Seventh Week Examples:
  Testing for Prime Numbers.
  Factoring

Following D. Milicic Eighth Week Examples.

Checking primality

The first program is inefficient. 
/**********************************************************************
A. Treibergs                                                      2-9-6

Silly prime checking

prime1.c
**********************************************************************/

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

int 
main(void) 
{

     int n, p;

     printf("Prime Checking\n\n Input an integer bigger than 1 : ");
     scanf("%d",&n);

      p = 2;
    
     while(p < n) 
     {
          if (n % p == 0) 
          {
             break;
          }
          else
             p = p+1; 
     }
    if (p == n)
         printf(" %d is prime!\n",n);
    else
         printf(" %d is not prime!\n",n);

    return EXIT_SUCCESS;
 
}

Checking primality 2

A better program. The loop continues to the square root of the number only, this time. 
/**********************************************************************
A. Treibergs                                                      2-9-6

Better prime checking

prime2.c
**********************************************************************/

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

int 
main ( void ) 
{
     int n, p, flag;

     printf ( "Input an integer bigger than 1 : ");
     scanf ( "%d", &n );

     flag = 0;
     p = 2;
     
     while ( p*p <= n ) 
     {
          if (n % p == 0) 
          {
             flag = 1;
             break;
          }
          else
             p = p+1; 
    }

    if (flag == 0)
         printf("%d is prime!\n",n);
    else
         printf("%d is not prime!\n",n);

    return EXIT_SUCCESS;
}

Factorization 

/**********************************************************************
A. Treibergs                                                      2-9-6

factoring a number

factor.c
**********************************************************************/

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

int 
factor ( int n )
{
   int p;
   p=2;
   
   while ( p*p <= n ) 
   {
	if ( n % p == 0) 
	      return(p);

	 p=p+1;
    }
         
    return(n);
}

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
   
int 
main(void) 
{
   int n, fact;

   printf ( "Input a number : " );
   scanf ( "%d", &n );

   printf("Prime factors of %d are: ",n);
	
   do 
   {
         fact = factor(n);
	 printf("%d  ",fact);
	 n = n/fact;
   } 
   while(n != 1);

   printf("\n");

   return EXIT_SUCCESS;
}