Assigning to C<$#days> actually changes the length of the array.
Shortening an array this way destroys intervening values. Lengthening
an array that was previously shortened does not recover values
-that were in those elements. (It used to do so in Perl 4, but we
-had to break this to make sure destructors were called when expected.)
+that were in those elements.
X<$#> X<array, length>
You can also gain some minuscule measure of efficiency by pre-extending
$0 and the $s variables in the (presumably) non-existent package
C<who>.
-In fact, an identifier within such curlies is forced to be a string,
-as is any simple identifier within a hash subscript. Neither need
+In fact, a simple identifier within such curlies is forced to be
+a string, and likewise within a hash subscript. Neither need
quoting. Our earlier example, C<$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
subroutine. 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. (But the empty C<package;> is no longer supported, as of version
+value.
(But the empty C<package;> is no longer supported, as of version
5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
is only available in 5.16 or higher, and only with a C<use v5.16> or
C<use feature "current_sub"> declaration.
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
+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
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:
+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
+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
+number of times the pattern matched) and assign that to $count. Note
that simply using
$count = $string =~ /\d+/g;
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 simple
-identifier. C<< => >> doesn't quote compound identifiers, that contain
-double colons. This makes it nice for initializing hashes:
+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.
+If a key appears more than once in the initializer list of a hash, the last
+occurrence wins:
+
+ %circle = (
+ center => [5, 10],
+ center => [27, 9],
+ radius => 100,
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+ # same as
+ %circle = (
+ center => [27, 9],
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+This can be used to provide overridable configuration defaults:
+
+ # values in %args take priority over %config_defaults
+ %config = (%config_defaults, %args);
+
=head2 Subscripts
An array can be accessed one scalar at a
@myarray = (5, 50, 500, 5000);
print "The Third Element is", $myarray[2], "\n";
-The array indices start with 0. A negative subscript retrieves its
+The array indices start with 0. A negative subscript retrieves its
value from the end. In our example, C<$myarray[-1]> would have been
5000, and C<$myarray[-2]> would have been 500.
Hash subscripts are similar, only instead of square brackets curly brackets
-are used. For example:
+are used. For example:
%scientists =
(
=head2 Multi-dimensional array emulation
Multidimensional arrays may be emulated by subscripting a hash with a
-list. The elements of the list are joined with the subscript separator
+list. The elements of the list are joined with the subscript separator
(see L<perlvar/$;>).
$foo{$a,$b,$c}
@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:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
+More generally, a slice yields the empty list if it indexes only
+beyond the end of a list:
+
+ @a = (1)[ 1,2]; # @a has no elements
+ @b = (1)[0,1,2]; # @b has three elements
+
This makes it easy to write loops that terminate when a null list
is returned:
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
-new file and directory handles into or out of functions. That's because
+new file and directory handles into or out of functions. That's because
C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
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
+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
+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:
https://perl5.git.perl.org / perl5.git / blobdiff