[Numpy-discussion] numarray.random_array number generation in C code

Todd Miller jmiller at stsci.edu
Wed Oct 6 23:47:31 CDT 2004

On Tue, 2004-10-05 at 21:10, Perry Greenfield wrote:
> Faheem Mitha wrote:
> > Dear People,
> >
> > I want to write some C++ code to link with Python, using the
> > Boost.Python interface. I need to generate random numbers in the C++
> > code, and I was wondering as to the best way of doing this.
> >
> > Note that it is important that the random number generation interoperate
> > seamlessly with Python, in the sense that the behavior of the calls to
> > the RNG is the same whether calls are made at the C level or the Python
> > level. I hope the reasons why this is important are obvious.
> >
> > I was thinking that the method should go like this.
> >
> > 1) When C/C++ code called, reads seed from python random state.
> >
> > 2) Does its stuff.
> >
> > 3) Writes seed back to python level when it exits.
> >
> > After doing a little investigation of the numarray.random_array python
> > library and associated extension modules, it seems possible that the
> > answer is simpler than I had supposed. However, I would appreciate it if
> > someone would tell me if my understanding is incorrect in some places.
> >
> > Summary: It seems that I can just call all the C entry point routines
> > defined in ranlib.h, without worrying about getting or setting seeds.
> >
> > Rationale:
> >
> > The structure of this random number facility has three parts, all
> > files in
> > Packages/RandomArray2/Src.
> >
> > 1) low-level C routines: Packages/RandomArray2/Src/com.c and
> > Packages/RandomArray2/Src/ranlib.c.
> >
> > com.c: basic RNG stuff; getting and setting seeds etc.
> > ranlib.c: Random number generator algorithms for different distributions
> > etc.
> >
> > 2) Python to C interface: Packages/RandomArray2/Src/ranlibmodule.c.
> >
> > This interfaces the stuff in com.c and ranlib.c.
> >
> > 3) Python wrapper: Packages/RandomArray2/Lib/RandomArray2.py.
> >
> > This wraps the C interface. In most cases it does not do much
> > else besides
> > some basic argument error checking.
> >
> > From my perspective, the important thing is that the random
> > number seed is
> > only defined at C level as a static object, all the RNG stuff
> > happens at C
> > level, and the Python code just calls the C code as necessary. (I'm
> > sketchy about the details of what is defined as the seed etc.)
> >
> > This is in contrast with the R RNG facility (the only other RNG
> > facility I
> > am familiar with), which uses macros SetRNGstate() and GetRNGstate() to
> > read and write the seed, which is defined at R level.
> >
> > Therefore, the upshot is that the C routines in ranlib.h read and write
> > the same seed as the python level functions do, so no special action is
> > necessary with regard to the seed.
> >
> > Is this correct?
> >
> > In any case, it would be nice if something like the above was documented,
> > so lost souls like myself don't have to go trawling through the source
> > code to figure out what is going on. Of course it is nice that the source
> > code is available, otherwise even that would be impossible.
> >
> > R documents this stuff in the "Writing R Extensions" manual, online at
> > http://cran.r-project.org/doc/manuals/R-exts.pdf. Perhaps the Numarray
> > manual could have a small section about this too.
> >
> >                                                          Regards, Faheem.
> >
> I'm not sure I understand what you want to do. Do you want to link
> directly to the extension code from your C++ code? If so I'm wondering
> why. It would make the most sense if the C++ code needed obtain
> small numbers of random numbers in some iterative loop, and you wish
> to use the same random number library that that numarray is using.
> Otherwise, I would normally obtain the random number array
> in python, then call the C++ extension. Perhaps I didn't read carefully
> enough. Normally linking to an extension module involves some hacks
> that I'm not sure were done for the randomarray module (the gory
> details are in the python docs for extension modules), Todd can
> check on that, 

I checked and there's no C level export of the ranlib interface, at
least not in the "hacked" sense of an extension module C-API where the
linkage is made indirect via an API pointer and bizarre macros.

> I'm not sure I will have time (a superficial check
> seems to indicate that it doesn't support direct linking, though
> one could link to the underlying library I suppose).

Ordinary C linkage to numarray.random_array.ranlib2 may be supported
since as an extension it is also a shared library, but I've never tried
it myself and I wonder if it would actually work. If anyone has tried
something like that I'd be interested in hearing how it turned out. 
Without a really compelling reason,  I'd avoid it myself.


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