language whose scalars can be strings, numbers, or references (which
includes objects). While strings and numbers are considered pretty
much the same thing for nearly all purposes, references are strongly-typed
-uncastable pointers with built-in reference-counting and destructor
+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
-Boolean context is just a special kind of scalar context.
+Boolean context is just a special kind of scalar context.
There are actually two varieties of null scalars: defined and
undefined. Undefined null scalars are returned when there is no real
use it as if it were defined, but prior to that you can use the
defined() operator to determine whether the value is defined or not.
-To find out whether a given string is a valid non-zero number, it's usually
+To find out whether a given string is a valid nonzero number, it's usually
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>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
- }
+ }
That's usually preferable because otherwise you won't treat IEEE notations
like C<NaN> or C<Infinity> properly. At other times you might prefer to
warn "has nondigits" if /\D/;
warn "not a whole number" unless /^\d+$/;
- warn "not an integer" unless /^[+-]?\d+$/
- warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
- warn "not a C float"
+ warn "not an integer" unless /^[+-]?\d+$/
+ warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
+ warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find the length of
I<NO LONGER> recovers the values that were in those elements. (It used to
in Perl 4, but we had to break this to make sure destructors were
called when expected.) You can also gain some measure of efficiency by
-preextending an array that is going to get big. (You can also extend
+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:
@whatever = ();
- $#whatever = $[ - 1;
+ $#whatever = -1;
If you evaluate a named array in a scalar context, it returns the length of
the array. (Note that this is not true of lists, which return the
newline, tab, etc., as well as some more exotic forms. See
L<perlop/Quote and Quotelike Operators> for a list.
+Octal or hex representations in string literals (e.g. '0xffff') are not
+automatically converted to their integer representation. The hex() and
+oct() functions make these conversions for you. See L<perlfunc/hex> and
+L<perlfunc/oct> for more details.
+
You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin. This is nice, but if you forget
your trailing quote, the error will not be reported until Perl finds
space, because single quote is a valid (though deprecated) character in
a variable name (see L<perlmod/Packages>).
-Two special literals are __LINE__ and __FILE__, which represent the
-current line number and filename at that point in your program. They
-may be used only as separate tokens; they will not be interpolated into
-strings. In addition, the token __END__ may be used to indicate the
-logical end of the script before the actual end of file. Any following
-text is ignored, but may be read via the DATA filehandle. (The DATA
-filehandle may read data from only the main script, but not from any
-required file or evaluated string.) The two control characters ^D and
-^Z are synonyms for __END__ (or __DATA__ in a module; see L<SelfLoader> for
-details on __DATA__).
+Three special literals are __FILE__, __LINE__, and __PACKAGE__, which
+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 a C<package;> directive), __PACKAGE__ is the undefined value.
+
+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, but may be read via a DATA filehandle: main::DATA for __END__,
+or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__.
+The two control characters ^D and ^Z are synonyms for __END__ (or
+__DATA__ in a module). See L<SelfLoader> for more description of
+__DATA__, and an example of its use. Note that you cannot read from the
+DATA filehandle in a BEGIN block: the BEGIN block is executed as soon as
+it is seen (during compilation), at which point the corresponding
+__DATA__ (or __END__) token has not yet been seen.
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
then any bareword that would NOT be interpreted as a subroutine call
produces a compile-time error instead. The restriction lasts to the
-end of the enclosing block. An inner block may countermand this
+end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
Array variables are interpolated into double-quoted strings by joining all
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 blank line.) The
+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;
+ print <<EOF;
The price is $Price.
EOF
Here's a line
or two.
THIS
- and here another.
+ 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
+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
assigns the value of variable bar to variable foo. Note that the value
of an actual array in a scalar context is the length of the array; the
-following assigns to $foo the value 3:
+following assigns the value 3 to $foo:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
-You may have an optional comma before the closing parenthesis of an
+You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:
@foo = (
array had been interpolated at that point.
A list value may also be subscripted like a normal array. You must
-put the list in parentheses to avoid ambiguity. Examples:
+put the list in parentheses to avoid ambiguity. For example:
# Stat returns list value.
$time = (stat($file))[8];
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
+You may assign to C<undef> in a list. This is useful for throwing
+away some of the return values of a function:
+
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+
Lists may be assigned to if and only if each element of the list
is legal to assign to:
or for using call-by-named-parameter to complicated functions:
- $field = $query->radio_group(
+ $field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
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
-it represents all types. This used to be the preferred way to
+it represents all types. This used to be the preferred way to
pass arrays and hashes by reference into a function, but now that
we have real references, this is seldom needed. It also used to be the
preferred way to pass filehandles into a function, but now
my $path = shift;
local *FH; # not my!
open (FH, $path) || return undef;
- return \*FH;
+ return *FH;
}
$fh = newopen('/etc/passwd');
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