Perl Cookbook

Reversing an Array

Problem

You want to reverse an array.

Solution

Use the reverse function:

# reverse @ARRAY into @REVERSED
@REVERSED = reverse @ARRAY;

Or use a for loop:

for ($i = $#ARRAY; $i >= 0; $i--) {
    # do something with $ARRAY[$i]
}

Discussion

The reverse function actually reverses a list; the for loop simply processes the list in reverse order. If you don’t need a reversed copy of the list, for saves memory and time.

If you’re using reverse to reverse a list that you just sorted, you should have sorted it in the correct order to begin with. For example:

# two-step: sort then reverse
@ascending = sort { $a cmp $b } @users;
@descending = reverse @ascending;

# one-step: sort with reverse comparison
@descending = sort { $b cmp $a } @users;

See Also

The reverse function in perlfunc (1) and Chapter 3 of Programming Perl ; we use reverse in Section 1.6