PHP implementation of custom number distribution

Question

I'm looking for a function to generate random numbers in [a,b]

Function must take one argument - seed and return random x є [a,b]. The probability of x = a is the lowest, but is increasing when moving to b. The probability of x = b should be the highest.

As far as I understand what I'm looking for is an implementation of cumulative distribution function, but can't say for sure. So i'll be glad for direct literature. Or maybe there's an in-box php implementation already?

Thanks

What have you tried? If you do not show some code, folks will close this faster than you can blink. — Buggabill
– Buggabill, Commented Dec 11, 2012 at 18:59
Possible duplicate of stackoverflow.com/questions/4304765/… — RonaldBarzell
– RonaldBarzell, Commented Dec 11, 2012 at 19:00

Žan Kusterle · Accepted Answer · 2012-12-11 19:40:47Z

1

First of all, you did not specify what an argument should be. The function below does not use comulative distribution. Incereasing the $n will cause values close to b more likely to be returned.

function almostRandom() {
    $a = 1;
    $b = 10;
    $n = 2.5;

    $random = rand(0, 1000000) / 1000000;

    $positionInInterval = 1 - pow($random, $n);

    $intervalLength = $b - $a;
    $value = $a + $intervalLength * $positionInInterval;

    return $value;
}

answered Dec 11, 2012 at 19:40

Žan Kusterle

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2 Comments

abr Over a year ago

This actually works, but I can't understand it at all. The argument is a "seed" (just recently found out how it's called)... Thanks!

Žan Kusterle Over a year ago

Use Wolfram Alpha to plot a graph of 1 - x^2.5 You will be able to see, that if x is random number between 0 and 1, the value of the graph is more likely to be closer to 1 than to 0. You can set the seed for random generator with srand function php.net/manual/en/function.srand.php

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