[GRASS5] r.random broken

[Michael Barton]

Hamish,

Thanks very much. I don't have the setup to compile to test. Though if a new
binary comes out in the next few days I will give it a go.

Michael


On 7/4/05 10:59 PM, "Hamish" <hamish_nospam at yahoo.com> wrote:

>> A month ago, I noted that r.random produces points all clustered along
>> the top of a region, instead of randomly distributed (see attached
>> output from r.random for 1000 points). You quickly identified the
>> problem. But it looks like it hasn¹t been fixed yet.
> 
> Following fix applied to 6.1-cvs.
> 
> In order to keep things generic, I applied this version:
>   (I have no idea what compilers people use)
> 
> #define lrand48() ((long)((double) rand() * (1<<31) / RAND_MAX))
> 
> 
>> Could this be fixed for 6.0.1?
> 
> Could someone please test it on Mac & Cygwin, if it works on both we can
> apply it to the 6.0 release branch too.
> 
> 
> 
> Hamish
> 
> ======================================================================
> http://grass.itc.it/pipermail/grass5/2005-June/018528.html
> [here slighly cropped -HB]
> 
> 
> From: Glynn Clements
> Date: Sun, 5 Jun 2005 01:12:05 +0100
> 
>>>>>> lrand48() is supposed to return random numbers between 0 and
>>>>>> 2^31. The above lrand48() macro will end up returning values
>>>>>> which are far too small, so, the random test will return true
>>>>>> too often, resulting in the desired number of random cells
>>>>>> being reached far too early.
> ..
>> lrand48() returns a value between 0 and 2^31.
>> 
>> rand() returns a value between 0 and RAND_MAX, where RAND_MAX is an
>> implementation-defined constant.
>> 
>> To emulate lrand48() using rand(), you would need something like:
>> 
>> #define lrand48() (rand() * (1<<31) / RAND_MAX)
>> 
>> Except that the above will overflow. To prevent that, convert to
>> double then back to long, e.g.:
>> 
>> #define lrand48() ((long)((double) rand() * (1<<31) / RAND_MAX))
>> 
>> Or, if your compiler supports it, use "long long":
>> 
>> #define lrand48() ((long)(rand() * (1LL<<31) / RAND_MAX))

____________________
C. Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
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