Prime Numbers C

[Doughnuts]

Hello, I am a newbie a C++. I was doing a program where you calculate prime numbers, and it prints prime and non-prime numbers. Here is my code,

#include <iostream>
#include <conio.h>
using namespace std;

main()
{
    int n = 1;
    int i = 2;
    bool IsPrime;
    int Max;
    int x = 0;
    
    cout << "Maximum number: ";
    cin >> Max;
    
    while (x < Max)
    {
        IsPrime = true;
    
        while ((i < n) &&)
        {
            if (n % i == 0) 
            {
                // Is not prime
                IsPrime = false;
                
            }
            else
            {
                // Is prime
                i++;
                
            }
        }
        
        if (IsPrime)
        {
            cout << "Prime: " << n << endl;
        }
        
        n++;
        x++;
    }

    getch();
}

Thanks,
~mr. doughnuts

[mrnutty]

simplify.

1) Make a bool function that takes a int i as a parameter and checks if it is a prime or not.
2) inside your loop, check if i to MAX is a prime.

your function prototype might look like this :

bool isPrime(int num);

HINTS :
a) How do you when a number is a prime ?
- When it is only evenly divisible by itself an 1.
b) Use the mod operator.

[Doughnuts]

firstperson, thanks for the quick reply.
What is a bool function? :)

~Mr doughnuts

[sfuo]

A bool function returns true or false.

[sfuo]

This is some really quick prime number checker that I made since the post above. It checks to see if the number can be divided by 2, 3, 5, 7 evenly and if it is not 1. If it passes through all this then it is prime.

#include <iostream>

using namespace std;

bool isPrime(int x)
{
	if( x == 1 )
		return false;
	if( x % 2 == 0 && x != 2 )
		return false;
	if( x % 3 == 0 && x != 3 )
		return false;
	if( x % 5 == 0 && x != 5 )
		return false;
	if( x % 7 == 0 && x != 7 )
		return false;
	return true;
}

int main()
{
	int max;
	cout << "Max?: ";
	cin >> max;
	for( int i = 1; i <= max; i++ )
	{
		if( isPrime(i) )
		{
			cout << "Prime: " << i << endl;
		}
	}
	system("PAUSE");
	return 0;
}

[mrnutty]

Fail.

[sfuo]

Lets see yours because last time I checked this works and its short.

[mrnutty]

#include <iostream>
#include <cmath>
using namespace std;

bool isPrime(int x)
{
	if( x == 1 )
		return false;
	if( x % 2 == 0 && x != 2 )
		return false;
	if( x % 3 == 0 && x != 3 )
		return false;
	if( x % 5 == 0 && x != 5 )
		return false;
	if( x % 7 == 0 && x != 7 )
		return false;
	return true;
}

bool is_prime(int i)
{
	if(i < 2) return false;

	if(i == 2) return true;


	float half = ceil( sqrt((float(i))) ) + 1;

	for(int j = 2; j < half; j++)
		if(i % j == 0) return false;

	return true;
}
int main()
{
	int i = 0;

	cout << boolalpha;

	while(true)
	{
		if(isPrime(i) != is_prime(i))
		{
			cout<<"When your function versus my simple brute force disagrees : \n\n";

			cout<<"Result :  from your function : "<<i<<" is prime = "<<isPrime(i)<<endl;
			cout<<"Result :  from my function   : "<<i<<" is prime = "<<is_prime(i)<<endl;
			cout<<"\nCheck what google says\n\n";
			cin.get();  
			break;
		}
		i++;
	}
	
	return 0;
}

[sfuo]

Flawed and bigger :( you win.

[mrnutty]

Flawed and bigger :( you win.

You learn from mistakes.

[Doughnuts]

Wow, that is smaller, masterful, flawless code. :|
Thanks, firstperson !!!

~mr. doughnuts