PureBasic Forums - English

Posted: **Sat Jan 06, 2007 1:40 pm**

hello,

sorry for the super noob question but,

how can i write my own random function??

thinking about it that a computer can pick a random number sounds
so unlogical to me!

i wish to also know how it is being done.

Posted: **Sat Jan 06, 2007 1:46 pm**

Most random functions are based on seed numbers and sometimes the time function get used, then some maths is performed on the seed, usually some kind of multiplication and an add and then the new number is used to make the new seed number.

Sounds complicated but it isn't really. The trick is trying to get the numbers to appear random.

Most computers return the same set of numbers each time if you set the seed to the same number.

Posted: **Sat Jan 06, 2007 1:48 pm**

yo hi.

i had some code on my harddisk (sent to me by a mate, not my code, but free-to-share)

Code: Select all

#m=100000000 
#m1=10000 
#b=31415821 

i.l 
Global a.l 
N.l 

Procedure mult(p.l,q.l) 
  p1.l 
  p0.l 
  q1.l 
  q0.l 
  p1=Int(p/#m1) 
  p0=p%#m1 
  q1=Int(q/#m1) 
  q0=q%#m1 
  ProcedureReturn (((p0*q1+p1*q0)%#m1)*#m1+p0*q0)%#m 
EndProcedure 

Procedure myrandom(r)  ;r=max 
  a=(mult(a,#b)+1)%#m 
  ProcedureReturn Int((Int(a/#m1)*r)/#m1)+1  ;numbers 1 to r 
EndProcedure 

N=10 
a=25;Random(12135)   ;seed 
For i=1 To N 
  Debug myrandom(11)  
Next

a random function is ANY function, that produces a row of numbers,
wich spreading is almost regular, meaning, none is significantly more often than another.

you could also use a sinus with a very short phase, to produce random numbers,
but with a sinus you will have significant more numbers at the border than in the middle...

PS:
historic remark:
the only true random ever was possible on the C64.
you could read the low-byte of the oscillator of the sound chip.
when you set the waveform to white noise, the values were perfectly random.

all other random functions are just seeming to be random, but are completely predictive.

(like Derek already stated, have read his post only after submitting mine)

Posted: **Sat Jan 06, 2007 2:14 pm**

Well there is near to true randomness under Linux aswell, there is the /dev/random dive that uses system noise to create random numbers and then there is /dev/urandom which usses the truely random seeds from the system to produce random numbers even when the /dev/random device is out of data. On some Intel MoBos these two functions may also use Hardware based Random number generators. If you want even more randomnes there are Hardware Random Number generators which for example use radioactive decay to generate absolutely unpredictable data. Because system noise can sometimes be tricked by putting the system in a vey loud environment playing montoneous sounds.

Posted: **Sat Jan 06, 2007 2:24 pm**

@KG: That was my code, translated from a Pascal code by Robert
Sedgewick, if I remember correctly

Posted: **Sat Jan 06, 2007 2:44 pm**

@remi
well, Leo gave it to me, i dunno, where it was from.
so, I hope it's ok that I posted it here.
I got it from Leo because I couldn't find some old C code i had in one of my books....
I was asked to create some Random-Class for the OOP-project in the Lounge...
(i haven't done it yet, because I cannot get familiar with the strange OOP-coding for PB)

Posted: **Sat Jan 06, 2007 2:53 pm**

No problem for me

. I just saw that you posted it somewhere else
in the past the same way, so I thought I would tell you. It's somewhere
deep in the Lounge

And I think it's a well-known algorithm, too. (just forgot the name of it)

Posted: **Sat Jan 06, 2007 11:18 pm**

Here's some slow code which generates a random number based on atmospheric noise. (connects to the net)

. To actually use this, you would have to create some sort of cache or something..

Code: Select all

NewList rand()
Procedure.l generaterandom(total,min,max)
  Shared rand()
  Url.s = "http://www.random.org/cgi-bin/randnum?num=1&min="+Str(min)+"&max="+Str(max)+"&Col=5"
  Buffer.s=Space(2048)  
  INET_RELOAD.l=$80000000 
  hInet.l=0: hURL.l=0: Bytes.l=0 
  hInet = InternetOpen_("PB@INET", OpenType, #Null, #Null, 0) 
  For x = 1 To total
    hURL = InternetOpenUrl_(hInet, Url, #Null, 0, INET_RELOAD, 0)
    res.s= ""
    Repeat 
      InternetReadFile_(hURL, @Buffer, Len(Buffer), @Bytes) 
      If Bytes > 0 
        res.s = res + Left(Buffer, Bytes) 
      EndIf 
    Until Bytes = 0
    InternetCloseHandle_(hURL) 
    AddElement(rand())
    rand() = Val(Trim(res))
  Next x
  InternetCloseHandle_(hInet)
EndProcedure 
Procedure.l getrandom()
  Shared rand()
  SelectElement(rand(),Random(CountList(rand())-1))
  ProcedureReturn rand()
EndProcedure
generaterandom(5,0,5)
MessageRequester("Random",Str(getrandom()))

Posted: **Sun Jan 07, 2007 5:52 am**

You may want to check out this well known website which deals with RNGs:
http://random.mat.sbg.ac.at/generators/

One of the best known algorithms is the Mersenne Twister:
http://www.math.sci.hiroshima-u.ac.jp/% ... 937ar.html