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In this example, you will learn to print all prime numbers between two numbers (entered by the user).

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To understand this example, you should have the knowledge of the following C programming topics:

• C for Loop
• C break and continue
• C Functions
• C User-defined functions

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To find all the prime numbers between the two integers, checkPrimeNumber() is created. This function checks whether a number is prime or not.

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Prime Numbers Between Two Integers

#include 
int checkPrimeNumber(int n);
int main() {
    int n1, n2, i, flag;
    printf("Enter two positive integers: ");
    scanf("%d %d", &n1, &n2);
    printf("Prime numbers between %d and %d are: ", n1, n2);
    for (i = n1 + 1; i < n2; ++i) {

        // flag will be equal to 1 if i is prime
        flag = checkPrimeNumber(i);

        if (flag == 1)
            printf("%d ", i);
    }
    return 0;
}

// user-defined function to check prime number
int checkPrimeNumber(int n) {
    int j, flag = 1;
    for (j = 2; j <= n / 2; ++j) {
        if (n % j == 0) {
            flag = 0;
            break;
        }
    }
    return flag;
}

Output

Enter two positive integers: 12
30
Prime numbers between 12 and 30 are: 13 17 19 23 29

If the user enters the larger number first, this program will not work as intended. To solve this issue, you need to swap the numbers first.