//100% c prog to check whether given interger>1 is prime or not

#include <stdio.h>
void main()
{
  int numb,i=2;  
 printf("\nenter a int>1 that has to be checked for prime or not:");
 scanf("%d",&numb);
 while(i<=numb/2)
 {
     if(numb%i==0)
     {
         printf("\n given number %d is not prime",numb);
         break;
     }
     i++;
 }
 if(i==(numb/2)+1)
    printf("\n given number %d is prime",numb);
} //end of main 

ఔట్పుట్  తెర  మీద ఇలా..

enter a int>1 that has to be checked for prime or not:19
given number 19 is prime

enter a int>1 that has to be checked for prime or not:15
given number 15 is prime

enter a int>1 that has to be checked for prime or not:2
given number 2 is prime

enter a int>1 that has to be checked for prime or not:4
given number 4 is not prime