# Common Prime Factors of a Two Number using C++

Suppose we are given two integers, our task is to find the common prime factors of those two numbers.

A basic method two achieve this comprises of two steps. Firstly, by finding the common factors between the two given numbers and then, checking each of these common factors for which of them are prime using C++ program.

An effective and optimized approach would be to:

- First, find the Greatest Common Divisor(GCD) of the two numbers.
- Then, we find the prime factors of the GCD of the two numbers.

We have to take two integer inputs from the user and then print the output for them.

For example:

- Input: 10, 20

Output: 2, 5

- Input: 34, 12

Output: 2

- Input: 6, 12

Output: 2, 3

## Algorithm for finding the Common Prime Factors of two numbers

- Take input of two numbers, m and n, from the user.
- Make a function, Sieve_Of_Eratosthenes(), which would create a boolean array and initializing all it’s values as “true”.
- After this, we will change all composite prime[i] values to “false”, using a loop.
- Create another function and in that, store the Greatest Common Divisor(GCD) of the two numbers in a temporary variable.
- Inside the same function start a loop from 2 till less than or equal to GCD.
- Inside the loop put an if condition to check if prime[i] && GCD%i = 0 and if this is true then print that i.
- All the values of i that have been printed are the Common Prime Factors of the two numbers.

## C++ code to find the Common Prime Factors of two numbers

#include<bits/stdc++.h> using namespace std; #define MAX_SIZE 1000 bool prime[MAX_SIZE]; void Sieve_Of_Eratosthenes() { //creating a boolean array and initializing all values as "true" memset(prime, true, sizeof(prime)); //now we will change all composite prime[i] values to "false" //as 0 and 1 are not primes prime[0]=false; prime[1]=false; //loop for the rest of the values for(int j=2 ; j*j<=MAX_SIZE ; j++) { if(prime[j] == true) { //changing the values of all composites to "false" for(int i= j*j ; i<=MAX_SIZE ; i += j) prime[i]=false; } } } //function to find the common prime factors void Common_Prime_Factors(int m, int n) { //variables used int i, GCD; //using library function to get GCD of two numbers GCD = __gcd(m, n); //loop to find prime factors of GCD for(i=2 ; i<=GCD ; i++) { // If i is prime and a divisor of gcd if(prime[i] && GCD%i == 0) { cout<<i<<endl; } } } //int main/driver code int main() { //variables used int m, n; //Taking the input from the user cout<<"Enter two numbers to find their common prime factors: "; cin>>m>>n; //calling function to create the Sieve of Eratosthenes Sieve_Of_Eratosthenes(); cout<<"\nThe common prime factors of "<<m<<" and "<<n<<" are:"<<endl; //calling the function to find the common prime factors Common_Prime_Factors(m, n); return 0; }

**Output**

Enter two numbers to find their common prime factors: 22 44 The common prime factors of 22 and 44 are: 2 11

## Note

- This solution only takes in account only two numbers. We can expand the code to calculate the common prime factors for more numbers as well.
- An optimised approach for the same would be to use the idea of Prime Factorization using Sieve, for multiple inputs.
- In case of no Common Prime Factors of the two numbers, we can also add an cout statement for the same. This case is not included right now.

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