Comparing Floating Point Numbers in C/C++

Comparing floating point numbers for equality can be problematic. It’s difficult because often we are comparing small or large numbers that are not represented exactly. There is also issues with rounding errors caused by not being able to represent and exact value.

Rather than doing a strict value comparison (==), we treat two values as equal if there values are very close to each other. So what does “very close” mean? Well to answer that we have to take a look at how the numbers are represented in memory.

Here is an example 32bit float:

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And here is the layout of a double precision (64bit) float:

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The number of bits in the fraction can be thought of as the number of significant bits (or, accuracy) of the number. We do not want to use all of the fraction bits otherwise we would be doing a strict comparison, but we will use most. For both sized float types I will use 4 less significant bits:

We will use this to calculate the epsilon (the small difference that is still considered equal). However, we have to be careful that the number of bits that we use to calculate the epsilon is based on the smallest precision value in the comparison. For example is we compare a 32bit float with a 64bit float we must use only the precision of the 32bit float.

For this was can use a macro:

There are two more gotchas:

  1. Zero is a special case because the epsilon would also be zero causing an exact comparison. So we need to treat this as a separate case.
  2. Non-rational numbers, such as NaNs and infinities are never equal to each other in any combination. If either side is non-rational the comparison is never equal.

You can find the full commented solution as part of the test suite in the c2go project.

Originally published at http://elliot.land on September 2, 2017.

Written by

I’m a data nerd and TDD enthusiast originally from Sydney. Currently working for Uber in New York. My thoughts here are my own. 🤓 elliotchance@gmail.com

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