Question

How to check whether a number can be expressed as sum of two prime numbers in C

Answer 1

#include <stdio.h>
#include <math.h>
#include <stdbool.h>

int isPrime(int n) {
    if (n < 2 || (n % 2 == 0 && n != 2)) {
        return 0;
    }

    int count = floor(sqrt(n));
    for (int i = 3; i <= count; i += 2) {
        if (n % i == 0) {
            return 0;
        }
    }
    return 1;
}

int main(void) {
    int n = 38;
    bool has_primes = false;

    for (int i = 2; i <= n / 2; i++) {
        if (isPrime(i)) {
            if (isPrime(n - i)) {
                printf("%d = %d + %d\n", n, i, n - i);
                has_primes = true;
            }
        }
    }

    if (!has_primes)
        printf("%d cannot be expressed as a sum of two prime numbers", n);

    return 0;
}




/*
run:

38 = 7 + 31
38 = 19 + 19

*/

 

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