=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.
int( <STDIN> )
-the integer operation provides scalar context for the E<lt><gt>
+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 E<lt><gt>, which
+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
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 Perl's B<-w> command-line option, you may see warnings
+When you use the C<use warnings> pragma or Perl's B<-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 C<"fred";> or C<getpwuid(0);>. It still
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
-"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 && $str ne "0") {
warn "That doesn't look like a number";
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find the length
-of array @days by evaluating C<$#days>, as in B<csh>. Technically
-speaking, this isn't the length of the array; it's the subscript
-of the last element, since there is ordinarily a 0th element.
+of array @days by evaluating C<$#days>, as in B<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 C<$#days> actually changes the length of the array.
Shortening an array this way destroys intervening values. Lengthening
an array that was previously shortened does not recover values
that were in those elements. (It used to do so in Perl 4, but we
had to break this to make sure destructors were called when expected.)
+X<$#> 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
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;
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 bucked to the next power of two:
+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:
12345
12345.67
.23E-10 # a very small number
- 4_294_967_296 # underline for legibility
+ 3.14_15_92 # a very important number
+ 4_294_967_296 # underscore for legibility
0xff # hex
- 0377 # octal
+ 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.
+X<number, literal>
+
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 C<\'> and
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 Quotelike Operators"> for a list.
+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
- $Price = '$100'; # not interpreted
- print "The price is $Price.\n"; # interpreted
+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. You must also do
+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";
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
+v-strings, provides an alternative, more readable way to construct
+strings, rather than use the somewhat less readable interpolation form
+C<"\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, 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 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. 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
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
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>.
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 B<-w> switch, Perl will warn you about any
-such words. Some people may wish to outlaw barewords entirely. If you
+words, and if you use the C<use warnings> pragma or the B<-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';
end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
+=head3 Array Joining Delimiter
+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:
$temp = join($", @ARGV);
system "echo $temp";
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<E<lt>E<lt>> 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<E<lt>E<lt>> and
-the identifier. (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 $Price.
- EOF
-
- print <<`EOC`; # execute commands
- echo hi there
- echo lo there
- EOC
-
- print <<"foo", <<"bar"; # you can stack them
- I said foo.
- foo
- I said bar.
- bar
-
- myfunc(<<"THIS", 23, <<'THAT');
- Here's a line
- or two.
- THIS
- and here's another.
- THAT
-
-Just don't forget that you have to put a semicolon on the end
-to finish the statement, as Perl doesn't know you're not going to
-try to do this:
-
- print <<ABC
- 179231
- ABC
- + 20;
-
-If you want your here-docs to be indented with the
-rest of the code, you'll need to remove leading whitespace
-from each line manually:
-
- ($quote = <<'FINIS') =~ s/^\s+//gm;
- The Road goes ever on and on,
- down from the door where it began.
- FINIS
+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):
length of the array; the following assigns the value 3 to $foo:
@foo = ('cc', '-E', $bar);
- $foo = @foo; # $foo gets 3
+ $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,
+ 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
+ normal tomato
+ spicy tomato
+ green chile
+ pesto
+ white wine
End_Lines
LISTs do automatic interpolation of sublists. That is, when a LIST is
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 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
+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:
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
+ $x = (($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.
-The final element may be an array or a hash:
+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, $b, @rest) = split;
my($a, $b, %rest) = @_;
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 C<=E<gt>> operator between key/value
-pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
+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:
+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,
+ 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',
+ witch => 'Mable the Merciless',
+ cat => 'Fluffy the Ferocious',
+ date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
$field = $query->radio_group(
- name => 'group_name',
+ name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
+=head2 Subscripts
+
+An array is subscripted by specifying a dollar sign (C<$>), 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", $myarray[2], "\n";
+
+The array indices start with 0. A negative subscript retrieves its
+value from the end. In our example, C<$myarray[-1]> would have been
+5000, and C<$myarray[-2]> would have been 500.
+
+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";
+
=head2 Slices
+X<slice> X<array, slice> X<hash, slice>
-A common way 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.
+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.
- $whoami = $ENV{"USER"}; # one element from the hash
- $parent = $ISA[0]; # one element from the array
- $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+ $whoami = $ENV{"USER"}; # one element from the hash
+ $parent = $ISA[0]; # one element from the array
+ $dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash
-simultaneously using a list of subscripts. It's a more convenient
-that writing out the individual elements as a list of separate
+simultaneously using a list of subscripts. It's more convenient
+than writing out the individual elements as a list of separate
scalar values.
- ($him, $her) = @folks[0,-1]; # array slice
- @them = @folks[0 .. 3]; # array slice
- ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
- ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
+ ($him, $her) = @folks[0,-1]; # array slice
+ @them = @folks[0 .. 3]; # array slice
+ ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
+ ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to
an array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
- = (0xff0000, 0x0000ff, 0x00ff00);
+ = (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
- = (0xff0000, 0x0000ff, 0x00ff00);
- ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
+ = (0xff0000, 0x0000ff, 0x00ff00);
+ ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
Since changing a slice changes the original array or hash that it's
-slicing, a C<foreach> construct will alter through some--or even
-all--of the values of the array or hash.
+slicing, a C<foreach> construct will alter some--or even all--of the
+values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
- foreach (@hash{keys %hash}) {
- s/^\s+//; # trim leading whitespace
- s/\s+$//; # 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$1/g; # "titlecase" words
}
-You couldn't just loop through C<values %hash> to do this because
-that function produces a new list which is a copy of the values,
-so changing them doesn't change the original.
-
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
- @b = (1,undef)[1,0,1]; # @b has three elements
+ @c = (0,1)[2,3]; # @c has no elements
+
+But:
+
+ @a = (1)[1,0]; # @a has two elements
+ @b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list
is returned:
while ( ($home, $user) = (getpwent)[7,0]) {
- printf "%-8s %s\n", $user, $home;
+ 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
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;
}
$fh = newopen('/etc/passwd');
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
+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
+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
+ }
+
+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
https://perl5.git.perl.org / perl5.git / blobdiff