How to compare float and double in C++?

In C++, floating-point numbers are used to represent decimal values. The most commonly used floating-point types are float and double. These data types differ in their size, precision, and use cases. Understanding how to use and compare them correctly is important for accurate computations.

Understanding Float and Double Precision

The float is a 32-bit single-precision floating-point type that can store approximately 7 decimal digits, while double is a 64-bit double-precision type that offers about 15 to 16 digits of precision. Due to these differences, double is generally preferred for higher accuracy in scientific and financial calculations.

Example: Printing Float and Double Values

This example demonstrates the difference in precision when storing the same decimal number using float and double:

#include<iostream>
using namespace std;

int main() {
   float a = 3.1415926535f;    // 'f' suffix denotes float
   double b = 3.1415926535;

   cout << "Float:  " << a << endl;
   cout << "Double: " << b << endl;

   return 0;
}

When you run the above code, the output will be:

Float:  3.14159
Double: 3.1415926535

Comparing Float and Double Using EPSILON

Floating-point numbers cannot always be compared reliably using the == operator due to rounding errors and precision limitations. A better approach is to use a small threshold value known as EPSILON to check if the absolute difference between two numbers is very small - indicating they are "close enough."

Example: Using EPSILON for Safe Comparison

The following example demonstrates how to compare float and double values using EPSILON:

#include<iostream>
#include<cmath>
using namespace std;

#define EPSILON 0.000001f 

bool areSameFloat(float a, float b) {
   return fabs(a - b) < EPSILON;
}

bool areSameDouble(double a, double b) {
   return fabs(a - b) < EPSILON;
}

int main() {
   float fa = 1.005f;
   float fb = 1.006f;

   cout << "Float comparison:" << endl;
   cout << "fa vs fa: " << areSameFloat(fa, fa) << endl;
   cout << "fa vs fb: " << areSameFloat(fa, fb) << endl;

   double da = 1.005;
   double db = 1.006;

   cout << "\nDouble comparison:" << endl;
   cout << "da vs da: " << areSameDouble(da, da) << endl;
   cout << "da vs db: " << areSameDouble(da, db) << endl;

   return 0;
}

When you run the above code, the output will be:

Float comparison:
fa vs fa: 1
fa vs fb: 0

Double comparison:
da vs da: 1
da vs db: 0

Float vs Double in C++: Feature Comparison Table

The table below compares key aspects of float and double in C++:

Conclusion

Choosing between float and double in C++ depends on your needs for precision and memory usage. While float is faster and uses less memory, double is more accurate and better suited for complex numerical computations.