Learning Perl, Second Edition

15.4. Advanced Sorting

Earlier, you learned that you could take a list and sort it in ascending ASCII order (like you do strings) using the built-in sort function. What if you don't want an ascending ASCII sort, but something else instead, like a numeric sort? Well, Perl gives you the tools you need to do the job. In fact, you'll see that the Perl sort is completely general and able to perform any well-defined sort order.

To define a sort of a different color, you need to define a comparison routine that describes how two elements compare. Why is this necessary? Well, if you think about it, sorting is putting a bunch of things in order by comparing them all. Since you can't compare them all at once, you need to compare two at a time, eventually using what you find out about each pair's order to put the whole kit'n'caboodle in line.

The comparison routine is defined as an ordinary subroutine. This routine will be called repeatedly, each time passing two elements of the list to be sorted. The routine must determine whether the first value is less-than, equal-to, or greater-than the second value, and return a coded value (described in a moment). This process is repeated until the list is sorted.

To save a little execution speed, the two values are not passed in an array, but rather are handed to the subroutine as the values of the global variables $a and $b. (Don't worry: the original values of $a and $b are safely protected.) The routine should return any negative number if