Calculating the 10001 prime number, no output is printed

Question

public class Prime 
{
    public static void main(String [] args)
    {
        int num = 3;
        int counter = 1;
        boolean flag = true;

        while(counter < 10001)
        {
            for(int i = 2; i < num; i++)
            {
                if(num%i==0)
                {
                    flag = false;
                }
            }
            if(flag)
            {
                counter++;
            }
            num++;
        }
        System.out.println(num);
    }
}

Whenever I run this code, no result is printed out. I'm assuming its because the code is inefficient, but I don't know what is wrong with this code.

Answer 1

Your problem is that you don't reset flag to true before the for loop; so once it is false , it remains false , so counter is never incremented, meaning the while loop guard never becomes false .

This is an example of why you should declare variables in the tightest possible scope (ie inside the while loop).

while(counter < 10001)
{
  int flag = true;
  for (...) {...}

  if (flag) { counter++; }
  // ...
}

It's also an example of why you should make sure your code is correct before you think about whether it is efficient . Had you debugged the code (or even just added in a few System.out.println s), you could have found that it wasn't actually doing the right thing.

Answer 2

As mentioned by @AndyTurner you should declare flag = true; in while loop and also there is a slight error in your logic for finding the prime number.

You are incrementing the num after it is checked for prime . So, if your 3rd prime is 5 , then it will print 6 as the answer. lly, if your 10001th prime is X , then it will print X + 1 as the answer.

I have posted the correct logic for your problem below. I hope it helps you.

public class Main
{
    public static void main(String [] args)
    {
        int num = 2;
        int counter = 1;
        boolean flag = true;

        while(counter < 10001)
        {
            flag = true;
            num++;
            for(int i = 2; i < num; i++)
            {
                if(num%i==0)
                {
                    flag = false;
                }
            }
            if(flag)
            {
                counter++;
            }
        }
        System.out.println(num);
    }
}

Answer 3

Its not printing cause its going in infinite loop as counter is not incremented once flag become false

public static void main(String [] args)
{
    int num = 3;
    int counter = 1;
    boolean flag = true;

    while(counter < 10001)
    {
        flag = true;
        for(int i = 2; i < num; i++)
        {
            if(num%i==0)
            {
                flag = false;
            }
        }
    if(flag)
    {
        counter++;
    }
    num++;
}
System.out.println(num);
}

Try this. hope it helped