I'm developing an online game and I need to generate some numbers randomly but they need to be slightly fixed in terms of probability...firstly the user has an already generated number between 1 and 5 (this is not the problem) but that number they are given is then generated to give another number between 1-5 but the chances of this has to be fixed so that the lower numbers often give out a lower number and the higher numbers often give a higher number....hopefully the sums below will make it clearer...

so for example if you have been give a 1 the chances of your next number being a 1 is 25/50

number 1
1 = 25/50
2 = 15/50
3 = 5/50
4 = 4/50
5 = 1/50

number 2
1 = 10/50
2 = 25/50
3 = 10/50
4 = 3/50
5 = 2/50

number 3
1 = 2/50
2 = 10/50
3 = 25/50
4 = 10/50
5 = 3/50

number 4
1 = 1/50
2 = 4/50
3 = 10/50
4 = 25/50
5 = 10/50

number 5
1 = 1/50
2 = 4/50
3 = 5/50
4 = 15/50
5 = 25/50

can anyone help?

I usually do something like this:

PHP Code:

<?php
$first_random_number = rand(1,5);

$random_probability_number = rand(1,50); //Since all your denominators are 50.

$second_random_number = 0; //Initialize variable.

switch($first_random_number) {
  case 1:
    if ($random_probability_number == 1) {
      $second_random_number = 5;
    } else if ($random_probability_number <= 5) { //5 = 4 + 1
      $second_random_number = 4;
    } else if ($random_probability_number <= 10) { //10 = 5 + 4 + 1
      $second_random_number = 3;
    } else if ($random_probability_number <= 25) { //25 = 15 + 5 + 4 + 1
      $second_random_number = 2;
    } else {
      $second_random_number = 1;
    }
    break;
  case 2:
    //I'll let you finish
    break;
}

echo $second_random_number;
?>

I'm not giving you all of the code as you didn't indicate if this was for a homework assignment and you didn't provide a try yourself.

What you need is a variant of a random number generator with gaussian distribution.

I modified this function from something I found on php.net:

PHP Code:

<?php

function gauss()
{   
    $x=random_0_1();
    $y=random_0_1();
    $u=sqrt(-2*log($x))*cos(2*pi()*$y);
    
    return $u;
}

function gauss_ms($mean=0.0,$standardDev=1.0)
{   
    return round(abs(gauss()*$standardDev+$mean));
}
//generates a random number between 0 and 1 with psudo-even distribution
function random_0_1()
{   
    return (float)rand()/(float)getrandmax();
}

?>

What you'll want to do is call gauss_ms with the mean parameter as the number you want your random number to be closest to, you'll also need to use the modulus operator because there is a chance that the number generated can be higher than your max.

So if you want to generate a number close to 2 with a max of 5 do the following:

PHP Code:

<?
$x = guass_ms(2)%5;
?>

I didn't really test this code but it should work, hope it helps.

Also the solution Jeremy provided may work better but it requires a bit more code.