Prime number

A prime number is an integer greater than 1 whose only factors are 1 and itself. A factor is an integer that can be divided evenly into another number.

Logic

To print all the prime numbers up to N, we start one loop from 2 to N and then inside the loop we check current number or “num” is prime or not. To check if it is prime or not we again need one nested loop. It is not an efficient way to check prime number but it is simpler to understand the basic of looping in C.

See also: Check whether a number is prime number or not

Program

#include <stdio.h>

int main() {

    int num, i, upto;

    // Take input from user
    printf("Find prime numbers upto : ");
    scanf("%d", &upto);

    printf("\nAll prime numbers upto %d are : \n", upto);

    for(num = 2; num <= upto; num++) {

        for(i = 2; i <= (num / 2); i++) {

            if(num % i == 0) {
                i = num;
                break;
            }
        }

        // If the number is prime then print it.
        if(i != num) {
            printf("%d ", num);
        }
    }

    return 0;
}

Output

Find prime numbers upto : 100
All prime numbers upto 100 are :
3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97