Perl Cookbook

Changing Array Size

Problem

You want to enlarge or truncate an array. For example, you might truncate an array of employees that’s already sorted by salary to list the five highest-paid employees. Or, if you know how big your array will get and that it will grow piecemeal, it’s more efficient to get memory for it in one step by enlarging it just once than it is to keep pushing values onto the end.

Solution

Assign to $#ARRAY :

# grow or shrink @ARRAY
$#ARRAY = $NEW_LAST_ELEMENT_INDEX_NUMBER;

Assigning to an element past the end automatically extends the array:

$ARRAY[$NEW_LAST_ELEMENT_INDEX_NUMBER] = $VALUE;

Discussion

$#ARRAY is the number of the last valid index in @ARRAY. If we assign it a number smaller than its current value, we truncate the array. Truncated elements are lost forever. If we assign $#ARRAY a number larger than its current value, the array grows. New elements have the undefined value.

$#ARRAY is not @ARRAY, though. Although $#ARRAY is the last valid index in the array, @ARRAY (in scalar context, as when treated as a number) is the number of elements. $#ARRAY is one less than @ARRAY because array indices start at 0.

Here’s some code that uses both:

sub what_about_that_array {
    print "The array now has ", scalar(@people), " elements.\n";
    print "The index of the last element is $#people.\n";
    print "Element #3 is `$people[3]'.\n";
}

@people = qw(Crosby Stills Nash Young);
what_about_that_array();

prints:

               
                  The array now has 4 elements.
               
                  The index of the last element is 3.
               
                  Element #3 is `Young'.

whereas:

$#people--;
what_about_that_array();

prints:

               
                  The array now has 3 elements.
               
                  The index of the last element is 2.
               
                  Element #3 is `'.

Element #3 disappeared when we shortened the array. If we’d used the -w flag on this program, Perl would also have warned “use of uninitialized value” because $people[3] is undefined.

$#people = 10_000;
what_about_that_array();

prints:

               
                  The array now has 10001 elements.
               
                  The index of the last element is 10000.
               
                  Element #3 is `'.
               

The "Young" element is now gone forever. Instead of assigning to $#people, we could have said:

$people[10_000] = undef;

Perl arrays are not sparse. In other words, if you have a 10,000th element, you must have the 9,999 other elements, too. They may be undefined, but they still take up memory. For this reason, $array[time], or any other construct that uses a very large integer as an array index, is a bad idea. Use a hash instead.

We have to say scalar @array in the print because Perl gives list context to (most) functions’ arguments, but we want @array in scalar context.

See Also

The discussion of the $#ARRAY notation in perldata(1), also explained in the “List Values and Arrays” section of Chapter 2 of Programming Perl