# C Program to Check Whether a Number Can Be Express as Sum of Two Prime Numbers

• Last Updated : 26 Jul, 2022

Here we will see whether a number can be expressed as the sum of two prime numbers using a C program. Below are the examples:

Input: 7
Output: Yes
Explanation: 7 can be expressed as sum of 2 and 5 which are prime

Input: 11
Output: No
Explanation: There are no two prime numbers such that their sum is 11

Approach: The idea is to loop from 2 to N  and check if i and N-i are prime

Below is the C program to check whether a number can be expressed as the sum of two prime numbers:

## C

 `// C program to check whether a``// number can be expressed as sum``// of two prime numbers``#include ` `// Function to check prime number``int` `isPrime(``int` `n)``{``    ``int` `i, isPrime = 1;` `    ``// 0 and 1 are not prime numbers``    ``if` `(n == 0 || n == 1)``    ``{``        ``isPrime = 0;``    ``}``    ``else``    ``{``        ``for` `(i = 2; i <= n / 2; ++i)``        ``{``            ``if` `(n % i == 0)``            ``{``                ``isPrime = 0;``                ``break``;``            ``}``        ``}``    ``}` `    ``return` `isPrime;``}` `// Driver code``int` `main()``{``    ``int` `n = 7, i, flag = 0;` `    ``for` `(i = 2; i <= n / 2; ++i)``    ``{``        ``// condition for i to be a``        ``// prime number``        ``if` `(isPrime(i) == 1)``        ``{``            ``// condition for n-i to``            ``// be a prime number``            ``if` `(isPrime(n - i) == 1)``            ``{``                ``printf``(``"Yes\n"``);``                ``return` `0;``            ``}``        ``}``    ``}` `    ``printf``(``"No\n"``);``    ``return` `0;``}`

Output

`Yes`

Time Complexity: O(N2)
Auxiliary Space: O(1)

My Personal Notes arrow_drop_up