Assertion in C with negative float

Question

I wrote a simple function and would like to get know the assert macro in use. When I test the function (moving_hysteresis) after the sixth assert function I get an abort and I could not find out why. The moving_hysteresis() function follows:

float moving_hysteresis(float Delta,float temp)
{    
float delta=Delta;
float temp_min=ZERO;
float temp_max=delta;
float result;

if(temp<=temp_max)
    {
        if(temp>=temp_min)
            {
                result=temp;
                return result;
            }
        else
            {
                result=temp+delta;
                return result;
            }
    }
else
    {
        result=temp-delta;
        return  result;
    }
 }

If I try to test this with

 assert(moving_hsysteresis(5.00,-5.01)==(float)-0.01);

I get an assertion and I could not find out why... For example, if I try to test this with

assert(moving_hsysteresis(5.00,-2.36)==(float)2.64);

it works correctly. Somebody have any idea?

Testing for exact floating-point values is almost ALWAYS a bad idea. — Scott Hunter, Oct 08 '17 at 15:53
Thank you all of your help and comment! It is clear now for me! — Ádám Herczig, Oct 08 '17 at 16:57

score 2 · Answer 1 · answered Oct 08 '17 at 16:00

If you compile the following code:

#include <stdio.h>
int main() {
    float a=5.00;
    float b=5.01;
    float c=0.01;
    printf("%.25f",a-b+c);
}

The value should be mathematically zero, but floating-point errors mean that the result will not be precisely zero. On my system this prints out -0.0000002291053533554077148.

For this reason, testing floats for inequality typically checks to within a small elipson instead of testing for exact inequality.

score 0 · Accepted Answer · answered Oct 08 '17 at 16:00

0

Your first assertion works because the difference is just too small for the precision to work (Floating point precision is limited).

That's why your second assertion works.

answered Oct 08 '17 at 16:00

gsamaras

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Assertion in C with negative float

2 Answers2