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11.1. Taking References to Arrays

11.1.3. Discussion

Here are array references in action:

# check whether $someref contains a simple array reference
if (ref($someref) ne "ARRAY") {
    die "Expected an array reference, not $someref\n";

print "@{$array_ref}\n";        # print original data

@order = sort @{ $array_ref };  # sort it

push @{ $array_ref }, $item;    # append new element to orig array

If you can't decide whether to use a reference to a named array or to create a new one, here's a simplistic guideline that will prove right more often than not. Only take a reference to an existing array to return the reference out of scope, thereby creating an anonymous array, or to pass the array by reference to a function. For virtually all other cases, use [@array] to create a new array reference with a copy of the old values.

Automatic reference counting and the backslash operator make a powerful combination:

sub array_ref {
    my @array;
    return \@array;

$aref1 = array_ref( );
$aref2 = array_ref( );

Each time array_ref is called, the function allocates a new piece of memory for @array. If we hadn't returned a reference to @array, its memory would have been freed when its block, the subroutine, ended. But here a reference to @array is still accessible, so Perl doesn't free that storage, and we wind up with a reference to a piece of memory no longer accessible through the symbol table. Such a piece of memory is called anonymous because it has no name associated with it.

To access a particular element of the array referenced by $aref, you could write $$aref[4], but $aref->[4] is the same thing, and clearer.

print $array_ref->[$N];         # access item in position N (best)
print $$array_ref[$N];          # same, but confusing
print ${$array_ref}[$N];        # same, but still confusing, and ugly to boot

If you have an array reference, you can only access a slice of the referenced array in this way:

@$pie[3..5];                    # array slice, but a little confusing to read
@{$pie}[3..5];                  # array slice, easier (?) to read

Array slices, even when accessed through array references, are assignable. In the next line, the array dereference happens first, then the slice:

@{$pie}[3..5] = ("blackberry", "blueberry", "pumpkin");

An array slice is just syntactic sugar for a list of individual array elements. Because you can't take a reference to a list, you can't take a reference to an array slice:

$sliceref = \@{$pie}[3..5];     # WRONG!

To iterate through the entire array, loop with foreach or for:

foreach $item ( @{$array_ref} ) {
    # $item has data

for ($idx = 0; $idx <= $#{ $array_ref }; $idx++) {
    # $array_ref->[$idx] has data

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