C program to print all prime numbers from 1 to N.

Problem statement

This program will read the value of N and print all prime numbers from 1 to N.

Logic

The logic behind implement this program - Run loop from 1 to N and check each value in another loop, if the value is divisible by any number between 2 to num-1 (or less than equal to num/2) - Here num is the value to check it is prime of not.

Print all PRIME numbers from 1 to N using C program

/*C program to print all prime numbers from 1 to N.*/

#include <stdio.h>

int checkPrime(int num)
{
    int i;
    int flg = 0;
    /*if number (num) is divisble by any number from 2 to num/2
      number will not be prime.*/
    for (i = 2; i < (num - 1); i++) {
        if (num % i == 0) {
            flg = 1;
            break;
        }
    }
    if (flg)
        return 0;
    else
        return 1;
}

int main()
{
    int i, n;

    printf("Enter the value of N: ");
    scanf("%d", &n);

    printf("All prime numbers are from 1 to %d:\n", n);
    for (i = 1; i <= n; i++) {
        if (checkPrime(i))
            printf("%d,", i);
    }

    return 0;
}

Output

Enter the value of N: 100
All prime numbers are from 1 to 100:
1,2,3,5,7,11,13,17,19,23,29,31,37,41,
43,47,53,59,61,67,71,73,79,83,89,97,