=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". Normal arrays
-are ordered lists of scalars indexed by number, starting with 0 and with
-negative subscripts counting from the end. Hashes are unordered
-collections of scalar values indexed by their associated string key.
+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 L<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
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
of this, see L<perlref>.
Names that start with a digit may contain only more digits. Names
-that do not start with a letter, underscore, or digit are limited to
-one character, e.g., C<$%> or C<$$>. (Most of these one character names
-have a predefined significance to Perl. For instance, C<$$> is the
-current process id.)
+that do not start with a letter, underscore, digit or a caret (i.e.
+a control character) are limited to one character, e.g., C<$%> or
+C<$$>. (Most of these one character names have a predefined
+significance to Perl. For instance, C<$$> is the current process
+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
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
+A scalar value is interpreted as FALSE in the Boolean sense
+if it is undefined, the null string or the number 0 (or its
+string equivalent, "0"), and TRUE if it is anything else. 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
-"0" (although this will cause B<-w> noises). That's because strings
-that aren't numbers count as 0, just as they do in B<awk>:
+"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 B<awk>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
an array that was previously shortened does not recover values
that were in those elements. (It used to do so in Perl 4, but we
had to break this to make sure destructors were called when expected.)
+X<$#> X<array, length>
-You can also gain some miniscule measure of efficiency by pre-extending
+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:
- my @whatever = ();
+ @whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the length
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;
set. For example, you stick 10,000 things in a hash, but evaluating
%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.
+10,000 of your items. This isn't supposed to happen. If a tied hash
+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:
- my %users = ();
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:
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
- 0377 # octal
+ 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
+between digits for legibility (but not multiple underscores in a row:
+C<23__500> is not legal; C<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.
+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>
- my $Price = '$100'; # not interpolated
+ $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>
- my $who = "Larry";
+ $who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
as is any simple identifier within a hash subscript. Neither need
quoting. Our earlier example, C<$days{'Feb'}> can be written as
C<$days{Feb}> and the quotes will be assumed automatically. But
-anything more complicated in the subscript will be interpreted as
-an expression.
+anything more complicated in the subscript will be interpreted as an
+expression. This means for example that C<$version{2.0}++> is
+equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
+
+=head3 Version Strings
+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
+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
+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 C<package;> directive), __PACKAGE__ is the undefined
-value.
+value. (But the empty C<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 C<use v5.16> or
+C<use feature "current_sub"> declaration.
+X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
+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
-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
-C<require> or C<do>) and leaves the remaining contents of the
-file accessible via C<main::DATA>.
+line after __DATA__. The program should C<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 C<require> or C<do>) and leaves the remaining
+contents of the file accessible via C<main::DATA>.
See L<SelfLoader> for more description of __DATA__, and
an example of its use. Note that you cannot read from the DATA
as it is seen (during compilation), at which point the corresponding
__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 Interpolation
+X<array, interpolation> X<interpolation, array> X<$">
+
Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the C<$">
-variable (C<$LIST_SEPARATOR> in English), space by default. The
-following are equivalent:
+variable (C<$LIST_SEPARATOR> if "use English;" is specified),
+space by default. The following are equivalent:
- my $temp = join($", @ARGV);
+ $temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
plain paranoid, you can force the correct interpretation with curly
braces as above.
-A line-oriented form of quoting is based on the shell "here-document"
-syntax. Following a C<< << >> you specify a string to terminate
-the quoted material, and all lines following the current line down to
-the terminating string are the value of the item. The terminating
-string may be either an identifier (a word), or some quoted text. If
-quoted, the type of quotes you use determines the treatment of the
-text, just as in regular quoting. An unquoted identifier works like
-double quotes. There must be no space between the C<< << >> and
-the identifier, unless the identifier is quoted. (If you put a space it
-will be treated as a null identifier, which is valid, and matches the first
-empty line.) The terminating string must appear by itself (unquoted and
-with no surrounding whitespace) on the terminating line.
-
- print <<EOF;
- The price is $Price.
- EOF
-
- print << "EOF"; # same as above
- The price is $Price.
- EOF
-
- print << `EOC`; # execute commands
- echo hi there
- echo lo there
- EOC
-
- print <<"foo", <<"bar"; # you can stack them
- I said foo.
- foo
- I said bar.
- bar
-
- myfunc(<< "THIS", 23, <<'THAT');
- Here's a line
- or two.
- THIS
- and here's another.
- THAT
-
-Just don't forget that you have to put a semicolon on the end
-to finish the statement, as Perl doesn't know you're not going to
-try to do this:
-
- print <<ABC
- 179231
- ABC
- + 20;
-
-If you want your here-docs to be indented with the
-rest of the code, you'll need to remove leading whitespace
-from each line manually:
-
- (my $quote = <<'FINIS') =~ s/^\s+//gm;
- The Road goes ever on and on,
- down from the door where it began.
- FINIS
-
-If you use a here-doc within a delimited construct, such as in C<s///eg>,
-the quoted material must come on the lines following the final delimiter.
-So instead of
-
- s/this/<<E . 'that'
- the other
- E
- . 'more '/eg;
-
-you have to write
-
- s/this/<<E . 'that'
- . 'more '/eg;
- the other
- E
-
-If the terminating identifier is on the last line of the program, you
-must be sure there is a newline after it; otherwise, Perl will give the
-warning B<Can't find string terminator "END" anywhere before EOF...>.
-
-Additionally, the quoting rules for the identifier are not related to
-Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
-in place of C<''> and C<"">, and the only interpolation is for backslashing
-the quoting character:
-
- print << "abc\"def";
- testing...
- abc"def
-
-Finally, quoted strings cannot span multiple lines. The general rule is
-that the identifier must be a string literal. Stick with that, and you
-should be safe.
+If you're looking for the information on how to use here-documents,
+which used to be here, that's been moved to
+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):
to be a list literal is simply the value of the final element, as
with the C comma operator. For example,
- my @foo = ('cc', '-E', $bar);
+ @foo = ('cc', '-E', $bar);
assigns the entire list value to array @foo, but
- my $foo = ('cc', '-E', $bar);
+ $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:
- my @foo = ('cc', '-E', $bar);
- my $foo = @foo; # $foo gets 3
+ @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:
- my @foo = (
- 1,
- 2,
- 3,
+ @foo = (
+ 1,
+ 2,
+ 3,
);
To use a here-document to assign an array, one line per element,
you might use an approach like this:
- my @sauces = <<End_Lines =~ m/(\S.*\S)/g;
- normal tomato
- spicy tomato
- green chile
- pesto
- white wine
+ @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
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 C<1,,3> is a
+multiple commas within lists are legal syntax. The list C<1,,3> is a
concatenation of two lists, C<1,> and C<3>, the first of which ends
with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
put the list in parentheses to avoid ambiguity. For example:
# Stat returns list value.
- my $time = (stat($file))[8];
+ $time = (stat($file))[8];
# SYNTAX ERROR HERE.
- my $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
+ $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
# Find a hex digit.
- my $hexdigit = ('a','b','c','d','e','f')[$digit-10];
+ $hexdigit = ('a','b','c','d','e','f')[$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:
- my($a, $b, $c) = (1, 2, 3);
+ ($a, $b, $c) = (1, 2, 3);
- ($map{red}, $map{blue}, $map{green}) = (0x00f, 0x0f0, 0xf00);
+ ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
An exception to this is that you may assign to C<undef> in a list.
This is useful for throwing away some of the return values of a
function:
- my($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+ ($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:
- my $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
- my $x = (($foo,$bar) = f()); # set $x to f()'s return count
+ $x = (($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,
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:
+assignment in scalar context. For example, this code:
- my $count = () = $string =~ /\d+/g;
+ $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
+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
+number of times the pattern matched) and assign that to $count. Note
that simply using
- my $count = $string =~ /\d+/g;
+ $count = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar context will
only return true or false, rather than a count of matches.
The final element of a list assignment may be an array or a hash:
- my($a, $b, @rest) = split;
- # or
+ ($a, $b, @rest) = split;
my($a, $b, %rest) = @_;
You can actually put an array or hash anywhere in the list, but the first one
items to be interpreted as a key and a value:
# same as map assignment above
- my %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
+ %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
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 identifier.
-This makes it nice for initializing hashes:
-
- my %map = (
- red => 0x00f,
- blue => 0x0f0,
- green => 0xf00,
+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,
+ blue => 0x0f0,
+ green => 0xf00,
);
or for initializing hash references to be used as records:
- my $rec = {
- witch => 'Mable the Merciless',
- cat => 'Fluffy the Ferocious',
- date => '10/31/1776',
+ $rec = {
+ witch => 'Mable the Merciless',
+ cat => 'Fluffy the Ferocious',
+ date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
- use CGI;
- my $query = CGI->new;
- my $field = $query->radio_group(
- name => 'group_name',
+ $field = $query->radio_group(
+ name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
- labels => \%labels,
+ labels => \%labels
);
Note that just because a hash is initialized in that order doesn't
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
-=head2 Slices
+If a key appears more than once in the initializer list of a hash, the last
+occurrence wins:
+
+ %circle = (
+ center => [5, 10],
+ center => [27, 9],
+ radius => 100,
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+ # same as
+ %circle = (
+ center => [27, 9],
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+This can be used to provide overridable configuration defaults:
+
+ # values in %args take priority over %config_defaults
+ %config = (%config_defaults, %args);
+
+=head2 Subscripts
+
+An array can be accessed one scalar at a
+time by specifying a dollar sign (C<$>), then the
+name of the array (without the leading C<@>), then the subscript inside
+square brackets. For example:
+
+ @myarray = (5, 50, 500, 5000);
+ print "The Third Element is", $myarray[2], "\n";
-A common way to access an array or a hash is one scalar element at a
-time. You can also subscript a list to get a single element from it.
+The array indices start with 0. A negative subscript retrieves its
+value from the end. In our example, C<$myarray[-1]> would have been
+5000, and C<$myarray[-2]> would have been 500.
- my $whoami = $ENV{"USER"}; # one element from the hash
- my $parent = $ISA[0]; # one element from the array
- my $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+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";
+
+You can also subscript a list to get a single element from it:
+
+ $dir = (getpwnam("daemon"))[7];
+
+=head2 Multi-dimensional array emulation
+
+Multidimensional arrays may be emulated by subscripting a hash with a
+list. The elements of the list are joined with the subscript separator
+(see L<perlvar/$;>).
+
+ $foo{$a,$b,$c}
+
+is equivalent to
+
+ $foo{join($;, $a, $b, $c)}
+
+The default subscript separator is "\034", the same as SUBSEP in B<awk>.
+
+=head2 Slices
+X<slice> X<array, slice> X<hash, slice>
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient
than writing out the individual elements as a list of separate
scalar values.
- my($him, $her) = @folks[0,-1]; # array slice
- my @them = @folks[0 .. 3]; # array slice
- my($who, $home) = @ENV{"USER", "HOME"}; # hash slice
- my($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
+ ($him, $her) = @folks[0,-1]; # array slice
+ @them = @folks[0 .. 3]; # array slice
+ ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
+ ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to
an array or hash slice.
- my( @days, %colors, @folks );
- @days[3..5] = qw(Wed Thu Fri);
+ @days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
- = (0xff0000, 0x0000ff, 0x00ff00);
+ = (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
- my( @days, %colors, @folks );
- ($days[3], $days[4], $days[5]) = qw(Wed Thu Fri);
- ($colors{red}, $colors{blue}, $colors{green})
- = (0xff0000, 0x0000ff, 0x00ff00);
+ ($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], $folks[0]);
Since changing a slice changes the original array or hash that it's
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
- foreach (@hash{keys %hash}) {
- s/^\s+//; # trim leading whitespace
- s/\s+$//; # trim trailing whitespace
- s/(\w+)/\u\L$1/g; # "titlecase" words
+ foreach (@hash{qw[key1 key2]}) {
+ s/^\s+//; # trim leading whitespace
+ s/\s+$//; # trim trailing whitespace
+ s/(\w+)/\u\L$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
- my @a = ()[1,0]; # @a has no elements
- my @b = (@a)[0,1]; # @b has no elements
- my @c = (0,1)[2,3]; # @c has no elements
+ @a = ()[1,0]; # @a has no elements
+ @b = (@a)[0,1]; # @b has no elements
+ @c = (0,1)[2,3]; # @c has no elements
But:
- my @a = (1)[1,0]; # @a has two elements
- my @b = (1,undef)[1,0,2]; # @b has three elements
+ @a = (1)[1,0]; # @a has two elements
+ @b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list
is returned:
- while ( my($home, $user) = (getpwent)[7,0] ) {
- printf "%-8s %s\n", $user, $home;
+ while ( ($home, $user) = (getpwent)[7,0]) {
+ printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list assignment
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
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
make @Here::blue an alias for @There::green, or %Here::blue an alias for
%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
of this. Strange though this may seem, this is the basis for the whole
-module import/export system. And none of it works under
-C<use strict 'vars'>.
+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:
- my $fh = *STDOUT;
+ $fh = *STDOUT;
or perhaps as a real reference, like this:
- my $fh = \*STDOUT;
+ $fh = \*STDOUT;
See L<perlsub> for examples of using these as indirect filehandles
in functions.
For example:
sub newopen {
- my $path = shift;
- local *FH; # not my!
- open (FH, $path) or return undef;
- return *FH;
+ my $path = shift;
+ local *FH; # not my!
+ open (FH, $path) or return undef;
+ return *FH;
}
- my $fh = newopen('/etc/passwd');
+ $fh = newopen('/etc/passwd');
Now that we have the C<*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
+new file and directory handles into or out of functions. That's because
C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
In other words, C<*FH> must be used to create new symbol table entries;
C<*foo{THING}> cannot. When in doubt, use C<*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
+them is an uninitialized scalar variable. This allows the constructs
such as C<open(my $fh, ...)> and C<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
+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 {
- my $filename = shift;
- open my $fh, $filename
- or die "Can't open '$filename': $!";
- return $fh;
+ open my $fh, "@_"
+ or die "Can't open '@_': $!";
+ return $fh;
}
{
my $f = myopen("</etc/motd");
- print <$f>;
- # $f implicitly closed here
+ print <$f>;
+ # $f implicitly closed here
}
+Note that if an initialized scalar variable is used instead the
+result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
+to C<open( *{'zzz'}, ...)>.
+C<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 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