Stuper Simple Ques - Random #'s

Agent420

I feel foolish that I can't figure this out after consulting both the manual and a forum search, but...

What is the best method to compute a random number for a desired range.· I'm not concerned about 'true' randomness, I just want to understand the ? operator.· I understand seed values and forward and backward sequences.

The manual states that the ? operator uses a variable's value as the seed and generates and number from NEGX to POSX.·

Say I want a random value from 1 to 10.· In the old Basic days, rnd returned a value from 0->1 that you simply mutliplied by the desired range.· Here it seems you would have to bit AND then modulus or something to scale the returned random value.

TIA


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heater

Just use the modulus operator on your random number

result = ?result // 10

Gives values 0 to 9 so just add 1.

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For me, the past is not over yet.

Agent420

Thank you.· I was totally overthinking this whole thing.· I am spending too much (or not enough) time calculating clock cycles and bit pattern masks to see the tree in the forest ;-)

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Ariba

But notice that the random number can be negativ and then also the modulo result is negativ, so you get a number in the range: -9..+9 with // 10 !

I had to learn this the hard way, with a program that generated 1 random number for every person on an art exibition. Because the first 5 numbers were all positiv, all my test runned OK, but then at the exibition the desaster started after a few persons.... (I got negative indexes into an array).

Andy

heater

Good point. Ouch!

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For me, the past is not over yet.

Roger Lee

wish I had a dime for every time I've "learned" that modulus trick.

I ask the same question a month or so ago.

All I needed was a bump in the right direction.

Roger

localroger

You could always mask the random number with something that eliminates the sign bit, THEN take the modulus for the range you want, e.g. result := (?seed & $7FFF_FFFF) // 10

You could also shift it right one bit, e.g. result := (?seed >> 1) // 10 which might take less token space since you don't need the 32 bit constant.

Or, probably even shorter, you could just take its absolute value before using it: result := (||(?seed))//10 Some of those parentheses might not be necessary, I'm doing this off the top of my head.

NetHog

Modulus gives a badly distributed random number unless its a power of 2 (but is often 'good enough')

On the subject of Random, I wrote this blog entry a year ago:

http://it.toolbox.com/blogs/codesharp/more-random-than-random-25106

Edit to add:

Fun exercise with random numbers

http://stackoverflow.com/questions/804316/how-do-i-implement-rand7-in-terms-of-rand5


Post Edited (NetHog) : 9/12/2009 2:18:07 AM GMT