Perl Hacks

Hack #53. Debug with Test Cases

Make exploratory code reusable.

Many programmers have subdirectories full of little test snippets; it's common to write a few programs to explore a feature of the language or a new library. It's also common to do this with false laziness, eyeballing output and tweaking an idea here or there.

Usually that's okay, but occasionally you know you wrote code to explore something you need to know right now—if only you could find it and decipher what you were thinking.

If you know how to write test cases with Perl's standard testing tools, you can end this madness and make even your experiments reusable and maintainable.

The Hack

Suppose you've just learned that Perl's default sorting algorithm changed from unstable to stable for Perl 5.8.0. The Internet reveals that, with a stable sorting algorithm, elements that have the same position in the sorting order will retain the positions relative to each other that they had in the input.

Writing test code

What does that really mean in practice? It's time to write some code:

my @elements =
(
    [ 2, 2 ], [ 2, 1 ], [ 2, 0 ],
    [ 1, 0 ], [ 1, 1 ], [ 1, 2 ],
);

my @sorted   = sort { $a->[0] <=> $b->[0] } @elements;

local $"     = ', ';
print "[ @$_ ]\\n" for @sorted;

A stable sorting algorithm should produce the output:

[ 1, 0 ]
[ 1, 1 ]
[ 1, 2 ]
[ 2, 2 ]
[ 2, 1 ]
[ 2, 0 ]

Because the algorithm sorts only on the first element, all of the ones should come before the twos. Because the algorithm is stable, all of the second values ...