CODEWITHSUNDEEP
c#probability
Steve
Steve

Reputation: 715

How to make a choice based on a percentage probability

I wish to use the answer provided in this post to randomly choose unique items from a list.

Following the method described, in each iteration of my loop I generate a probability value which is the percentage chance of the current item being picked from the list.

What I need to know is how do I use this percentage value to pick the item (or not).

Here is the code I have, with remainingIndices being a List<int>

for (var i = 0; i < remainingIndices.Count; i++)
{
    var probability = pixelsToAdd / (float)(remainingIndices.Count - i);
}

pixelsToAdd is 120 and remainingIndices.Count is 3600. The probability values I am getting start at 0.0333333351

The solution should be flexible to work with a much wider range of values, preferably any values.

Thanks

Comment

For future readers of this question I should clarify that at first I thought the probability value was some percentage between 0 and 100 but in reality it's a value between 0 and 1 and so matches up perfectly with the return value of Random.NextDouble() which therefore can be used for comparison as described in the answers below.

Upvotes: 5

Views: 2924

Answers (1)

C&#233;dric Bignon
C&#233;dric Bignon

Reputation: 13022

To use your probability, compare it with a sample from random variable following a uniform distribution on [0, 1].

if (Random.NextDouble() <= probability)
    // Take the ith element in the list

You resulting loop will be:

List<???> selectedItems = new List<???>();
for (var i = 0; i < remainingIndices.Count; i++)
{
    var probability = pixelsToAdd / (float)(remainingIndices.Count - i);
    if (Random.NextDouble() <= probability)
    {
        selectedItems.Add(items[i]);
        pixelsToAdd--;
    }
}

Upvotes: 3

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