shuffling

blah.java

import java.util.Random;

public class Person2 {
  /**
   * This method should take the string
   * input and return its characters in
   * random order.
   * given "gtg123b" it should return
   * something like "g3tb1g2".
   *
   * @param input the string to be modified
   * @return the modified string
   */
  private String calc(String input) {
    //http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#The_modern_algorithm
    //To shuffle an array a of n elements (indices 0..n-1):
      //for i from n - 1 downto 1 do
         //j = random integer with 0 <= j <= i
         //exchange a[j] and a[i]
    StringBuilder randomizer = new StringBuilder(input);

    for (int i = randomizer.length() - 1; i > 0; i--){
      int j = new Random().nextInt( i + 1 );

      //swap
      char tmp = randomizer.charAt(i);
      randomizer.setCharAt(i, randomizer.charAt(j));
      randomizer.setCharAt(j, tmp);
    }

    return randomizer.toString();
  }

}

Well, if I'm understanding you correctly, if a string is constructed randomly (which I think your implementation was), then backwards vs forwards makes no difference right?

However, if you still decide to use mine, I've discovered a bug in it. Random().nextInt(n) chooses from [0, n). But according to the algorithm, this needs to be [0, n] (inclusive of both zero and n), so you need to change the nextInt call to nextInt( i + 1 ).

This is a prime example of "bias" that I was talking about about the meeting. Little bugs like this can cause a certain set of permutations to be more likely than others. In this example, there would always be 1 position that was improperly excluded from the possibility of of being swapped.

Hehe, now, do you need this kind of randomness for something as silly as this assignment? Nope.