Thread: Help me understand.

Code:

// This is the main project file for VC++ application project
// generated using an Application Wizard.

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

#using <mscorlib.dll>

using namespace System;
using namespace std;

bool checkprime(int, int);

int k=3;


int _tmain()
{
       int n;
	  
       cout << "Input Number: "; cin >> n;
       
       

	   if(checkprime(n,k))
		   cout << "Yes"<<endl;
	   else 
		   cout << "no" <<endl;
	

		  

return 0;

}


bool checkprime(int nv, int kv) {
	
	
	if (nv%2 ==0)
		return false;
	
	if (nv %k ==0){
		return false;
		checkprime(nv, k+1);
	}
	else
		return true;
}

I wrote this program using recursion. I know what I did for the most part, but I am not clear on when it returns true. The program runs alright and does what its supposed to do. But I didnt set any boundary on k so I am wondering when it breaks out and returns true. If k didnt break then the program wouldnt display the correct answer. Does it break when k =nv? If so, why? Thanks for the help.

Apart from the points made above, you should also double check your programs logic. If someone enters a prime number bigger than 2 (eg. 3) it will return false. In fact, this function (without the errors pointed out already) will return false for every number entered, because eventually k will become the same value as nv, and nv % k == nv % nv == 0, so you need to make sure that you stop looping/recursing when hit the number that is one below nv.

As a side note, you don't need to check for nv % 2 every time you call the function, this is one of the inefficiencies that I'm sure CornedBee was referring to.

A small suggestion, instead of starting from k = 3 and go upwards, why not start at k = n - 1 and stop when you get to 2?

And as a big suggestion, do some reading up on the Sieve of Eratosthenes, it might just help you find a faster solution