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How to use the ratio_subtract() function in C++

Harsh Jain

In this shot, we will learn how to use the ratio_subtract() function. This template alias is used to subtract two ratios.

The ratio_subtract() function is available in the <ratio> header file in C++.

What is a ratio?

A ratio is a representation of a fraction in which the numerator and denominator are differentiated by a colon (:) symbol. The numerator and denominator can be of float data type.

Let’s understand with the help of an example. Suppose that the first ratio is 1 : 3 and the second ratio is 5 : 2. So, the result of the ratio_subtract() function on these two ratios gives -13 : 6. Let’s explore how the addition took place.

  • First, we need to find the LCMLowest Common Multiple of the denominators of the two fractions which will be the denominator of the subtraction.

  • The LCM of 3 and 2 is 6.

  • Now subtraction = (1253)6\frac{(1*2-5*3)}{6} = 136\frac{-13}{6}.


The ratio_subtract() method takes the following parameters:

  • Ratio1: A ratio object for addition.
  • Ratio2: Another ratio object to perform the addition with the first ratio object.

Return value

This function returns the result of the subtraction in the simplest form. It returns two member constants:

  • num: The simplified numerator of the ratio after the subtraction of the two ratios.
  • den: The simplified denominator of the ratio after the subtraction of the two ratios.


Let’s have a look at the code.

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

int main()
  typedef ratio<1, 3> ratio1;
  typedef ratio<5, 2> ratio2;

  typedef ratio_subtract< ratio1, ratio2 > diff;
  cout << "The ratio after addition is : ";
  cout << diff::num << "/" << diff::den;
  return 0;
Use ratio_subtract() function in C++


  • In lines 1 and 2, we import the required header files.

  • In line 5, we make a main() function.

  • From lines 7 and 8, we declare two ratios.

  • In line 10, we perform the subtraction of the two declared ratios by using the ratio_subtract() function.

  • In line 12, we display a message regarding the result.

  • In line 13, we display the numerator and denominator by accessing them through the sum variable and display the result.

In this way, we can use the ratio_subtract() function to add two ratios and get the subtraction in the simplest form.



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