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Approximate a float using a minimal fraction

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This sounds like it's probably a well-known problem, but I haven't been able to find references to it by searching.

Given a floating point value $x$ and an error range $\varepsilon$, how can I efficiently find a minimal positive integer $q$ such that $\left|x - \frac{p}{q}\right| < \varepsilon$ for some integer $p$?

The approach that seems obvious is to iterate from $q = 1$ upward (skipping composite numbers), computing $p := round(x * q)$ and checking whether $\left|x - \frac{p}{q}\right| < \varepsilon$. But I'm wondering if there's a smarter way.

Asked By : LarsH

Answered By : Yuval Filmus

The partial convergents of the continued fraction of $x$ consists of all the best rational approximations of $x$; see Wikipedia, for example. A best rational approximation of $x$ is a rational number $p/q$ such that $\left|x-\frac{p}{q}\right| \leq \left|x-\frac{p'}{q'}\right|$ for all $q' \leq q$. Your $p/q$ is in particular a best rational approximation, so it would be one of the convergents.

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