Within a List of Elements, choose one Element randomly based on the probability of each element in the List.

``````// C# Random Elements
//
// Simple problem: I have a list of elements and I want to choose one randomly
// based on the probability of each element.
//
// The nature of the "randomness" is not important here, more important is the
// probabiliy that each element (in an array for example) is selected compared
// to the other elements. For the random aspect, check out the article on
// generating actually random numbers in C#.
//
// Example
//
// So let's say we have four elements, and each has a 25% chance of being
// chosen:
//
//      A - 25%
//      B - 25%
//      C - 25%
//      D - 25%
//
// How do we randomly pick one? In .NET it is easy, using the Random class we
// can generate a pseudo-random number between 0 and 3 (index) and that will be
// the element we randomly selected. This works because the chance of picking
// one of the four elements is the same for all of them.
//
// But now let's modify the list:
//
//      A - 35%
//      B - 15%
//      C - 40%
//      D - 10%
//
// We can't just randomly pick an index anymore.
//
// Statistics
//
// The answer is pretty simple with some basic statistics knowledge. Instead of
// randomly selecting an index, we generate a random number between 0 and 100
// (or 0.0 and 1.0) and compare that to the cumulative probability of each
// element. The cumulative probability is simply the probability of the element
// plus the probability of all the elements before it.
//
// Let's apply that to our example, first let's sort the elements:
//
//      Element - Probability - Cumulative Probability
//         D          10%               10%
//         B          15%               25%
//         A          35%               60%
//         C          40%               100%
//
// Now let's roll the die... let's say we get 53. We compare that number to each
// element's cumulative probability and select the first one that is within
// range. So D is 10%, not within 53. Next is B, 25%, no good. Next is A, 60%,
// that is greater than 53! Thus the element randomly chosen with our random
// number 53 is A.
//
// This makes sense because elements with low probability will need a small
// random number so they can be chosen right off the bat. Elements with higher
// probability will "leverage" the probability of the smaller ones to have a
// better chance of getting chosen.
//
// C# Code
//
// This is a coding website, so let's write down some simple code:

List<KeyValuePair<string, double>> elements = ...some data
Random r = new Random();
double diceRoll = r.NextDouble();

double cumulative = 0.0;
for (int i = 0; i < elements.Count; i++)
{
cumulative += elements[i].Value;
if (diceRoll < cumulative)
{
string selectedElement = elements[i].Key;
break;
}
}

// In this case, each element is a keyvalue pair, with the key being the element
// name and the value being the probability of that element (between 0.0 and
// 1.0).
//
// One final note. It is pretty obvious, but make sure the probabiliy of all the
// elements add up to 1 (or 100 depending on your range). Simple fact, but can
// cause some headaches if you forget.
//``````