C Program to Find Whether a Number is Prime or Not

In this example, We Check inputted number is prime or not and print prime numbers between range

What is Prime Number?

A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself.

C Program to Check Whether a Number is Prime or Not Using Loop

	
#include<stdio.h>  
int main(){    
    int no,i,m=0,flag=0;    
    printf("Enter the number to check prime:");    
    scanf("%d",&no);    
    m=no/2;    
    for(i=2;i<=m;i++)    
    {    
        if(no%i==0)    
        {    
            printf("Number is not prime");    
            flag=1;    
            break;    
        }    
    }    
    if(flag==0)    
        printf("Number is prime");     
    return 0;  
}
	

Output :

	
Enter the number to check prime:87
Number is not prime
	

C Program to Check Whether a Number is Prime or Not Using User Define Function

	
#include <stdio.h>
int prime(int n);
int main()
{
    int num,res=0;
    printf("\nENTER A NUMBER: ");
    scanf("%d",&num);
    res=prime(num);
    if(res==0)
        printf("Number is prime");
    else
        printf("Number is not prime");
    return 0;
}
int prime(int n)
{
    int i;
    for(i=2;i<=n/2;i++)
    {
        if(n%i!=0)
            continue;
        else
            return 1;
    }
    return 0;
}
	

Output :

	
ENTER A NUMBER: 869
Number is not prime
	

C Program to Check Whether a Number is Prime or Not Using Recursion

	
#include <stdio.h>
int prime(int, int);
int main()
{
    int num, check;
    printf("Enter a number: ");
    scanf("%d", &num);
    check = prime(num, num / 2);
    if (check == 1)
    {
        printf("%d is a prime number\n", num);
    }
    else
    {
        printf("%d is not a prime number\n", num);
    }
    return 0;
}
 
int prime(int num, int i)
{
    if (i == 1)
    {
        return 1;
    }
    else
    {
       if (num % i == 0)
       {
         return 0;
       }
       else
       {
         return prime(num, i - 1);
       }       
    }
}
	

Output :

	
Enter a number: 29
29 is a prime number
	

C Program to Display Prime Numbers Between Two Intervals

	
#include <stdio.h>

int main() {
    int start, end, i, flag;
    printf("Enter Starting Point :: ");
    scanf("%d", &start);

    printf("Enter Ending Point :: ");
    scanf("%d", &end);

    printf("Prime numbers between %d and %d are: ", start, end);

    while (start < end) {
        flag = 0;

        if (start <= 1) {
            ++start;
            continue;
        }
        for (i = 2; i <= start / 2; ++i) {
            if (start % i == 0) {
            flag = 1;
            break;
            }
        }
        if (flag == 0)
            printf("%d ", start);
        ++start;
    }
    return 0;
}
	

Output :

	
Enter Starting Point :: 1
Enter Ending Point :: 50
Prime numbers between 1 and 50 are: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47