Pick a random position in the word - IllegalArgumentException?

Question

I have to implement the following instructions (pseudocode):

Read a word.
Repeat word.length() times
   Pick a random position i in the word, but not the last position.
   Pick a random position j > i in the word. (this is a tricky point!)
   Swap the letters at positions j and i.
Print the word.

My code throws - IllegalArgumentException here:

j = i + 1 + generator.nextInt( word.length() - i - 1 );

I stack here and don't know how to circumvent this point.

Code:

public String scramble(String word) {
        Random generator = new Random(42);
        int x, i = 0, j = 0, wordLen = word.length();

        for (x = 0; x < wordLen; x++) {                
            i = generator.nextInt(wordLen);            
            j = i + 1 + this.generator.nextInt( word.length() - i - 1 );
        }

I can't find a good solution for this step:

Pick a random position j > i in the word.

• How to solve this issue?

Answer 1

The line

generator.nextInt( word.length() - i - 1 );

would throw IllegalArgumentException incase the argument <= 0..

From nextInt Docs

Throws: IllegalArgumentException - if n is not positive

Check the length of a string before calling the line

---

You could do it this way

for (x = 0; x < wordLen; x++) {
      i = generator.nextInt(wordLen - 1); 
      j = generator.nextInt(wordLen);           
      if (j <= i)
        j = i + generator.nextInt(wordLen - i);
}

Answer 2

String word = "Example"
int len = word.length(); //len = 7
int placeHolder1 = (int) (Math.random() * length); //generates number between 0 and 6
int placeHolder2 = (int) 
int diff = length - placeHolder1;
placeHolder2 = (int) (Math.random() * diff + (length - diff + 1)); //Generates random between placeHolder1 and the length of the String 

This gives you the indices of two characters on the String, and you can swap them however you prefer.