99 problems

There is a classic set of programming exercises called “Ninety-Nine Prolog Problems”. Though somewhat tailored to logic programming, they form an interesting set of exercises for other languages. I’ve seen adaptations of varying completeness for Haskell, Lisp, Perl 6, and Python.

I was reminded of this all by a recent blog post by some bloke called Dave who was using the problems as a way to become more familiar with Python. He used Python’s unittest module to test his solutions.

I actually played with these same problems in Python a while ago (before I had found that collection on the Python wiki), and like Dave I decided to incorporate tests. Instead of unittest, though, I used doctest, so the test cases live in the docstring with the description of the problem. For example:

def p31(n):
    """
    P31 (**) Determine whether a given integer number is prime.

    >>> p31(7) and p31(2) and p31(31)
    True
    >>> p31(1) or p31(4) or p31(9)
    False
    """
    if n < 2:
        return False
    for i in range(2, n/2 + 1):
        if n % i == 0:
            return False
    return True

(No, not the most efficient algorithm!)

I collected all the problems in a single module. At the end is the code that invokes the doctest runner:

if __name__ == '__main__':
    import doctest
    doctest.testmod()

One motivation for this structure was the idea that I could build a version of the module with just the docstrings and pass function bodies as a blank slate for people who wanted to try their hand at the problems.

I got through about 40 problems, but it’s unlikely I’ll have the time to finish them any time soon (life got busy, and they get harder as you go). The source is here for anyone who feels like continuing the project, or using the doctest idea to enhance the version on the Python wiki. Keep me posted if you do!

Pistahh commented :

One trivial optimization is to terminate the loop after sqrt(n) instead of n/2.

mabuse commented :

just one advice. There is no need to go from 2 to n/2. You only need from 2 to sqrt(n).

Paul commented :

I’m trying to come up with a good excuse. sqrt not a Python built-in? Not enough coffee? Too much coffee?