Question

So I’m trying out Problem 7 of Project Euler.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
What is the 10 001st prime number?

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

bool isPrime(int a){
    if (a==2||a==3){
        return true;
    }
    if (a%2==0){
        return false;
    }

    bool prime=true;
    for (int b=2;b<sqrt(a);b++){
        if (a%b==0)
            prime=false;

    }
    if (prime==true)
        return true;
    else 
        return false;
}

int main(){
    int infinite=0;
    long long int primecounter=0;
    for (int c=2;infinite==0;c++){
        if (isPrime(c)==true){
            primecounter++;
            //cout<<c<<endl;
            if (primecounter==10001)
                {cout<<c;

            break;}
        }
    }
    return 0;}

This is what I’ve come up with so far. It works for the few numbers that I tested, like the 6th prime number etc. However, when I run it for the 10001st prime, it gives me 104021, and the answer is wrong. Can someone tell me what is wrong with my code?

Answer 1

Where you get it wrong is b < sqrt(a). Think of a=25, what happens in this case?

rest of answer already pointed by comments.