# Weighted Random In C# (csharp)

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weighted random function in c# 3.0. For c# 2.0 just remove 'this' before WeightedRandom rnd argument.
``````
public static partial class Utils
{
static string WeightCountMustBeGreaterThanZero="Weight count must be greater than zero.";
static string ElementWeightMustBeGreaterThanOrEqualToZero="Weight must be greater than or equal to zero";
///
```
/// Returns random index in weights list with probability based on its weight value.
///
///
/// List of weights.
///
/// Throws an exception if weights is null.
///
/// Throws an exception if weights count is zero.
/// Throws an exception if any weight is less than zero.
///
///
/// Returned values are within range of zero and weights.Count (exclusive).
/// Chance of returned value to be i is weights[i]/weights.Sum().
/// Any weight can be equal to zero. Such index is never selected.
///
///
/// < code>
/// var weights=new List<int>(new int[]{2,3,5,0});
/// int v=new Random().WeightedRandom(weights);
///
/// 20% chance for v==0
/// 30% chance for v==1
/// 50% chance for v==2
/// 0% chance for v==3
///
public static int WeightedRandom(this Random rnd, IList weights)
{
if (weights == null)
{
throw new ArgumentNullException("weights");
}
if (weights.Count == 0)
{
throw new ArgumentOutOfRangeException("weights", WeightCountMustBeGreaterThanZero);
}
List total_weights = new List();
for (int i = 0; i < weights.Count; i++)
{
if (weights[i] < 0)
{
throw new ArgumentOutOfRangeException("weights", ElementWeightMustBeGreaterThanOrEqualToZero);
}
int last;
if (total_weights.Count > 0)
{
last = total_weights[total_weights.Count - 1];
}
else
{
last = 0;
}
int w = checked(last + weights[i]);
total_weights.Add(w);
}
int total_random = rnd.Next(total_weights[total_weights.Count - 1]);
for (int i = 0; i < weights.Count; i++)
{
if (weights[i] > total_random)
{
return i;
}
total_random -= weights[i];
}
throw new Exception();
}
}

NUnit tests:
```
[TestFixture]
public class TWeightedRandom
{
[Test]
public void WeightedRandom()
{
var weights = new List
```(new int[] { 1, 0, 2, 3 });
List l = new List();
int n = 1000 * 1000;
Random rnd = new Random();
for (int i = 0; i < n; i++)
{
l.Add(rnd.WeightedRandom(weights));
}
int a = l.Where(v => v == 0).Count();
int b = l.Where(v => v == 2).Count();
int c = l.Where(v => v == 3).Count();
int z = l.Where(v => v == 1).Count();
Assert.AreEqual(n, a + b + c);
Assert.AreEqual(0, z);
Assert.Less(Math.Abs((double)b / a - 2), 0.1);
Assert.Less(Math.Abs((double)c / a - 3), 0.1);
}
[Test]
public void WeightedRandomOverflow()
{
int num = 1000 * 1000 * 1000;
var weights = new List(new int[] { 2 * num, 2 * num });
try
{
new Random().WeightedRandom(weights);
Assert.Fail("overflow not thrown");
}
catch (ArithmeticException e) { }
}
}

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