# 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 on the percentage chance given.

how do I approach this?

my attempt so far looks like this:

``````r = random.random()
if r <= .7:
return a
elif r <= .9:
return b
else:
return c
``````

I'm stuck coming up with an algorithm to handle this. How should I approach this so it can handle larger sets of values without just chaining together if-else flows.

(any explanation or answers in pseudo-code are fine. a python or C# implementation would be especially helpful)

1
27
9/7/2010 2:52:10 AM

Here is a complete solution in C#:

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

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

static Random random = new Random();
public static T ChooseByRandom<T>(
this IEnumerable<ProportionValue<T>> 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());
``````
36
9/7/2010 3:22:32 AM