how for loop works in this program ?

Question

3.33/5 (3 votes)

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i want some explanation of this program and how it works.

C#

#include <stdio.h>

int main(void)
{
  int num, i, is_prime;

  printf("Enter a number: ");
  scanf("%d", &num);


  is_prime = 1;

  for(i = 2; i <= num / 2; i = i + 1)
    if((num%i)==0)
         is_prime = 0;

  if(is_prime==1)
      printf(" is prime.");
  else
      printf("not prime.");

  return 0;
}

Posted 22-Oct-12 1:30am

rhmoony

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Stefan_Lang 22-Oct-12 10:33am

If the person who downvoted this question reads this: everyone has to start somewhere! Just because it is trivial to a seasoned programmer, is no reason to downvote this question.

Also: if there is a valid reason for downvoting, at least post a comment and point it out! Otherwise the vote is utterly pointless.

P.S. to the author: Your question wasn't particularly specific, and that may have been the reason why someone voted it down. In any case, now that you have some solutions, please accept them as solution if they answer your question, or enter a comment if you have further questions.

2 solutions

Solution 1

It's basically a simple trial division prime finder. See here: http://en.wikipedia.org/wiki/Trial_division[^] which explains it very well.

Posted 22-Oct-12 1:35am

OriginalGriff

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psychic6000 · Accepted Answer · 2012-10-22T03:52:00

C#

int num, i, is_prime; // declare three variables, num for user input, i for loop and is_prime used as boolean for the result

//get number from user
 printf("Enter a number: ");
 scanf("%d", &num);

//set is_prime to true
 is_prime = 1;

 for(i = 2; i <= num / 2; i = i + 1) //start from 2 and reach until the half the number
   if((num%i)==0)// check if any of the number in for loop range successfully divides the number, if so set is_prime to false, as dividable no are not prime
        is_prime = 0;

 if(is_prime==1) // check if Is_prime was never changed in loop, if s
     printf(" is prime.");
 else // if the is_prime was changed
     printf("not prime.");

most of time we use

C#

for(i = 2; i <= num / 2; i = i + 1)
    if((num%i)==0){
         is_prime = 0;
         break; }

this stops the for loop as soon as the no is found not to be prime, save remaining iterations of loop