filehandles is with the
Symbol module or with the IO::Handle module and its ilk.
These modules have the advantage of not hiding different
types of the same name during the local(). See the bottom
of "open()" in perlfunc for an example.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
See perlvar for a description of Perl's built-in variables
and a discussion of legal variable names. See perlref,
perlsub, and "Symbol Tables" in perlmod for more discussion
on typeglobs and the *foo{THING} syntax.
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/.
perl v5.12.5 Last change: 2012-11-03 18