Whats wrong with my Prime number Checker? - c++

Question

I created a prime number checking program which checks the user entered number prime or not.
It detects non prime numbers easily, but when we type prime numbers, it crashes!
I think I know why, but don't know how to rectify them...
Here's my Program:
#include "stdafx.h"
#include <iostream>
#include<iomanip>
#include <cmath>
using namespace std;
float Asker()
{
float n;
cin >> n;
return n;
}
int Remainder(int n, int x)
{
int q = n%x;
if (q == 0)
return 1;
else
Remainder(n, x + 1 > n);
/*
Here is the PROBLEM
*/
return 0;
}
int main()
{
cout << "Enter your Number : ";
float n = Asker();
int r = Remainder(n, 2);
if (r == 1)
cout << "That Ain't Prime!\n";
else
cout << "Yep Thats Prime!\n";
main();
return 0;
}
Suppose, when I enter 7, I know that, it checks upto 6, then it should crash!(due to x + 1 > n condition). I don't know how to return 0 when it fails the else condition...

Answer 1

To answer to your question "Whats wrong with my Prime number Checker?" a lot of things are wrong:
Don't call main() in main. That's not how you do recursion
int Remainder(int n, int x) and you call it with a float (cast is missing) then with a bool : Remainder(n, x + 1 > n);
Your asker doesn't need to be a float
About the recursion within main there is two reason:
With this config you'll get an endless loop;
ISO C++ forbids taking address of function '::main'

Answer 2

//#include "stdafx.h" //This is an invalid header.
#include <iostream>
#include<iomanip>
#include <cmath>
using namespace std;
float Asker()
{
float n;
cin >> n;
return n;
}
int Remainder(int n, int x)
{
int q = n%x;
if (q == 0 && n>2 )//'2' have to be excluded.
//otherwise 2%2==0 can set
//'2' as a non prime which is wrong
return 1;
else if(x+1<n)
Remainder(n, x + 1);
/*
Here was the PROBLEM
Remainder(n, x + 1 > n) 'x + 1 > n ' is an invalid paramrter.
*/
else
return 0;
}
int main()
{
cout << "Enter your Number : ";
float n=Asker();
int r=1; //It is essential to initialize r to 1
if(n!=1) //Have to exclude '1'. Otherwise
//It will assign '1' as prime which is wrong
r = Remainder(n, 2);
if (r == 1 )
cout << "That Ain't Prime!\n";
else
cout << "Yep Thats Prime!\n";
//main(); //Why are you calling main again?
return 0;
}
Your first error was " #include "stdafx.h" ". Where'd you get this header?
Then inside int Remainder(int n, int x) function you used recursion and sent an invalid syntax " Remainder(n, x + 1 > n) ". You can't use syntax like x+1>n in a parameter.
After that why are you calling main() inside main function?
And your algorithm needed some touch which I have added and explained in comment.
But you should know that the shortest way to check a prime number is to check n%x==0 till x<=square_root(n).

Answer 3

First of all you don't have to check modulo for all numbers up to n-1: it is sufficient to check modulo up to sqrt(n). Second, you should return 0 from the function if the next divisor to check is larger than sqrt(n). Here is the corrected Remainder function.
int Remainder(int n, int x)
{
int q = n%x;
if (q == 0)
return 1;
else
{
if(x+1 > std::sqrt(n)) return 0;
else return Remainder(n, x + 1);
}
}
Finally, it is better to change the type of n in main and Asker from float to int, and return type of Asker should be int too.
This is not an exhausting list of what's wrong with the prime number checker in focus - just a way to fix it quickly. Essentially, such prime number checker shouldn't use recursion - it's more neat to just iterate over all potential divisors from 2 to sqrt(n).