c-programs-basics
In the following program, we are going to find if the number entered by the user is a prime number or not. We can use different methods to do so.
What is a prime number?
A Prime Number is the one which can be evenly divided only by 1 and Itself.

C Program To Find Prime Number or Not

#include<stdio.h>
#include<conio.h>
void main()
{

 int n,i;
 clrscr();

 printf("\n Enter Any Number : ");
 scanf("%d",&n);

 for(i=2;i<=n-1;i++)
   {
     if(n%i==0)
     {
       break;
     }
   }

 if(n==i)
    printf("\n Prime Number");

 else
    printf("\n Not Prime Number");


  getch();

 }

Output :

Output-Prime-Number-or-not

Output-Given-Number-Is-Prime-Or-Not







In the above program what we do is, 
  • we get a number from the user.
  • Start a loop which divides the entered number by 2,3,4... and so on until the number one less than the entered number.(since any number is completely divisible by 1 we start the loop from 2)
  • Here is the tricky part.
    If the entered number isn't completely divisible by any of the number(which suggest the number is prime), that means the loop will run until the number one less than the entered number and value of 'i' will be equal to the entered number in the end. i.e(n==i) 
  • Otherwise the number will be not prime.
However, this method is not so efficient when finding any bigger number such as 500 or 1000. As in that case, the loop has to run 499 and 999 times respectively. 
One way mentioned in the video below can be more effective to find the given number is a prime number or not.


C Program To Find The Prime Number Or Not Using The Function

#include<stdio.h>
#include<conio.h>

int prime(int m);

void main()
{

 int n;
 clrscr();

 printf("\n Enter Any Number : ");
 scanf("%d",&n);

 prime(n);

 getch();

 }

 int prime(int m)
 {
  int i,temp;
  for(i=2;i<=m/2;i++)
   {
     if(m%i==0)
     {
       temp=0;
       break;
     }
     else
      temp=1;
   }

 if(temp)
    printf("\n Prime Number");

 else
    printf("\n Not Prime Number");

   return 0;
 }
In this way we, we can find a number is prime or not using this simple method.