Question

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Biased Random Number Generator

for once I need a random number generator with at least two numbers that have higher probabality than the others.

i.e for example: random 1->10 in a sequence of 1000. Numbers A=3 and B=7.

A – should repeat approx. at least 20% of the time.
B – should repeat approx. at least 30% of the time.

That should cover at least 50% of the 1000 sequence. Also the insertion of A and B should be somewhat probable/random in themselves. Not just add A and B every Nth step.
Total/exact control isn’t needed.

Any Ideas?

I’m a noob – c++ style code would be greatly appreciated!

Answer 1

One way you can do this is to randomly generate a number between 0.0 and 1.0, and choose what number to generate based on that number. For example, to implement your example scenario (pseudocode):

let "result" be an array of 1000 integers
let "i" be an integer
for i = 1 to 1000:
    let "j" be a random number between 0.0 and 1.0
    if j < 0.2:
        let result[i] be 3
    else if j < 0.5:
        let result[i] be 7
    else:
        let "k" be an integer
        do, while k = 3 or k = 7:
            let "k" be a random number in the range 1 to 10
        let result[i] be k
end

Basically, j is used to partition the range 1 to 10 into three parts – one part covering from 0% to 20% of the range (the first if), the second covering from 20% to 50% of the range (i.e. 30% wide, the second if), and the last covering the remaining 50%. Depending on which part we randomly fall in, we chose the number to generate as appropriate.