How do I do this?

**ldb88** · 07-24-2006

Is it possible (or practical) to do this without using exit() or goto? And do I need the default case, since there is only one case I am worried about? I have a version using if(), would it be more efficient to use that instead? Do I need the break after exit() since the program will have ended anyway?

Code:

#include<iostream>
using namespace std;

int main()
{
	int n;
	int i;
	printf("Please enter an integer\n");
	scanf("%d",&n);
	for(i=2;i<n;i++)
	{
		switch(n%i)
		{
		case 0:
			printf("%d is not a prime\n",n);
			exit(0);
			break;
		default:
			break;
		}
	}
	printf("%d is a prime\n",n);
	return 0;
}

Thanks.

**Mario F.** · 07-24-2006

Code:

int main()
{
	int n;
	int i;
	printf("Please enter an integer\n");
	scanf("%d",&n);
	for( i=2; i < n; ++i ) {
            if( n % i == 0 ) {
		printf("%d is not a prime\n",n);
                break;
            }
        }
	if( i == n )
            printf("%d is a prime\n",n);
	return 0;
}

Note: no error checking is done

**twomers** · 07-24-2006

Something like this ?

Code:

#include<iostream>
using namespace std;

int main()
{
	int n;
	int i;

	bool isprime = true;

	cout<< "Please enter an integer\n";
	cin >> d;

	for( i=2; i<n; i++ )	// I think it only needs be n/2 really. 
	{	// If a number is being divided by something that's over half its value, it can't be whole!

		if ( ( n%i ) == 0 )
		{
			isprime = false;
		}
	}

	switch ( isprime )
	{
		case true;
			cout<< d << " is a prime!";
			break;

		case false;
			cout<< d << "is not a prime number :(";
			break;
	}

	return 0;
}

I haven't compiled it, so I don't know if it works. Why did you use printf and scanf?

EDIT - so much for going to sleep Mario F.

**ldb88** · 07-24-2006

Thanks to both of you. Especially Mario F. - my if() version was exactly like that, except I didn't think to put an if() for the bottom printf. And I used printf and scanf because I thought of this while doing Thinking in C by Bruce Eckel and he uses a lot of printf and scanf in the lecture.

**Mario F.** · 07-24-2006

Originally Posted by twoners

EDIT - so much for going to sleep Mario F.

I am, dad!
Just finishing some stuff now that I finally had the whole thing working. It may not be much, but it really felt good to finally understand about memory management and have put it to work

**twomers** · 07-24-2006

You could always put in something to check if it's an even number so that you wouldn't have to go through the whole checking all the variable malarky

**Richie T** · 07-24-2006

There's a pretty big mix up in variables in twomers' code, change all occurances
of d to n, also in the switch cases, it should be

case true:

not

case true;

Other than those it works fine (assuming a well behaved user

)

EDIT: If you are reading "Thinking In C", why did you post in the C++ forum?
Make sure you know what language you are trying to learn!!!

**ldb88** · 07-24-2006

I'm used to C++, and it works in C and C++, so I didn't figure it would matter.

**Dave_Sinkula** · 07-24-2006

Putting your code into a function would be something else to consider.

Code:

#include <cstdio>
using namespace std;

bool foo(int n)
{
   for ( int i = 2; i < n; ++i )
   {
      if ( n % i == 0 )
      {
         return 0;
      }
   }
   return 1;
}

int main()
{
   for ( int n = 1; n < 20; ++n )
   {
      printf("%d is %sa prime\n", n, foo(n) ? "" : "not ");
   }
   return 0;
}

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