=head2 Variable names
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
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
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";
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 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
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
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, a fatal error will result, since this
+bucket usage information is currently not available for tied hashes.
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
+ $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.
+
As in some shells, you can enclose the variable name in braces to
disambiguate it from following alphanumerics (and underscores).
You must also do
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 ${who}'s here.\n";
anything more complicated in the subscript will be interpreted as
an expression.
+=head3 Version Strings
+
+B<Note:> Version Strings (v-strings) have been deprecated. They will
+be removed in some future release after Perl 5.8.1. The marginal
+benefits of v-strings were greatly outweighed by the potential for
+Surprise and Confusion.
+
A literal of the form C<v1.20.300.4000> is parsed as a string composed
of characters with the specified ordinals. This form, known as
v-strings, provides an alternative, more readable way to construct
Note that using the v-strings for IPv4 addresses is not portable unless
you also use the inet_aton()/inet_ntoa() routines of the Socket package.
+Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
+are not v-strings before the C<< => >> operator (which is usually used
+to separate a hash key from a hash value), instead they are interpreted
+as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
+Perl 5.8.0, but that caused more confusion and breakage than good.
+Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
+be v-strings always.
+
+=head3 Special Literals
+
The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package name at that
point in your program. They may be used only as separate tokens; they
as it is seen (during compilation), at which point the corresponding
__DATA__ (or __END__) token has not yet been seen.
+=head3 Barewords
+
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
"barewords". As with filehandles and labels, a bareword that consists
end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
+=head3 Array Joining Delimiter
+
Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the C<$">
-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
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
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
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 = $query->radio_group(
+ name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
- labels => \%labels,
+ labels => \%labels
);
Note that just because a hash is initialized in that order doesn't
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
+=head2 Subscripts
+
+An array is subscripted by specifying a dollary sign (C<$>), 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", $myarray[2], "\n";
+
+The array indices start with 0. A negative subscript retrieves its
+value from the end. In our example, C<$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
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, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to
an array or hash slice.
- my( @days, %colors, @folks );
- @days[3..5] = qw(Wed Thu Fri);
+ @days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
- = (0xff0000, 0x0000ff, 0x00ff00);
+ = (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
- my( @days, %colors, @folks );
- ($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 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
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
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