nth prime number

[FraidaL]

This code is supposed to find the nth prime number (n being input by the user). I have the code to calculate whether a number is prime or not, but I don't know how to get the code to display the nth prime number.

#include <iostream>
using namespace std;

void isprime(int);

int main () 
{
    int x;
    cout << "Which prime number would you like to display? ";
    cin >> x;

    if (x==0)
    {
        cout << "That is not a valid choice. Please enter another number." << endl;
        cin >> x;
    }

    if (x<0)
    {
        cout << "That is not a valid choice. There are no negative prime numbers. Please enter another number." << endl;
        cin >> x;
    }


    return 0;
}

void isprime(int x)
{
    cout << "Enter a number: ";
    int n;
    cin >> n;

    int isprime;

    if(n<2 || n>2 && n%2==0)
    {
        isprime=0;

        for(int i=2; i<n; i++)
        {
            if((n%i)==0)
            {
                isprime=0;
            }
        }
    }

    else isprime=1;
}   

[WaltP]

Start calcumating prime numbers. When you've calculated x of them, there you are...

And your tests for bad values of x would be better if you used a do-while loop.

[FraidaL]

That's the part I am stuck with. Should I be calculating the prime numbers in my isprime function or main? Also I'm not sure how to write that in the code. Is a do-while loop similar to a while loop? I never heard of do-while loop before. How would that differ from what I have now, with regards to the code?

[CeilingSpy]

You should calculate the prime numbers inside of main.
Do-While loops are the same as While loops but Do-While loops go through the loop once no matter what, then checks the condition, ie:

do
{
    // This will run once, even though the condition is false.
}while(1 == 2);

Hope this Helps!

[FraidaL]

The trouble I am having with the coding is this: My isprime function computes composite numbers as 0 and prime numbers as 1. I want the code to say that when isprime=1, it should count all those until it gets to the x-th value and output that.

[scudzilla]

Use a counter that increments whenever isprime is 1. Once the counter is equal to the input x, display the value you sent to isprime.

[FraidaL]

This is my code now, but it always calculates the answer as 0. Where am I going wrong?

#include <iostream>
using namespace std;

int isprime(int&);

int main () 
{
    int i;
    int x;
    cout << "Which prime number would you like to display? ";
    cin >> x;

    if (x==0)
    {
        cout << "That is not a valid choice. Please enter another number." << endl;
        cin >> x;
    }

    if (x<0)
    {
        cout << "That is not a valid choice. There are no negative prime numbers. Please enter another number." << endl;
        cin >> x;
    }

    while (isprime(x)==1)
    {
        for (i=2; i<=x; i++)
        {
            i;
        }
    }

    cout << "The " << x << "th prime number is " << i;

    return 0;
}

int isprime(int&x)
{
    int n;
    int isprime;

    if(n<2 || n>2 && n%2==0)
    {
        isprime==0;

        for(int i=2; i<n; i++)
        {
            if((n%i)==0)
            {
                isprime==0;
            }
        }
    }

    else isprime==1;

    return isprime;
}

[WaltP]

You need to think through your problem more carefully, Given any input:
1) How many numbers do you have to test to find x primes? What kind of loop is best for this?
2) How can you count the primes you find so you know when x have been calculated?

And for your input, use while since you know how it works. Right now the user can break your input easily.