What is the most effective way for float and double comparison in C/C++?

Anvi Jain

Answered 17h 20m ago

Here we will see how to compare two floating point data or two double data using C or C++. The floating point / double comparison is not similar to the integer comparison.

To compare two floating point or double values, we have to consider the precision in to the comparison. For example, if two numbers are 3.1428 and 3.1415, then they are same up to the precision 0.01, but after that, like 0.001 they are not same.

To compare using this criteria, we will find the absolute value after subtracting one floating point number from another, then check whether the result is lesser than the precision value or not. By this we can decide that they are equivalent or not.

Example

#include <iostream>
#include <cmath>
using namespace std;
bool compare_float(float x, float y, float epsilon = 0.01f){
   if(fabs(x - y) < epsilon)
      return true; //they are same
      return false; //they are not same
}
bool compare_float(double x, double y, double epsilon = 0.0000001f){
   if(fabs(x - y) < epsilon)
      return true; //they are same
      return false; //they are not same
}
int main() {
   float x, y;
   x = 22.0f/7.0f;
   y = 3.1415f;
   if(compare_float(x, y)){
      cout << "They are equivalent" << endl;
   } else {
      cout << "They are not equivalent" << endl;
   }
   if(compare_float(x, y, 0.001f)){
      cout << "They are equivalent" << endl;
   } else {
      cout << "They are not equivalent" << endl;
   }
   double a, b;
   a = 2.03547415;
   b = 2.03547428;
   if(compare_float(a, b)){
      cout << "They are equivalent" << endl;
   } else {
      cout << "They are not equivalent" << endl;
   }
   if(compare_float(a, b, 0.000001f)){
      cout << "They are equivalent" << endl;
   } else {
      cout << "They are not equivalent" << endl;
   }
}

Output

They are equivalent
They are not equivalent
They are not equivalent
They are equivalent