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Perl Cookbook
book

Perl Cookbook

by Tom Christiansen, Nathan Torkington
August 1998
Intermediate to advanced content levelIntermediate to advanced
800 pages
39h 20m
English
O'Reilly Media, Inc.
Content preview from 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

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Publisher Resources

ISBN: 1565922433Supplemental ContentCatalog PageErrata