PHP Random Number with specific range and near to a specific number

Question

As far as you know PHP has a random function which everyone can use it.

<?php  rand(min,max);   ?>

Now My question is How can I define gravity to specific number?

for example, rand(0,100) returns really random number between min and max and unforeseeable what number will be chosen,I use rand in 10 loops and result is:

28,41,2,94,65,23,47,97,59,69,32

Now I want to have a rand number near to specific number for example, rand(0,20). Near to 10 with for example a rage with 4 numbers length.

1,6,12,15,7,8,20,12,10,9,20

As you can See most of numbers is near to 10 but there is 1 and even 20.

I have no idea to write a random function with Below criteria:

1- to what number must be near? 10

2- What range is near? 4

3- How much percentage of this number near to specific number? 70%

possible duplicate of Generating random numbers with known mean and variance — Barmar
– Barmar, Commented Feb 1, 2014 at 12:59
@Barmar I already read it but I don't understand much what he is idea. and Doesn't cover all my question. Also It's not practical for PHP. — user3113782
– user3113782, Commented Feb 1, 2014 at 13:03
@user1844933 Yes it's still randome because not so much perdictable but can say guided random number. — user3113782
– user3113782, Commented Feb 1, 2014 at 13:04
@user1844933 It's random, but it has normal distribution rather than linear distribution. — Barmar
– Barmar, Commented Feb 1, 2014 at 13:07

Reeno · Accepted Answer · 2014-02-01 13:16:20Z

4

You want to create a normal random variable. the following function creates one variable using the Marsaglia polar method:

 function rand_polar($m = 0.0, $s = 1.0){
       do {
             $x = (float)mt_rand()/(float)mt_getrandmax();
             $y = (float)mt_rand()/(float)mt_getrandmax();

             $q = pow((2 * $x - 1), 2) + pow((2 * $y - 1), 2);
       }
       while ($q > 1);

       $p = sqrt((-2 * log($q))/$q);

       $y = ((2 * $y - 1) * $p);
       $x = ((2 * $x - 1) * $p);

       return $y * $s + $m;
 }

Usage: rand_polar(MEAN, STANDARD_VARIANCE), in your case rand_polar(10, 4)

With boundaries:

function rand_polar($m = 0.0, $s = 1.0, $min = 0, $max = 20){
    do {
        do {
            $x = (float)mt_rand()/(float)mt_getrandmax();
            $y = (float)mt_rand()/(float)mt_getrandmax();

            $q = pow((2 * $x - 1), 2) + pow((2 * $y - 1), 2);
        }
        while ($q > 1);

        $p = sqrt((-2 * log($q))/$q);

        $y = ((2 * $y - 1) * $p);
        $x = ((2 * $x - 1) * $p);
        $rand = $y * $s + $m;
    }
    while($rand > $max || $rand < $min);
    return $rand;
}

Usage: rand_polar(10, 4, 0, 20)

edited Feb 1, 2014 at 13:16

answered Feb 1, 2014 at 13:05

Reeno

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5 Comments

Barmar Over a year ago

Since he also wants the result bounded, you should add $min and $max parameters.

Reeno Over a year ago

It's not specified in the criteria at the end of the text, so I wasn't sure about this.

Barmar Over a year ago

He said "rand(0, 20). Near to 10". Why would he give those boundaries if he didn't want them?

user3113782 Over a year ago

@Reeno Yes I still need it be in min and max

Reeno Over a year ago

I included a second function with boundaries.

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