=head1 DESCRIPTION
=head2 Variable names
+X<variable, name> X<variable name> X<data type> X<type>
Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes". A scalar is a
for a simple identifier, an expression that produces a reference
to the value at runtime. This is described in more detail below
and in L<perlref>.
+X<identifier>
Perl also has its own built-in variables whose names don't follow
these rules. They have strange names so they don't accidentally
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 L<perlvar>.
+X<variable, built-in>
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.
+X<scalar>
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@',
-which works much like the word "these" or "those" does in English,
+which works much as the word "these" or "those" does in English,
in that it indicates multiple values are expected.
+X<array>
@days # ($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 '%':
+X<hash>
%days # (key1, val1, key2, val2 ...)
and @foo are two different variables. It also means that C<$foo[1]>
is a part of @foo, not a part of $foo. This may seem a bit weird,
but that's okay, because it is weird.
+X<namespace>
Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable
from conflict with future reserved words. Case I<is> significant--"FOO",
"Foo", and "foo" are all different names. Names that start with a
letter or underscore may also contain digits and underscores.
+X<identifier, case sensitivity>
+X<case>
It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type. For a description
id.)
=head2 Context
+X<context> X<scalar context> X<list context>
The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
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.
+anyway) also evaluates the right-hand side in list context.
When you use the C<use warnings> pragma or Perl's B<-w> command-line
option, you may see warnings
context.
=head2 Scalar values
+X<scalar> X<number> X<string> X<reference>
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
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.
+X<boolean> X<bool> X<true> X<false> X<truth>
There are actually two varieties of null strings (sometimes referred
to as "empty" strings), a defined one and an undefined one. The
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.
+X<defined> X<undefined> X<undef> X<null> X<string, null>
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
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.)
+X<$#> X<array, length>
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
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:
+X<array, length>
scalar(@whatever) == $#whatever - $[ + 1;
the value of C<$[> no longer need to worry about whether another
file changed its value. (In other words, use of C<$[> is deprecated.)
So in general you can assume that
+X<$[>
scalar(@whatever) == $#whatever + 1;
%HASH in scalar context reveals C<"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.
+is evaluated in scalar context, the C<SCALAR> method is called (with a
+fallback to C<FIRSTKEY>).
+X<hash, scalar context> X<hash, bucket> X<bucket>
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
=head2 Scalar value constructors
+X<scalar, literal> X<scalar, constant>
Numeric literals are specified in any of the following floating point or
integer formats:
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.
+X<number, literal>
String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells:
C<\\>). The usual C-style backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic
forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
+X<string, literal>
Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
+X<interpolation>
$Price = '$100'; # not interpolated
print "The price is $Price.\n"; # interpolated
There is no double interpolation in Perl, so the C<$100> is left as is.
+By default floating point numbers substituted inside strings use the
+dot (".") as the decimal separator. If C<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 L<perllocale> and L<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:
+X<interpolation>
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
=head3 Version Strings
-
-B<Note:> Version Strings (v-strings) have been deprecated. They will
-be removed in some future release after Perl 5.8.1. The marginal
-benefits of v-strings were greatly outweighed by the potential for
-Surprise and Confusion.
+X<version string> X<vstring> X<v-string>
A literal of the form C<v1.20.300.4000> is parsed as a string composed
of characters with the specified ordinals. This form, known as
comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
more dots in the literal, the leading C<v> may be omitted.
- print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
+ print v9786; # prints SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both C<require> and C<use> for
-doing a version check. The C<$^V> special variable also contains the
-running Perl interpreter's version in this form. See L<perlvar/$^V>.
-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.
+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 C<v65>)
are not v-strings before the C<< => >> operator (which is usually used
-to separate a hash key from a hash value), instead they are interpreted
+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 C<v65.66> and C<65.66.67> continue to
be v-strings always.
=head3 Special Literals
+X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
+X<end> X<data> X<^D> X<^Z>
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 C<package;> directive), __PACKAGE__ is the undefined
-value.
+value. (But the empty C<package;> is no longer supported, as of version
+5.10.)
+X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
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__
but may be read via the filehandle C<PACKNAME::DATA>,
+Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
where C<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
+line after __DATA__. It is the program's responsibility to
C<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 toplevel script (but not in files loaded with
+like __DATA__ in the top level script (but not in files loaded with
C<require> or C<do>) and leaves the remaining contents of the
file accessible via C<main::DATA>.
__DATA__ (or __END__) token has not yet been seen.
=head3 Barewords
+X<bareword>
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 C<use warnings> pragma or the B<-w> switch,
-Perl will warn you about any
-such words. Some people may wish to outlaw barewords entirely. If you
+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';
end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
-=head3 Array Joining Delimiter
+=head3 Array Interpolation
+X<array, interpolation> X<interpolation, array> X<$">
Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the C<$">
L<perlop/Quote and Quote-like Operators>.
=head2 List value constructors
+X<list>
List values are denoted by separating individual values by commas
(and enclosing the list in parentheses where precedence requires it):
It is often more readable to use the C<< => >> operator between key/value
pairs. The C<< => >> 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 (C<< => >> doesn't quote compound identifiers, that contain
-double colons). This makes it nice for initializing hashes:
+interpreted as a string if it's a bareword that would be a legal simple
+identifier. C<< => >> doesn't quote compound identifiers, that contain
+double colons. This makes it nice for initializing hashes:
%map = (
red => 0x00f,
=head2 Subscripts
-An array is subscripted by specifying a dollar sign (C<$>), then the
+An array can be accessed one scalar at a
+time by specifying a dollar sign (C<$>), then the
name of the array (without the leading C<@>), then the subscript inside
square brackets. For example:
@myarray = (5, 50, 500, 5000);
- print "Element Number 2 is", $myarray[2], "\n";
+ print "The Third Element is", $myarray[2], "\n";
The array indices start with 0. A negative subscript retrieves its
value from the end. In our example, C<$myarray[-1]> would have been
print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
-=head2 Slices
+You can also subscript a list to get a single element from it:
+
+ $dir = (getpwnam("daemon"))[7];
+
+=head2 Multi-dimensional array emulation
+
+Multidimensional arrays may be emulated by subscripting a hash with a
+list. The elements of the list are joined with the subscript separator
+(see L<perlvar/$;>).
-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.
+ $foo{$a,$b,$c}
- $whoami = $ENV{"USER"}; # one element from the hash
- $parent = $ISA[0]; # one element from the array
- $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+is equivalent to
+
+ $foo{join($;, $a, $b, $c)}
+
+The default subscript separator is "\034", the same as SUBSEP in B<awk>.
+
+=head2 Slices
+X<slice> X<array, slice> X<hash, slice>
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient
The null list contains no elements, so when the password file is
exhausted, the result is 0, not 2.
+Slices in scalar context return the last item of the slice.
+
+ @a = qw/first second third/;
+ %h = (first => 'A', second => 'B');
+ $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.
scalar) or a plural one (a list).
=head2 Typeglobs and Filehandles
+X<typeglob> X<filehandle> X<*>
Perl uses an internal type called a I<typeglob> to hold an entire
symbol table entry. The type prefix of a typeglob is a C<*>, because
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 L<perlfunc/open()> for an
+during the local(). See the bottom of L<perlfunc/open> for an
example.
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff