Question

Please explain why the following pieces of code behave differently.

#include<stdio.h>
int main(){
 float a=0.1;
 if(a<0.1)
  printf("less");
 else 
  printf("greater than equal");
getchar();
}

Output:greater than equal

 #include<stdio.h>
 int main(){
 float a=0.7;
 if(a<0.7)
  printf("less");
 else 
  printf("greater than equal");
getchar();
}

Output:less contrary to what i expected.

PS: This is NOT homework.

Answer 1

You cannot use comparison operators on floating point numbers reliably.

A good way of comparing two floating point numbers is to have a accuracy threshold which is relative to the magnitude of the two floating point numbers being compared.

Something like:

#include < math.h >
if(fabs(a - b) <= accurary_threshold * fabs(a))

Good Read:

• What Every Computer Scientist Should Know About Floating-Point Arithmetic
• Comparing Floating Point Numbers