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std::gcd | C++ inbuilt function for finding GCD

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  • Difficulty Level : Basic
  • Last Updated : 20 Oct, 2022
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In many competitive programming problems, we need to find greatest common divisor also known as gcd. Euclids algorithm to find gcd has been discussed here.
C++ has the built-in function for calculating GCD. This function is present in header file. 
Syntax for C++14 :

 Library: 'algorithm'
 __gcd(m, n) 
Parameter :  m, n
Return Value :  0 if both m and n are zero, 
else gcd of m and n.

Syntax for C++17 :

Library: 'numeric'
gcd(m, n)
Parameter :  m, n
Return Value :  0 if both m and n are zero,
else gcd of m and n.

CPP




// CPP program to illustrate
// gcd function of C++ STL
#include <iostream>
#include <algorithm>
// #include<numeric> for C++17
 
using namespace std;
 
int main()
{
    cout << "gcd(6, 20) = " << __gcd(6, 20) << endl; // gcd(6,20) for C++17
}

Output

gcd(6, 20) = 2

Time Complexity: O(logn)
Auxiliary Space: O(1)

Note: If either M or N is not an integer type, or if either is (possibly cv-qualified) bool, the program is ill-formed. Also, If either |m| or |n| is not representable as a value of type std::common_type_t, the behavior is undefined. 
 

CPP




// CPP program to illustrate
// undefined behavior of
// gcd function of C++ STL
#include <iostream>
#include <algorithm>
// #include<numeric> for C++17
 
using namespace std;
 
int main()
{
    cout << "gcd(2.0, 8) = " << __gcd(2.0, 8) << endl; // gcd(2.0,8) for C++17
}

Output: 
Error, As the data type float is not supported by std::gcd.

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