Chapter 7. Practical Reference TricksContents:
Review of Sorting This chapter looks at optimizing sorting and dealing with recursively defined data. 7.1. Review of SortingPerl'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 $b, like so: my @numerically_sorted = sort { if ($a < $b) { -1 } elsif ($a > $b) { +1 } else { 0 } } 1, 2, 4, 8, 16, 32; Now you have a proper numeric comparison, so you have a proper numeric sort. Of course, this is far too much typing, so you can use the spaceship operator instead: my @numerically_sorted = sort { $a <=> $b } 1, 2, 4, 8, 16, 32; The spaceship operator returns -1, 0, and +1, according to the rules discussed. A descending sort is as simple as reversing the position of $a and $b: my @numerically_descending = sort { $b <=> $a } 1, 2, 4, 8, 16, 32; In every place the previous sort expression returned -1, this expression returns +1, and vice versa. Thus, the sort is in the opposite order. It's also easy to remember because if $a is to the left of $b, you get out the lower items first, just like a and b would be in the resulting list. Likewise, a string sort can be indicated with cmp, although this is used less often because it is the default comparison. The cmp operator is handier when you have a more complex comparison, as you'll see shortly. Copyright © 2003 O'Reilly & Associates. All rights reserved. |
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