CODEWITHSUNDEEP
ccomparison
kate
kate

Reputation: 201

Compare two floats

#include <stdbool.h>

bool Equality(double a, double b, double epsilon)
{
  if (fabs(a-b) < epsilon) return true;
  return false;
}

I tried this method to compare two doubles, but I always get problems since I don't know how to chose the epsilon, actually I want to compare small numbers (6 6 digits after the decimal point) like 0.000001. I tried with some numbers, sometimes I get 0.000001 != 0.000001 and sometimes 0.000001 == 0.000002 Is there another method else than comparing with the epsilon?

My purpose is to compare two doubles (which represent the time in my case). The variable t which represents the time in milliseconds is a double. It is incremented by another function 0.000001 then 0.000002 etc. each time t changes, I want to check if it is equal to another variable of type double tt, in case tt == t, I have some instructions to execute..
Thanks for your help

Upvotes: 20

Views: 56359

Answers (4)

Roddy
Roddy

Reputation: 68114

Look here: http://floating-point-gui.de/errors/comparison/

Due to rounding errors, most floating-point numbers end up being slightly imprecise. As long as this imprecision stays small, it can usually be ignored. However, it also means that numbers expected to be equal (e.g. when calculating the same result through different correct methods) often differ slightly, and a simple equality test fails.

And, of course, What Every Computer Scientist Should Know About Floating-Point Arithmetic

Upvotes: 24

Chameleon
Chameleon

Reputation: 2147

Keep in mind that when float a = +2^(254-127) * 1.___22 zeros___1 and float b = +2^(254-127) * 1.___23 zeros___ then we expect abs(a-b) < epsilon but instead a - b = +2^(254-127-23) * 1.___23 zeros___ = 20282409603651670423947251286000 which is much bigger than epsilon...

Upvotes: 1

rebs01
rebs01

Reputation: 11

Alternatively, you could compare two integers instead. Just multiply your two floats by the desired precision and cast them to integers. Be sure to round up/down correctly. Here is what it looks like:

BOOL floatcmp(float float1, float float2, unsigned int precision){
   int int1, int2;

   if (float1 > 0)
      int1 = (int)(float1 * precision + .5);
   else
      int1 = (int)(float1 * precision - .5);

   if (float2 > 0)
      int2 = (int)(float2 * precision + .5);
   else
      int2 = (int)(float2 * precision - .5);

   return (int1 == int2);
}

Upvotes: 1

unwind
unwind

Reputation: 400159

First: there's no point in computing a boolean value (with the < operator) and then wrapping that in another boolean. Just write it like this:

bool Equality(float a, float b, float epsilon)
{
  return fabs(a - b) < epsilon;
}

Second, it's possible that your epsilon itself isn't well-represented as a float, and thus doesn't look like what you expect. Try with a negative power of 2, such as 1/1048576 for instance.

Upvotes: 4

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