[GRASS-dev] using rand(x,y) in r.mapcalc (grass7)
Paulo van Breugel
p.vanbreugel at gmail.com
Thu Jul 3 00:39:58 PDT 2014
Just a quick additional question, how to set this GRASS_RND_SEED from
within a python script (I want to add the option to set the seed with a
seed parameter in my script, as suggested in the previous email).
On Thu, Jul 3, 2014 at 8:55 AM, Paulo van Breugel <p.vanbreugel at gmail.com>
> On 03-07-14 03:43, Vaclav Petras wrote:
> On Wed, Jul 2, 2014 at 8:15 PM, Glynn Clements <glynn at gclements.plus.com>
>> > Shouldn't the seed not be generated on e.g, OS time,
>> > which would ensure that each run would give a different result?
>> No. The reason is to provide reproducibility. Anyone running the same
>> command with the same data should obtain the same result.
> It is certainly be good to be able to reproduce commands. However, I
> think in most (statistical) software the default / expected behaviour is to
> have a new automatically generated seed at each run. In R for example, if
> you have to explicitly specify the seed using the function set.seed(). I
> would think therefore what most users will expect a similar behaviour in
> GRASS. It would certainly be my personal preference to have the option to
> set the seed explicitly if you want reproducibility, but have it generated
> automatically otherwise. But that is just a personal preference.
>> Does the reproducibility go behind one operating system, compiler or
> library? I don't think that the first random number is specified by the C
> language standard. If the results would be really reproducible it would be
> good for testing framework but I'm afraid that they are not (with my
> limited knowledge about the topic).
>> If you want a different result each time, set GRASS_RND_SEED to a
>> different value each time, e.g.
>> GRASS_RND_SEED=`date +%N` r.mapcalc "a = rand(0,100)"
>> [%N is the nanoseconds portion of the current time; this is a GNU
> Perhaps this can be explained like this in the manual page? A far
> better option would be to provide this as a normal parameter so it can be
> set from the gui interface or command line like any other variable.
> I've heard that this is not enough on powerful computers/clusters, that
> you have to use also PID because nanoseconds might be the same (I think I
> rememberer that it was nanoseconds not seconds).
>> > On a related note, it would be nice to be able to set the seed (I think
>> > there has been such a request before, but not sure about the answer at
>> > time).
>> GRASS_RND_SEED was the answer.
> I think there should be some possibility of randomization (auto-setting
> of seed) build-in the modules providing random(ized) results. Perhaps a
> flag which would turn it on. It can be also an option which would behave
> like GRASS_RND_SEED but would have one special value for auto-generating
> the seed. (GRASS_RND_SEED if present would override this option.) With the
> default value of the option we should ask a question what is actually the
> expected behavior of the module giving random results.
> Yes, that would be great. As for the default value, see my earlier
> This would provide a nicer interface in Python, standard interface in
> command line, and possibility to set it in the GUI (which means possibility
> to set it for users which don't use command line.) Moreover, it would
> provide all users with the way of setting the random seen in the manner
> which we consider the best according to our knowledge.
> Agree. The way to set the seed now may not be understood by everybody and
> with all the work going into streamlining the GUI, this kind of fairly
> important options should also be available through the GUI
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