Suppose you have a number x between 0 and 1. You want to find a rational approximation for x, but you only want to consider fractions with denominators below a given limit.

For example, suppose x = 1/e = 0.367879… Rational approximations with powers of 10 in the denominator are trivial to find: 3/10, 36/100, 367/1000, etc. But say you’re willing to have a denominator as large as 10. Could you do better than 3/10? Yes, 3/8 = 0.375 is a better approximation. What about denominators no larger than 100? Then 32/87 = 0.36781… is the best choice, much better than 36/100.

How do you find the best approximations? You could do a brute force search. For example, if the maximum denominator size is N, you could try all fractions with denominators less than or equal to N. But there’s a much more efficient algorithm. The algorithm is related to the Farey sequence named after John Farey, though I don’t know whether he invented the algorithm.

The idea is to start with two fractions, a/b = 0/1 and c/d = 1/1. We update either a/b or c/d at each step so that a/b will be the best lower bound of x with denominator no bigger than b, and c/d will be the best upper bound with denominator no bigger than d. At each step we do a sort of binary search by introducing the mediant of the upper and lower bounds. The mediant of a/b and c/d is the fraction (a+c)/(b+d) which always lies between a/b and c/d.

Here is an implementation of the algorithm in Python. The code takes a number x between 0 and 1 and a maximum denominator size N. It returns the numerator and denominator of the best rational approximation to x using denominators no larger than N.

def farey(x, N): a, b = 0, 1 c, d = 1, 1 while (b <= N and d <= N): mediant = float(a+c)/(b+d) if x == mediant: if b + d <= N: return a+c, b+d elif d > b: return c, d else: return a, b elif x > mediant: a, b = a+c, b+d else: c, d = a+c, b+d if (b > N): return c, d else: return a, b

In Python 3.0, the float statement could be removed since the division operator does floating point division of integers.

Read more about rational approximation in Breastfeeding, the golden ratio, and rational approximation.

Here’s an example of a situation in which you might need rational approximations. Suppose you’re designing an experiment which will randomize subjects between two treatments A and B. You want to randomize in blocks of size no larger than N and you want the probability of assigning treatment A to be p. You could find the best rational approximation a/b to p with denominator b no larger than N and use the denominator as the block size. Each block would be a permutation of a A’s and b–a B’s.

Update: Eugene Wallingford wrote a blog post about implementing the algorithm in Klein, a very restricted language used for teaching compiler writing.

Update: There’s a bug in the code. See discussion below.