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
which return whatever they feel like returning.) The following is
always true:
+ scalar(@whatever) == $#whatever - $[ + 1;
+
+Version 5 of Perl changed the semantics of C<$[>: files that don't set
+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
+
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as to
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
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
- my $Price = '$100'; # not interpolated
- print "The price is $Price.\n"; # interpolated
+ $Price = '$100'; # not interpreted
+ print "The price is $Price.\n"; # interpreted
As in some shells, you can enclose the variable name in braces to
disambiguate it from following alphanumerics (and underscores).
variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:
- my $who = "Larry";
+ $who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
variable (C<$LIST_SEPARATOR> in English), space by default. The
following are equivalent:
- my $temp = join($", @ARGV);
+ $temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
rest of the code, you'll need to remove leading whitespace
from each line manually:
- (my $quote = <<'FINIS') =~ s/^\s+//gm;
+ ($quote = <<'FINIS') =~ s/^\s+//gm;
The Road goes ever on and on,
down from the door where it began.
FINIS
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 = (
+ @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;
+ @sauces = <<End_Lines =~ m/(\S.*\S)/g;
normal tomato
spicy tomato
green chile
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,
return values, by assigning to an empty list and then using that
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
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
interpreted as a string--if it's a bareword that would be a legal identifier.
This makes it nice for initializing hashes:
- my %map = (
+ %map = (
red => 0x00f,
blue => 0x0f0,
green => 0xf00,
or for initializing hash references to be used as records:
- my $rec = {
+ $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(
+ $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
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.
- 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
+ $whoami = $ENV{"USER"}; # one element from the hash
+ $parent = $ISA[0]; # one element from the array
+ $dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient
than writing out the individual elements as a list of separate
scalar values.
- my($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, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to
an array or hash slice.
- my( @days, %colors, @folks );
- @days[3..5] = qw(Wed Thu Fri);
+ @days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
= (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
- my( @days, %colors, @folks );
- ($days[3], $days[4], $days[5]) = qw(Wed Thu Fri);
- ($colors{red}, $colors{blue}, $colors{green})
+ ($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], $folks[0]);
+ ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
Since changing a slice changes the original array or hash that it's
slicing, a C<foreach> construct will alter some--or even all--of the
A slice of an empty list is still an empty list. Thus:
- my @a = ()[1,0]; # @a has no elements
- my @b = (@a)[0,1]; # @b has no elements
- my @c = (0,1)[2,3]; # @c has no elements
+ @a = ()[1,0]; # @a has no elements
+ @b = (@a)[0,1]; # @b has no elements
+ @c = (0,1)[2,3]; # @c has no elements
But:
- my @a = (1)[1,0]; # @a has two elements
- my @b = (1,undef)[1,0,2]; # @b has three elements
+ @a = (1)[1,0]; # @a has two elements
+ @b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list
is returned:
- while ( my($home, $user) = (getpwent)[7,0] ) {
+ while ( ($home, $user) = (getpwent)[7,0]) {
printf "%-8s %s\n", $user, $home;
}
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.
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
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': $!";
+ open my $fh, "@_"
+ or die "Can't open '@_': $!";
return $fh;
}
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