Learning Perl Objects, References, and Modules

Chapter 7. Practical Reference Tricks

This chapter looks at optimizing sorting and dealing with recursively defined data.

Review of Sorting

Perl’s built-in sort operator sorts text strings in their natural text order, by default.^[29]

This is fine if you want to sort text strings:

my @sorted = sort qw(Gilligan Skipper Professor Ginger Mary_Ann);

but gets pretty messy when you want to sort numbers:

my @wrongly_sorted = sort 1, 2, 4, 8, 16, 32;

The resulting list is 1, 16, 2, 32, 4, 8. Why didn’t sort order these properly? It treats each item as a string and sorts them in string order. Any string that begins with 3 sorts before any string that begins with 4.

You can fix this by overriding how Perl compares pairs of items in the list. By default, as Perl orders the items, a string comparison is used. A new comparison is specified using a sort block, placed between the sort keyword and the list of things to sort.^[30]

Within the sort block, $a and $b stand in for two of the items to be sorted. The last evaluated expression must return a -1, 0, or +1 value.^[31] If the value is -1, the value currently in $a must appear before the value in $b in the final sorted list. If the value is +1, then the value in $a must appear after the value in $b in the final sorted list. If the value is 0, you don’t know or can’t tell, so the results are unpredictable.^[32]

For example, to sort those numbers in their proper order, you can use a sort block comparing $a and $b, like so:

my @numerically_sorted = sort ...