# C Program to Check Prime Number using For loop, Recursion and Function

C program for prime number : The following program checks and finds the prime numbers from 1 to 100. It also checks prime numbers using function and recurison in C.

## Algorithm to check prime number:

Any number is divisible by other numbers which are less than or equal to half that number. For example, number 14 is divisible by numbers which are less equal 7 i.e 1 and 7. In general a number `n` is divisible by numbers less than or equal to `n/2`. So we can use this concept to check whether a given number is prime or not.

## C program to check prime number

``````/* C program to check whether a number is prime using for loop and if-else*/

#include<stdio.h>
#include<stdlib.h>

int main()
{
int n,i,flag=0;

printf("\n Enter any number to check whether it is prime or not:- ");
scanf("%d",&n);

if (n<2 0="" 1="" 29="" a="" above="" any="" break="" cases:="" check="" code="" d="" else="" enter="" exit="" flag="=0)" for="" i="=0)" if="" is="" it="" n="" not:-="" not="" number="" of="" or="" output="" prime="" printf="" return="" runtime="" test="" to="" whether="" x9=""``````
factorial program in C

Now we will write a C program to find prime numbers between two intervals, that is between a given range. For example prime numbers between 1 to n, prime numbers between 1 to 100 or 1 to 500 or 1000.

## C program to find prime numbers between 1 to 100 (a range)

``````/* C program to find prime numbers between a range */

#include<stdio.h>

int main()
{
int lower_limit,n,upper_limit,i,prime;

printf("\n \n Enter the range x and y : ");
scanf("%d%d",&lower_limit,&upper_limit);

for(n=(lower_limit+1);n<upper_limit;n++)
{
prime=1;

for(i=2;i<=n/2;++i)
{
if (n%i==0)
{
prime=0;
break;
}
}

if (prime==1)
printf("%d ",n);
}

return 0;
}

/* Output of above code:-

Enter the range x and y : 1 100

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 */``````

## C program to check prime number using function

``````/* C program to check prime number using function */

#include<stdio.h>
#include<stdlib.h>

void isPrime(int n); // isPrime Function Prototype

int main()
{
int n;

printf("\n Enter any value:- ");
scanf("%d",&n);

isPrime(n);
}

// isPrime Function
void isPrime(int n)
{
int i,LED=0;

if(n<2 0="" 2="" above="" any="" break="" code:-="" code="" d="" else="" enter="" exit="" for="" i="=0)" if="" is="" n="" not="" number="" of="" output="" prime="" printf="" return="" value:-="" x9:printf="" x9=""``````

## C program to check prime number using recursion

``````/* C Program to find whether a Number is Prime or Not using Recursion */

#include<stdio.h>

int prime_number(int, int);

int main()
{

int num, check;

printf("Enter a number: ");
scanf("%d", &num);

check = prime_number(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_number(int num, int i)
{
if (i == 1)
{
return 1;
}
else
{
if (num % i == 0)
{
return 0;
}
else
{
return primeno(num, i - 1);
}
}
}

/* Output of above code / Runtime Cases :-

Enter a number: 789
789 is not a prime number

Enter a number: 751
751 is a prime number

*/``````