[C language]Checking whether an integer is a prime number

Definition of a prime number

A prime number is an integer, which is divided only by 1 and itself.

Algorithm 1: dividers between 2 and N-1 will be tested

/**************************\

 *  prime_number1.c     *
\**************************/

/* algorithm : test all the dividers */
#include <stdio.h>


int main (void)
{
  int i, nb, count, test;
  test = count = 0;
  printf ("enter integer: ");
  if (scanf ("%d", &nb) != 1)
    return -1;

  for (i = 2; i < nb; i++, count++)
    if (nb % i == 0)
      test = 1;
  if (!test)
    printf ("%d prime number, number of iterations = %d\n", nb, count);
  else
    printf ("%d is not a prime number, number of iterations = %d\n", nb,count);
  return 0;
}

Algorithm 2: Even dividers will not be tested, research is limited to odd dividers

/**************************\

 *    prime_number2.c   *
\**************************/

/* algorithm : exclude even numbers and

 *    test all the dividers */
#include <stdio.h>


int main (void)
{
  int i, nb, count, test;
  test = count = 0;
  printf ("enter integer: ");
  if (scanf ("%d", &nb) != 1)
    return -1;

  if (nb % 2 == 0)
          test = 1;
  else{
      for (i = 3 ; i < nb; i+=2, count++)
        if (nb % i == 0)
          test = 1;
  }
  if (!test)
          printf ("%d prime number, number of iterations = %d\n",
                          nb, count);
  else
          printf ("%d is not a prime number, number of iterations = %d\n",nb, count);
  return 0;
}

Algorithm 3: All odd dividers up to the square root of N will be tested

/**************************\

 *    prime_number3.c   *
\**************************/

/* algorithm : exclude all even numbers and

 * test all the dividers up to square root */
#include <stdio.h>
#include <math.h>

int main (void)
{
  int i, nb, count, test,limit;
  test = count = 0;
  printf ("enter integer : ");
  if (scanf ("%d", &nb) != 1)
    return -1;
  limit = sqrt(nb) + 1;

  if (nb % 2 == 0)
          test = 1;
  else{
      for (i = 3 ; i < limit; i+=2, count++)
        if (nb % i == 0)
          test = 1;
  }
  if (!test)
          printf ("%d prime number, number of iterations = %d\n", nb, count);
  else
          printf ("%d not a prime number, number of iterations = %d\n",nb, count);
  return 0;
}

Algorithme4: stop the program when a divider is found

/**************************\

 *    prime_number4.c   *
\**************************/

/* algorithm : exclude all even numbers and

 * test all dividers up to the square root
 * exit loop when first divider is found */
#include <stdio.h>
#include <math.h>

int main (void)
{
  int i, nb, count, test,limit;
  test = count = 0;
  printf ("Enter integer: ");
  if (scanf ("%d", &nb) != 1)
    return -1;
  limit = sqrt(nb) + 1;

  if (nb % 2 == 0)
          test = 1;
  else{
      for (i = 3 ; i < limit && ! test; i+=2, count++)
        if (nb % i == 0)
          test = 1;
  }
  if (!test)
          printf ("%d prime number, number of iterations = %d\n", nb, count);
  else
          printf ("%d not prime number, number of iterations = %d\n", nb, count);
  return 0;
}