Question

Lets assume:

List<element> which element is:

public class Element {
   int Weight { get; set; }
}

What I want to achieve is, select an element randomly by the weight.
For example:

Element_1.Weight = 100;
Element_2.Weight = 50;
Element_3.Weight = 200;

So

• the chance Element_1 got selected is 100/(100+50+200)=28.57%
• the chance Element_2 got selected is 50/(100+50+200)=14.29%
• the chance Element_3 got selected is 200/(100+50+200)=57.14%

I know I can create a loop, calculate total, etc…

What I want to learn is, whats the best way to do this by Linq in ONE line (or as short as possible), thanks.

UPDATE

I found my answer below. First thing I learn is: Linq is NOT magic, it’s slower then well-designed loop.

So my question becomes find a random element by weight, (without as short as possible stuff 🙂

Answer 1

If you want a generic version (useful for using with a (singleton) randomize helper, consider whether you need a constant seed or not)

usage:

randomizer.GetRandomItem(items, x => x.Weight)

code:

public T GetRandomItem<T>(IEnumerable<T> itemsEnumerable, Func<T, int> weightKey)
{
    var items = itemsEnumerable.ToList();

    var totalWeight = items.Sum(x => weightKey(x));
    var randomWeightedIndex = _random.Next(totalWeight);
    var itemWeightedIndex = 0;
    foreach(var item in items)
    {
        itemWeightedIndex += weightKey(item);
        if(randomWeightedIndex < itemWeightedIndex)
            return item;
    }
    throw new ArgumentException("Collection count and weights must be greater than 0");
}