prime numbers

[ft95]

this program should display the prime numbers between 1 and 1000
I don't understand what is the problem.

#include "stdafx.h"
#include<iostream>
#include<cmath>

using namespace std;

void prime(int);

int _tmain(int argc, _TCHAR* argv[])
{ 
	int number;

	for(number=2;number<=1000;number++)
	{
		prime(number);
	}

	return 0;
}

void prime(int number)
{
	
	for(int counter=2;counter<=(number/2);counter++);
	{
		if(number%counter!=0)
		{
			if(counter==(number/2))
			{
				cout<<number<<",";
			}
		}
		
	}

}

[Chervil]

This line has a semicolon which should be removed:
for(int counter=2;counter<=(number/2);counter++);

The logic is also faulty. As written, it outputs a list of the odd numbers.

[ft95]

How can I fix it?

[Chervil]

The test you have where you test the remainder is "not equal to zero" doesn't work. The usual problem is that you need every remainder to be non-zero. In your code, you filter the result so you are not checking all of them, only the case where counter = n/2, in other words you verify that it isn't divisible by 2.


You need to check that number cannot be exactly divided by any integer. So, if the remainder number%counter is equal to zero, the number is not prime. You could test that condition, if it's true, exit (return) from the function.

When the for-loop makes it all the way through to the end, you will know the number is prime. So put the cout statement after the end of the for loop.

[ft95]

thank you so much...