Kata Kickin’ Some Prime Numbers

Lately I have been interviewing people for prospective positions in the company I work for.  In the various questions that I and others pose during these interviews one popped up as a prospective Kata.  The challenge is really quit simple, “Write a method that would take a int and check if it is a prime number.”  The following snippet is what I came up with.

protected bool IsPrime(int theNumber) 
{ 
    bool isPrime = true;

    int theHalf = theNumber / 2;

    for (int i = 2; i < theHalf; i++) 
    { 
        var remainder = theNumber % i; 
        if (remainder == 0) 
        { 
            isPrime = false; 
            break; 
        } 
    }

    return isPrime; 
}

The next idea that popped up was to find X prime number in order.  As the answer a question like, “What is the 6th prime number?”  I came up with the following method for that.

protected int GetPrimeByPosition(int position) 
{ 
    int counter = 1; 
    int thePosition = 0; 
    while (true) 
    { 
        if(IsPrime(counter)) 
        { 
            if(thePosition == position) 
                return counter; 
            thePosition++; 
        }

        counter++; 
    } 
}

In the end the Kata is a simple 2 step problem.

  1. Write a method that determines if an int is a prime number.  i.e. “Is 5 a prime number?”
  2. Write a method that determines the prime number based on X order.  i.e. “What is the 3rd prime number?”

So based on these two steps, what would be some other steps to add to this Kata?  What are some other good interview questions that are short and simple for prospective employees to code through?

One thought on “Kata Kickin’ Some Prime Numbers

  1. I’ve been asked this question in an interview before, too. The trouble is there isn’t a right answer.

    You can come up with distinctly different looking implementations depending on if you’re optimizing for small memory footprint (as yours is) vs. bulk speed. Then within there, you can critique it (e.g. yours could skip testing division by even numbers since you already tested 2).

    So maybe you can add “how would you write this differently if you wanted to determine the first N prime numbers? What if memory was no object?” etc.

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