Question

I have a requirement in my .NET project where I need to select an item from a collection, each item has a Weight (integer from 1 to 10) assigned to it.

I need a random generator that would take this weight into consideration i.e., the higher the weight, the more chances the object would be selected.

Quick copy/paste C# code in case someone stumbles upon this.

    class RandomWeightedSelector<T>
    {
        private List<T> items = new List<T>();

        public void Add(T item, uint weight = 1)
        {
            for (int i = 0; i < weight; i++)
                items.Add(item);
        }

        public T GetRandom()
        {
            return items[new Random().Next(0, items.Count)];
        }
    }

Answer 1

Here’s an algorithm which doesn’t require adding the items multiple times to a list. It can also work with non-integer weights, although if you’re using NextDouble from System.Random, you’ll have to scale all of the weights to add up to 1, or multiply the value from NextDouble with S to get it in the desired range.

Given a list L of items (I,W), where I is the item and W is the weight:

1. Add all of the weights together. Call this sum S.
2. Generate a random number between 0 and S (excluding S, but including 0). Call this value R.
3. Initialize a variable to 0 to keep track of the running total. We’ll call this T.
4. For each item (I,W) in L:
   1. T=T+W
   2. If T > R, return I.