[Numpy-discussion] Strange behavior of the RandomArray module

[David M. Cooke]

On May 19, 2005, at 23:13, Tom Denniston wrote:

> I am running under python 1.4 and windows XP.

I hope you mean 2.4 :-)

> I get the following results in my python interpretter:
>
> --------------------------------------------
> n=0
> import RandomArray
> while 1:
>        arr = RandomArray.randint(0, 2509, 252)
>        for x in arr:
>                if x>2508:
>                        raise "Found number greater than 2508: " +
> str(x) + " on iteration " + str(n)
>        n=n+1
>
>
>
> Traceback (most recent call last):
>  File "<pyshell#43>", line 5, in -toplevel-
>    raise "Found number greater than 2508: " + str(x) + " on iteration
> " + str(n)
> Found number greater than 2508: 2509 on iteration 91719
>
> -------------------------------------------
>
> Even though the Numeric reference (see below) on RandomArray class
> claims that the number returned is always strictly less than the max.
> Does anyone
> know what might cause this problem?

This looks like it may be the same problem as bug #545336:

http://sourceforge.net/tracker/ 
index.php&func=detail&aid=545336&group_id=1369&atid=101369

I ran the above (generating 1e10 numbers). The above problem happened  
280 times, so it comes up at a rate of 2.8e-8 -- about what you'd  
expect for something going wrong with a last-bit error in rounding  
floats.

There is indeed some float-double casting going on in ranlibmodule.c;  
that's probably it. I've gone and replaced that in CVS, generating an  
array of doubles directly instead for random() and randint(). This  
fixes it for me.

[As an aside: note that the other random functions use floats.  
Really, an array of floats should be returned for those instead of an  
array of doubles. Next thing on the list :-)]

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|David M. Cooke              http://arbutus.physics.mcmaster.ca/dmc/
|cookedm at physics.mcmaster.ca