How can i replace index values with other ones

Question

Let's say i have an array of 6 elements of int type.

It looks like this

var array = new int [] { 0, 1, 2, 3, 4, 5 };

How can I randomly shuffle my array ensuring every index have a new value.

// Bad
// The number at index 3 did not change and still has a value of 3
new int [] { 1, 0, 5, 3, 2, 4 } 

// Good:
// All the index have a new value
new int [] { 4, 2, 0, 5, 3, 1 }

I've tried to Shuffle, but sometimes some values will have the same index position

@Ed Plunkett The suggested post is not the solution to the current problem, but thanks anyways :) — Charles, Jun 12 '19 at 19:32
What about another array with final positions ? If you fill it with random numbers for particular positions, there are only 2 rules to check - number != position and was not used yet ? — Jan, Jun 12 '19 at 19:58
possible duplicate https://stackoverflow.com/questions/108819/best-way-to-randomize-an-array-with-net — Zain Arshad, Jun 12 '19 at 20:10
@ZainArshad I don't think it's a dupe since OP has the additional requirement of making sure each index has a new value. — Justin Lessard, Jun 12 '19 at 20:23

Zotta · Accepted Answer · 2019-06-12T20:34:06.307

You could iterate over the array and always swap with a randomly choosen index which is bigger than the current one. That way the numbers get shuffled, but no element can be shuffled back.

Edit: This acutally works, try it:

using System;

class Program {
    private static Random rnd = new Random();

    public static void Main() {

        for (var i = 0; i < 10000; i++) {
            int[] array = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            Shuffle(array);
            Console.WriteLine(String.Join(", ", array));
        }
    }

    public static void Shuffle(int[] array) {
        for (var i = 0; i + 1 < array.Length; i++) {
            // generate j so that i < j < length
            int j = rnd.Next(i + 1, array.Length);

            // swap elements at i and j
            var tmp = array[j];
            array[j] = array[i];
            array[i] = tmp;

            // check
            if (array[i] == i) {
                throw new Exception(":(");
            }
        }
    }
}

Also its very similar to Sattolo's algorithm, which would also work.

How can i replace index values with other ones

1 Answers1