How to stop the loop after printing one?

user_16100119 0
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So here is the problem: Write a program that accept an integer n, print out the largest number but smaller or equal n that is the product of two consecutive even number. Example: Input: 12, Output: 8 ( 2x4 ) Here is my code :

#include <stdio.h>

int main()
{
  int n;
  scanf("%d", &n);
  for (int i = n; i >= 0; i--)
      {
        for (int j = 0; j <= n; j = j + 2)
            {
              if ( i == j * (j+2) )
                  {
                    printf("%d ", i);
                    break;
                  }
            }
      }
return 0;
}

So if i input 20, it will print out 8 and 0 instead of 8, if i input 30, it will print out 24,8 and 0 instead of just 24. How do i make it stop after printing out the first number that appropriate ?

c
3 Answers

You need to stop an outer loop from processing, for example by using a boolean flag (meaning "solution found, we finish work") or a goto statement.

#include <stdio.h>

int main() {
  int n;
  scanf("%d", &n);
  int solutionFound = 0;
  for (int i = n; i >= 0; i--) {
      // this could also be put into for's condition i.e. "i >= 0 && !solutionFound"
      if (solutionFound) {
          break;
      }
      for (int j = 0; j <= n; j = j + 2) {
          if ( i == j * (j+2) ) {
              printf("%d ", i);
              solutionFound = 1;
              break;
          }
      }
  }
  return 0;
}

EDIT: immediate return as noted in the comments is also a nice idea, if you don't need to do anything later.

user_62046120

Break only breaks you out of immediate loop, so either use flags or just use return to terminate the execution. Or you can even use following code:

#include <stdio.h>

int main()
{
  int n;
  scanf("%d", &n);

        for (int j = 0; j <= n; j = j + 2)
            { 
              if ( n < j * (j+2) )
                  {
                    printf("%d ", j*(j-2));
                    break;
                  }
            }
return 0;
}
user_153873410

Your problem is that you are nested - in a for loop which is inside another for loop - when you want to stop processing.

Some languages would let you code break 2; to indicate that you want to break out of 2 loops. Alas, C i snot such a language.

I would recommend that you code a function. That would serve a few porpoises: 1) your main should be "lean & mean" 2) as your programs get larger, you will learn the benefits of putting individual coding tasks into functions 3) you can use return; instead of break; and it will exit the function immediately.

Something like this:

#include <stdio.h>

void FindNeighbouringDivisors(int n)
{
    for (int i = n; i >= 0; i--)
    {
        for (int j = 0; j <= n; j = j + 2)
        {
        if ( i == j * (j+2) )
          {
            printf("%d times %d = %d", j, j + 2, i);
            return;
          }
        }
    }
    
    printf("There are no two adjacent even numbers which can be multiplied to give %d", n);
}

int main()
{
  int n;
  scanf("%d", &n);      /* could get from comamnd line */
  
  FindNeighbouringDivisors(n);
  
  return 0;    /* should be EXIT_SUCCESS */
}

Btw, when you have a problem with your code, ask a question here. When you have it working, consider posting it at our code review site where more experienced programmers can give you advice on how to improve it. It's a great way to learn

user_1929100
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