C Program to Display Prime Numbers Between Two Intervals

Example to print all prime numbers between two numbers (entered by the user). This problem is solved using nested for loop and if...else statement.

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

Prime Numbers

Example #1: Display Prime Numbers Between two Intervals

#include <stdio.h>
int main()
{
    int n1, n2, i, flag;
    printf("Enter two numbers(intevals): ");
    scanf("%d %d", &n1, &n2);

    printf("Prime numbers between %d and %d are: ", n1, n2);

    while (n1 < n2)
    {
        flag=0;

        for(i=2; i<=n1/2; ++i)
        {
            if(n1%i == 0)
            {
                flag=1;
                break;
            }
        }

        if (flag == 0)
            printf("%d ",n1);

        ++n1;
    }

    return 0;
}

Output

Enter two numbers(intervals): 20 50 Prime numbers between 20 and 50 are: 23 29 31 37 41 43 47

In this program, the while loop is iterated (n2 - n1 - 1) times.

In each iteration, whether n1 is a prime number or not is checked and the value of n1 is incremented by 1 until n1 is equal to n2.

Visit this page to learn more on how to check whether a number is prime or not.

If the user enters larger number first, this program doesn't work as intended. You can solve this issue by swapping the numbers if the user enters larger number first.

Example #2: Display Prime Numbers Between two Intervals When Larger Number is Entered first

#include <stdio.h>
int main()
{
    int n1, n2, i, flag, temp;
    printf("Enter two numbers(intevals): ");
    scanf("%d %d", &n1, &n2);

    //swapping numbers if n1 is greater than n2
    if (n1 > n2) {
        temp = n1;
        n1 = n2;
        n2 = temp;
    }

    printf("Prime numbers between %d and %d are: ", n1, n2);

    while (n1 < n2)
    {
        flag=0;

        for(i=2; i<=n1/2; ++i)
        {
            if(n1%i == 0)
            {
                flag=1;
                break;
            }
        }

        if (flag == 0)
            printf("%d ",n1);

        ++n1;
    }

    return 0;
}

Visit this page to learn how you can display all prime numbers between two intervals by using user-defined function.