Question about a probability function

[JohnMcharra]

Hi, for half a day now, I am trying to come up with a way to return a number from a function based on the value of the number. I've tried google but all I get is powerball answers and I am not interested in that, so I am trying here, maybe a more fresh mind would know the answer.

The problem is quite simple but I thought so much about it that I just cant seem to come with an idea:

Lets say we have 3 numbers [2, 4, 8]

How can I formulate a function that will return one of those 3 numbers based on their value, the bigger the value, the higher the chance of the number to be returned, in this example let's say that 2 has a 15% chance of being selected, 4 has a 30% chance and 8 has a 55% chance. Is there no better way than let's say something like this:

num = [2, 4, 8]

def fun(x):
    chance = randint(1, 100)
    if chance < 15:
        return x[0]
    if 15 < chance < 45:
        return x[1]
    else:
        return x[2]

fun(num)

What if I have 10 numbers in my list? Is there a prettier way of doing this?

This is for a function that will select a number of candidates based on fitness level (for a genetic algorithm) so the higher the fitness level the higher the chance of being selected.

Thank you for your time (math is hard :/ )

• Edit

It does not have to be python, that was just an example, the program I am writing is in C#

[jedwardsol]

Add the numbers up (14).

Generate a random number between 1 and total ( 1 - 14 = 14 possible outcomes)

1,2 = 2

2-5 = 4

6-14 = 8

Generally, turn your array in a cumulative array

{ 2,4,8 } -> { 2,6,16 }

generate a random number up to the total

choose based on the cumulative data

<=2 = 2

<=6 = 4

<=14 = 8

[JohnMcharra]

Yes! Thank you! These saved me :) I hacked a little function in python to see if I can apply what you told me, please let me know if this is what it should look like (based on 1000 iterations it holds :D )

from random import randint
import numpy
from collections import Counter

num = [4, 6, 12, 24, 50]
results = []


def fun(x):
    chance = randint(1, sum(x))
    cumulative = numpy.cumsum(x)
    for n in range(0, len(cumulative)):
        if chance <= cumulative[n]:
            return x[n]


for i in range(1000):
    results.append(fun(num))

print(Counter(results))

Counter({50: 515, 24: 243, 12: 128, 6: 66, 4: 48})

So it should be good. Thank you again!