A program in the C language can be used to check if an integer is a prime number. Some knowledge of programming concepts and languages like C is required to write a program code in C. Basic concepts such as looping, including for loops, while do loops, if else loops, functions, etc, are necessary to write programs. A program to check if an integer is a prime number in C can be written by using nested for loops. Nested for loops contain one for loop inside another. Some knowledge of C functions such as scanf and printf will also prove to be helpful when programming in C.

[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: divisors 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 = %dn", nb, count);    
  else    
    printf ("%d is not a prime number, number of iterations = %dn", nb,count);    
  return 0;    
}    

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

/**************************    

    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 = %dn",    
                          nb, count);    
  else    
          printf ("%d is not a prime number, number of iterations = %dn",nb, count);    
  return 0;    
}    

Algorithm 3: All odd divisors 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 = %dn", nb, count);    
  else    
          printf ("%d not a prime number, number of iterations = %dn",nb, count);    
  return 0;    
}    

Algorithm 4: 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 = %dn", nb, count);    
  else    
          printf ("%d not prime number, number of iterations = %dn", nb, count);    
  return 0;    
}    

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