Perl How to get the index of last element of array reference?

If we have array then we can do following:

my @arr = qw(开发者_如何学CField3 Field1 Field2 Field5 Field4);
my $last_arr_index=$#arr;

How do we do this for array reference?

my $arr_ref = [qw(Field3 Field1 Field2 Field5 Field4)];
my $last_aref_index; # how do we do something similar to $#arr;

my $arr_ref = [qw(Field3 Field1 Field2 Field5 Field4)];
my ($last_arr_index, $next_arr_index);

If you need to know the actual index of last element, for example you need to loop over the array's elements knowing the index, use $#$:

$last_arr_index = $#{ $arr_ref };
$last_arr_index = $#$arr_ref; # No need for {} for single identifier

If you need to know the index of element after the last, (e.g. to populate next free element without push()),

OR you need to know the amount of elements in the array (which is the same number) as above:

my $next_arr_index = scalar(@$arr_ref);
$last_arr_index = $next_arr_index - 1; # in case you ALSO need $last_arr_index
# You can also bypass $next_arr_index and use scalar, 
# but that's less efficient than $#$ due to needing to do "-1"
$last_arr_index = @{ $arr_ref } - 1; # or just "@$arr_ref - 1"
   # scalar() is not needed because "-" operator imposes scalar context 
   # but I personally find using "scalar" a bit more readable
   # Like before, {} around expression is not needed for single identifier

If what you actually need is to access the last element in the arrayref (e.g. the only reason you wish to know the index is to later use that index to access the element), you can simply use the fact that "-1" index refers to the last element of the array. Props to Zaid's post for the idea.

$arr_ref->[-1] = 11;
print "Last Value : $arr_ref->[-1] \n";
# BTW, this works for any negative offset, not just "-1". 

my $last_aref_index = $#{ $arr_ref };

The reason you probably need to access the last index is to get the last value in the array reference.

If that is the case, you can simply do the following:

$arr_ref->[-1];

The -> operator dereferences. [-1] is the last element of the array.

If you need to count the number of elements in the array, there's no need to do $#{ $arr_ref } + 1. DVK has shown a couple of better ways to do it.

my $arr_ref = [qw(Field3 Field1 Field2 Field5 Field4)];

my $last_aref_index = $$arr_ref[$#{$arr_ref}];
print "last element is: $last_aref_index\n";