[PHP] Random Number Problem

[Mr Happy]

Basically I want to generate a Random number but increase the chances of lower numbers appearing more.

if I just use mt_rand(1, 1000) the chances of getting the number 1 is the same as 468 which is the same as 1000.

I want to increase the chances of lower numbers appearing.

This poor way shows what I mean

if (mt_rand(1, 2) == 1)   // 50-50 chance
{
    mt_rand(1, 10);
}
else if (mt_rand(1, 2) == 1)
{
    mt_rand(10, 100);
}
else
{
    mt_rand(100, 1000);
} 

So the chance of the number between 1 and 10 is higher than between 10 and 100 which is higher than 100 and 1000.

It's still pretty crap though as the chance of getting 124 is the same as 975 (not to mention it's generating upto 5 times to get one result).

I want a gradual increase in chance. I don't even know the mathematical term. Searching "php random sliding scale" gives random image scripts and "php random heavy slide" gives articles on the difference between rand() and mt_rand()

Can anyone help with an fast efficient function or even tell me the mathematical term for what I want to make searching easier?

Mr Happy Reviewed by Mr Happy on 2nd Jul 2010. [PHP] Random Number Problem Basically I want to generate a Random number but increase the chances of lower numbers appearing more. if I just use mt_rand(1, 1000) the chances of getting the number 1 is the same as 468 which is the same as 1000. I want to increase the chances of lower numbers appearing. This poor way shows what I mean if (mt_rand(1, 2) == 1) // 50-50 chance { Rating: 5

[jayfella]

hmmm. i have an idea.

lemme have a go.

[Hyperz]

$num = (time() & 1) ? mt_rand(1, 10) : mt_rand(1, 1000); 

Or Maybe

$num = (time() & 1) ? mt_rand(1, 10) : mt_rand(1, mt_rand(10, 1000)); 

[jayfella]

a bit more complicated, but you get the idea. Sorry, i dont know php, but you can of course see the logic.

static void DoWork()
        {
            Random rnd = new Random(5000);
            string output = string.Empty;

            int num1 = rnd.Next();
            int num2 = num1;
            num1 = rnd.Next();

            if (num1 > num2)
            {
                for (int i = 0; i < 5; i++) // 5 chances to be smaller than previous.
                {
                    num1 = rnd.Next();
                    if (num1 < num2)
                        break;
                }
            }

            output += num1.ToString() + Environment.NewLine;
        } 

[litewarez]

if(substr(time(),0,-1) > 7)
{
    //LOWER NUMBER (Chances are 7/10) of being lower
}else
{
   //Higher
} 

But the problem with the above is it will be lower for every 7 out of 10 seconds.

[jayfella]

why am i the only one writing the whole thing makes me look bloated

[litewarez]

function doWork()
{
    $rnd = rand(0,5000);

    $num1 = $num2 = $rnd+1;


    if(($num1+1) > $num2)
    {
        for($i=0;$i<5;$i++)
        {
            $num1 = $rnd+1;
        }
    }
    return $num1;
} 

Translated version for JayFella.

[Hyperz]

why am i the only one writing the whole thing makes me look bloated

My 1 line of code is the whole thing . It has 50% chance of generating a number between 1 and 10 and 50% chance of generating one between 1 and 1000 (using the 1st one). And its fast because its only 1 binary AND operation, one if-else and one rand().

[Mr Happy]

Ok so these are all far superior on what I used as an example but their's no way of doing this so
1 is bigger than 2 which is bigger than 3 which is bigger than 4 which is bigger than 5 .....

I asked a friend (Phd Mathematical Student) after making this topic and said I need to set up a matrix and add a curve or something where x=0 and y=1000 where points were picked but I didn't really understand what he was saying.

Anyway it's not that important as long as it's roughly weighted which we've all got. Just though their was a maths way. Maths is seriously not my strong point.

See while Hyperz way is fast the chances of getting 116 are the same as 959 which isn't exactly what I want.


Thanks for all the quick replies.

[Hyperz]

Oh I didn't think of a matrix, that would actually make it fairly effective and also easy to implement. I'll write another example with once I finish my coffee.