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 my version that can apply to any IList and normalize the weight. It is based on Timwi's solution : selection based on percentage weighting

/// <summary>
/// return a random element of the list or default if list is empty
/// </summary>
/// <param name="e"></param>
/// <param name="weightSelector">
/// return chances to be picked for the element. A weigh of 0 or less means 0 chance to be picked.
/// If all elements have weight of 0 or less they all have equal chances to be picked.
/// </param>
/// <returns></returns>
public static T AnyOrDefault<T>(this IList<T> e, Func<T, double> weightSelector)
{
    if (e.Count < 1)
        return default(T);
    if (e.Count == 1)
        return e[0];
    var weights = e.Select(o => Math.Max(weightSelector(o), 0)).ToArray();
    var sum = weights.Sum(d => d);

    var rnd = new Random().NextDouble();
    for (int i = 0; i < weights.Length; i++)
    {
        //Normalize weight
        var w = sum == 0
            ? 1 / (double)e.Count
            : weights[i] / sum;
        if (rnd < w)
            return e[i];
        rnd -= w;
    }
    throw new Exception("Should not happen");
}