I teach a general programming class, and I'm looking for interesting tasks to teach programming concepts.

One example is "produce English date suffixes from numbers", i.e.

for(1 .. 31){
  print $_ . suffix($_) . "/n";
}
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where "suffix()" is a sub that the students have to come up with.

I like this task because

• it's relatively simple, although
• there's a gotcha because all numbers ending in "1" take "st" except 11, and the same for 2/12 and 3/13
• it kind of involves math but mostly just logical thinking
• TMTOWTDI -- I can think of about ten ways to do it, from the straightforward but clunky:

      @suffixes = ( undef, 'st','nd','rd' ... etc);
      return $suffixes[$_[0]];
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  to the golf-y

      $_[0]=~/(\b|[^1])(\d)$/;qw(0 st nd rd)[$2]or'th'
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• the content is familiar and practical, not abstract -- you might actually want someday to produce output like "last edited: 1st of April".

So, have monks got any examples of similar exercises which would be suitable for learners?