C语言练习：打印出一个数的所有两个素数之和

在此示例中，您将学习检查用户输入的整数是否可以表示为所有可能组合的两个素数之和。

• C if…else Statement
• C for Loop
• C Functions
• C User-defined functions

#include <stdio.h>
int checkPrime(int n);
int main() {
  int n, i, flag = 0;
  printf("Enter a positive integer: ");
  scanf("%d", &n);

  for (i = 2; i <= n / 2; ++i) {
    // condition for i to be a prime number
    if (checkPrime(i) == 1) {
      // condition for n-i to be a prime number
      if (checkPrime(n - i) == 1) {
        printf("%d = %d + %d\n", n, i, n - i);
        flag = 1;
      }
    }
  }

  if (flag == 0)
    printf("%d cannot be expressed as the sum of two prime numbers.", n);

  return 0;
}

// function to check prime number
int checkPrime(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;
}

结果：

Enter a positive integer: 34
34 = 3 + 31
34 = 5 + 29
34 = 11 + 23
34 = 17 + 17