How to randomize array items and then crop the array by required percent

Question

What is the best way to randomize items in an array and also remove random items to get to a required array length?

Let's say I have an array with 100 items.

First, I want to randomize it.

Then, I have three percent coefficients: 10%, 30%, 60%.

I want to remove items from the original array so the new array I get will be 60% of the original size (60 items), then 30%, then 10%.

Answer 1

The best way to randomise the array would be to use a Fisher-Yates-Durstenfeld shuffle. You could either do this in-place (ie, shuffle the existing array) or create a new collection containing the items in random order.

Here's how I'd probably do it, creating a new shuffled collection rather than mutating the original array, and then using Skip and Take to get the required sub-sequences of the shuffled array. (And it would be easy enough to alter this code to perform an in-place shuffle instead, if you prefer.)

// uses Shuffle extension from https://stackoverflow.com/a/1653204/55847
var temp = original.Shuffle().ToArray();
var sixty = temp.Take((int)(temp.Length * 0.6)).ToArray();
var thirty = temp.Skip(sixty.Length).Take((int)(temp.Length * 0.3)).ToArray();
var ten = temp.Skip(sixty.Length + thirty.Length).ToArray();

Answer 2

Well, first of all you can't remove items from an array. The size of an array is determined when it's created, and can never change after that. If you want a smaller array, you have to create a new one.

You can shuffle the items using an extension like this:

public static class IEnumerableExtensions {

  public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> list, Random rnd) {
    List<T> items = new List<T>(list);
    for (int i = 0; i < items.Count; i++) {
      int pos = rnd.Next(i, items.Count);
      yield return items[pos];
      items[pos] = items[i];
    }
  }

}

I'm not exactly sure what you mean by the coefficients, but I guess that you want to get the first 60% in one array, then another 30% in second array, then another 10% in a third array:

IEnumerable<int> shuffled = originalArray.Shuffle(new Random()).ToList();

int cnt60 = (int)Math.Round(originalArray.Length * 0.6);
int cnt30 = (int)Math.Round(originalArray.Length * 0.3);
int cnt10 = (int)Math.Round(originalArray.Length * 0.1);

int[] first60 = shuffled.Take(cnt60).ToArray();
int[] next30 = shuffled.Skip(cnt60).Take(cnt30).ToArray();
int[] next10 = shuffled.Skip(cnt60+cnt30).Take(cnt10).ToArray();