Prime Number Function

[sha92]

Hello,
My code doesn't give the output, prime numbers 1-100. What have I done wrong?

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

int n, i;

bool isPrime(int number)
{
	for (i = 1; i <= sqrt(n); i++)
		n = n / i;
	if (n / i == 0)
		return false;
else return true;
}

int main()
{
	for (i = 1; i <= 100; i++)
	{
		if (isPrime(i)==true)
			cout << i<<endl;
	}
	system("pause");
	return 0;
}

[TarikNeaj]

Are you expecting the program to magically know what "n" is equal to? because you never tell the program what n is equal to.

Edit: Try this

bool isPrime(int number)
{
	bool prime = true;
	for (int j = 2; j*j <= number; j++)
	{
		if (number % j == 0)
		{
			prime = false;
			break;
		}
		return true;
	}
}

int main(){

	for (int i = 2; i<500; i++)
	{
		if (isPrime(i) == true)
			cout << i << endl;
	}
	system("pause");
	return 0;
}

[sha92]

i see, so it should change to bool isPrime(int n)
But then the program gives an error.
btw your words are a bit offensive.

[TarikNeaj]

btw your words are a bit offensive.

It wasnt meant offensive. Sorry.

i see, so it should change to bool isPrime(int n)

No not that n. This n int n, i; Here you create n. But then you do this
for (i = 1; i <= sqrt(n); i++)

sqrt(n); But n has no value, its currently garbage becuase you only created the variable n, but never gave it a value.

Read the code Ive provided, it gives you the first 500 prime numbers, you only have to change the for-loop if you want 1000.

[sha92]

It wasnt meant offensive. Sorry.

it's ok.

something is off. your program outputs some of the non prime numbers too, like 21 etc

[TarikNeaj]

Oh ye lol. Just noticed. Try this

bool isPrime(int number)
{
	for (int i = 2; i < sqrt(number); i++)
	{
		if (number%i == 0)
		{
			return false;
		}
	}
	return true;
}

int main(){

	for (int i = 2; i<500; i++)
	{
		if (isPrime(i) == true)
			cout << i << endl;
	}
	system("pause");
	return 0;
}

[sha92]

I think it's still not right in 1 to 10

[fg109]

i <= sqrt(number)

[TarikNeaj]

Its supposed to start at 2, not 10. So 2-10. Yeh still not quite correct, forgot one thing.

Add a +1 in the sqrt.

for (int i = 2; i < sqrt(number + 1); i++) // added (number +1)

[sha92]

great! it works. Thank you so much :)