PERLDATA(1) 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
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@', which works much as the word "these" or
"those" does in English, in that it indicates multiple values are expected.
@days # ($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
of these one character names have a predefined significance to Perl. For instance, $$ is the current process
id.)
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 right-hand 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 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.
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:
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;
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, the "SCALAR" method is called (with a fallback to
.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 (but not
multiple underscores in a row: "23__500" is not legal; "23_500" is). 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.
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 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
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. __SUB__ gives a reference to the current subroutine. 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. (But the empty "package;" is no longer
supported, as of version 5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__ is only
available in 5.16 or higher, and only with a "use v5.16" or "use feature "current_sub"" declaration.
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 line after
__DATA__. The program should "close DATA" when it is done reading from it. (Leaving it open leaks
filehandles if the module is reloaded for any reason, so it's a safer practice to close 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 Interpolation
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:
@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
A list value may also be subscripted like a normal array. You must put the list in parentheses to avoid
ambiguity. For example:
# 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 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;
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:
$rec = {
witch => 'Mable the Merciless',
cat => 'Fluffy the Ferocious',
date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
$field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order doesn't mean that it comes out in that order. See
"sort" in perlfunc for examples of how to arrange for an output ordering.
Subscripts
An array can be accessed one scalar at a time by specifying a dollar sign ("$"), 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",
);
is equivalent to
$foo{join($;, $a, $b, $c)}
The default subscript separator is "\034", the same as SUBSEP in awk.
Slices
A slice accesses several elements of a list, an array, or a hash simultaneously using a list of subscripts.
It's more convenient than writing out the individual elements as a list of separate scalar values.
($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
This makes it easy to write loops that terminate when a null list is returned:
$t = @a[0, 1]; # $t is now 'second'
$u = @h{'first', 'second'}; # $u is now 'B'
If you're confused about why you use an '@' there on a hash slice instead of a '%', think of it like this.
The type of bracket (square or curly) governs whether it's an array or a hash being looked at. On the other
hand, the leading symbol ('$' 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;
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(),
{
my $f = myopen("</etc/motd");
print <$f>;
# $f implicitly closed here
}
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.
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.
perl v5.16.3 2013-03-04 PERLDATA(1)
Back to main site | Back to man page index