=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
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";
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:
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
- $Price = '$100'; # not interpreted
- print "The price is $Price.\n"; # interpreted
+ $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).
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:
$temp = join($", @ARGV);
system "echo $temp";
It is often more readable to use the C<< => >> operator between key/value
pairs. The C<< => >> operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to be
-interpreted as a string--if it's a bareword that would be a legal identifier.
-This makes it nice for initializing hashes:
+interpreted as a string -- if it's a bareword that would be a legal simple
+identifier (C<< => >> doesn't quote compound identifiers, that contain
+double colons). This makes it nice for initializing hashes:
%map = (
red => 0x00f,
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
+=head2 Subscripts
+
+An array is subscripted by specifying a dollary sign (C<$>), 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 ", $scientists{"Darwin"}, "\n";
+
=head2 Slices
A common way to access an array or a hash is one scalar element at a
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
- foreach (@hash{keys %hash}) {
+ foreach (@hash{qw[key1 key2]}) {
s/^\s+//; # trim leading whitespace
s/\s+$//; # trim trailing whitespace
s/(\w+)/\u\L$1/g; # "titlecase" words
# $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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