selection based on percentage weighting

Question

I have a set of values, and an associated percentage for each:

a: 70% chance
b: 20% chance
c: 10% chance

I want to select a value (a, b, c) based

Answer 1

Here is a complete solution in C#:

public class ProportionValue
{
    public double Proportion { get; set; }
    public T Value { get; set; }
}

public static class ProportionValue
{
    public static ProportionValue Create(double proportion, T value)
    {
        return new ProportionValue { Proportion = proportion, Value = value };
    }

    static Random random = new Random();
    public static T ChooseByRandom(
        this IEnumerable> collection)
    {
        var rnd = random.NextDouble();
        foreach (var item in collection)
        {
            if (rnd < item.Proportion)
                return item.Value;
            rnd -= item.Proportion;
        }
        throw new InvalidOperationException(
            "The proportions in the collection do not add up to 1.");
    }
}

Usage:

var list = new[] {
    ProportionValue.Create(0.7, "a"),
    ProportionValue.Create(0.2, "b"),
    ProportionValue.Create(0.1, "c")
};

// Outputs "a" with probability 0.7, etc.
Console.WriteLine(list.ChooseByRandom());