Prime numbers in order

Greeting to all of you, who wish to help me.

I tried to make a program, in which you could say which prime number you wanted to know, for example the 23rd , and it would then tell you which prime number is the 23rd.

The theory goes like this:
You input the number, the program will check every prime number (excluding 2 and 3 so it would work a bit faster), and for every prime number it finds it would mark n as the n-th prime number. When n reaches the desired input, y in this matter, it would output the message "The y-th number is x".

I assume that I booched something up, or that my theory doesn't equal my program. Any help would be gladly appreciated.

Thank you in advance.

And here's the code:

#include <iostream>
using namespace std;

int main()
{
    int p;
    int y;
    int n=2;
    cout << "Katero zaporedno prastevilo zelite izvedeti (vnesite samo stevilo):"; //"Which prime number in order do you want?"
    cin >> y;
    if (y==1)
    {
       cout << y << ". prastevilo je 2\n";
    }
    if (y==2)
    {
       cout << y << ". prastevilo je 3\n";
    }
    for(int x=5; x<10000; x+=2) // I, skipped 4 because it would be just a waste and I limited it to 10000 just for testing.
    {
            for(int i=3; i<x; i++)
            {
                    if (x%i==0)
                    {
                               x = p;
                               n++;
                               if (n==y)
                               {
                                        cout << y << ". prastevilo je " << p << "\n"; // The y. prime nubmer is p
                                        break;      
                               }
                               else
                                        continue;
                    }
                    else
                        continue;

            }
    } 
    system("PAUSE");
    return 0;
}

Last edited on

ats15 (423)

Your condition on line 23 is wrong. If x%i==0, x is not a prime

MazzFraps (5)

Ah, that seemed to be one of the problems, but now any number that I put in like 4 or 5, it outputs the number 1991220788, for any number...

uzferry (88)

it's because p has no value, so it shows random number

Last edited on

MazzFraps (5)

How else could I track the number the program is testing?

ats15 (423)

you don't need p at all. Just print x. Or in your program p=x, not x=p

Last edited on

MazzFraps (5)

One problem fixed and another surfaces.
The good thing is that it actually works, but not as it should.
For the 3rd it outputs 5
For the 4th it outputs 7
For the 5th it outputs 7
For the 6th it outputs 9

I have no idea why it is doing this.

dmpaul26 (33)

When I read this, I tried my own version of the same program just for practice. I ran into the exact same problem you did, and my code was almost identical. I did however get it to work, and this is what I came up with. Hope it helps. Also, didn't see the point of the else continues.

#include <iostream>
using namespace std;

int main()
{
	int y, p=2, n, counter;
	
	cout << "Which prime number do you want? ";
	cin >> y;

	if (y==1)
		cout << "2 is the first prime number";

	if (y==2)
		cout << "3 is the second prime number";

	for(int x=5; x<10000; x+=2)
	{
		counter = 0;

		for(n=2; n<x; n++)
		{
			
			if (x % n != 0)  //keeps track of the amt. of numbers not divisible
				counter++;
			
			if (counter == (x - 2))  //checks if all numbers were not divisible
			{
				p++;
				
				if (p == y)
					cout << x << " is the " << y << " prime number.\n";
			}
		}
	}
	return 0;
}

MazzFraps (5)

Thank you, your code helped me a lot and now it finally works! :D

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Prime numbers in order