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 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
} |
gcd(6, 20) = 2
Time Complexity: O(logn)
Auxiliary Space: O(1)
Program to Find the gcd of all numbers in a vector.
#include<bits/stdc++.h> #include <iostream> #include <algorithm> using namespace std;
int main()
{ vector< int > numbers = { 12, 15, 18, 21, 24 };
int ans =__gcd(numbers[0], numbers[1]);
for ( int i = 2; i < numbers.size(); i++)
{
ans = __gcd(ans, numbers[i]);
}
cout << "The GCD of the numbers in the vector is: " <<ans<<endl;
return 0;
} |
The GCD of the numbers in the vector is: 3
Time Complexity: O(k*logn)
Auxiliary Space: O(k)
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 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.