How do I find the nth prime number?

Question

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I have done the program with my knowledge and by not peeking into others' code. I feel that the logic is fine but whenever I enter the nth number I am not getting the corresponding prime number.

What I have tried:

Java

import java.util.*;
public class Main
{
	public static void main(String[] args) {
	 int count=0;
	 int flag;
	 Scanner obj=new Scanner(System.in);
	 int num=obj.nextInt();
	 int i;
	 while(count<=num)
	 {
	     flag=0;
	     for(i=2;i<=num/2;i++)
	     {
	         if(num%i==0)
	         {
	             flag=1;
	             break;
	         }
	     }
	     if(flag==0)
	     {
	         count++;
	     }
	    if(count==num) 
	    {
	        System.out.println(i);
	    }
	 }
	}
}

Posted 7-Jul-21 18:40pm

Nandini Sathyan S

Updated 7-Jul-21 19:41pm

Dave Kreskowiak

v2

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Greg Utas 8-Jul-21 7:38am

You test divisors from 2 to num/2. You can improve that. Hint: multiplication is commutative.

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Patrice T · Accepted Answer · 2021-07-07T19:41:00

Quote:
How do I find the nth prime number?

By testing every integer 2, 3, 4, 5, 6, 7, 8 ... to see which are primes.
Your code miss this logic, in fact you continuously check if num is prime or not.

C++

flag=0;
for(i=2;i<=num/2;i++)
{
    if(num%i==0)
    {
        flag=1;
        break;
    }
}

To help you understand what is going on in your code, the tool of choice is the debugger.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Dave Kreskowiak · Accepted Answer · 2021-07-07T19:17:00

If the "logic was fine" you wouldn't be having problems. Your implementation for finding a prime number is, shall we say, naive at best.

You're testing every single number up to the number you enter in the console. That's NOT the nth prime number. For example, if you enter 10, your code is testing the numbers 2, 3, 4, and 5. It's NOT testing more and more numbers until the count of primes found is reached.

How do I find the nth prime number?

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