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Modulus Operator

Take a simple arithmetic problem: what's left over when you divide 11 by 3? The answer is easy to compute: divide 11 by 3 and take the remainder: 2. But how would you compute this in a programming language like C or C++? It's not hard to come up with a formula, but the language provides a built-in mechanism, the modulus operator ('%'), that computes the remainder that results from performing integer division.

The modulus operator is useful in a variety of circumstances. It is commonly used to take a randomly generated number and reduce that number to a random number on a smaller range, and it can also quickly tell you if one number is a factor of another. 

If you wanted to know if a number was odd or even, you could use modulus to quickly tell you by asking for the remainder of the number when divided by 2.

#include <iostream>

using namespace std;

int main()
    int num;
    cin >> num;
    // num % 2 computes the remainder when num is divided by 2
    if ( num % 2 == 0 )
        cout << num << " is even ";

    return 0;

The key line is the one that performs the modulus operation: "num % 2 == 0". A number is even if and only if it is divisible by two, and a number is divisible by another only if there is no remainder. 


"Tutorial from by Alexander Allain. serves programming audiences with tutorials and articles on programming, along with free source code and an active message board." 

Last edited Sep 21, 2015 at 6:17 PM by vineetchoudhary, version 2