On Thu, Jul 3, 2014 at 9:39 AM, Paulo van Breugel <p.vanbreu...@gmail.com> wrote:
> 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). > Concerning the question above, I found out how to do so. I used it in my r.random.weight script (in grass 7 addons svn). This script uses the rand() function in r.mapcalc. But rather than using the same seed (1), there is the option to set the seed, while as default the a time-dependent seed is set. I am sure there are better ways to do this, but it works. > > > On Thu, Jul 3, 2014 at 8:55 AM, Paulo van Breugel <p.vanbreu...@gmail.com> > wrote: > >> >> On 03-07-14 03:43, Vaclav Petras wrote: >> >> >> On Wed, Jul 2, 2014 at 8:15 PM, Glynn Clements <gl...@gclements.plus.com> >> wrote: >> >>> > 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 >>> extension.] >>> >> 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 >>> that >>> > 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 >> argument. >> >> >> 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 >> >> >> Vaclav >> >> >> >
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