Convert string to mathematical function

[Nick Vatamaniuc]

Jeremy,

Which method of extension are you using? For example you can pass the
function to C++/C directly using weave, without the need to convert to
Python first. Python has a different syntax than C++. Also I assume you
want exponentiation in you example "x^2..." and not 'xor', well, in
Python that would be "x**2" and in C you would have to include math.h
and then do pow(x,2). In other words there is no easy way to map Python
expressions to C++ (if it would be, Python could just all be mapped to
C++ and run at C++ speed!).

Below is my attempt at using weave (I prefer pyrex myself...). Note:
this won't work too fast if your function changes on-the-fly because
each new version will need to be compiled (gcc will be called and so
on...). So it would only pay off if your function gets compiled ones
and then takes a __very__ long time to compute.

Anyway here is my example:
--------------------------------------------------------------------------------------------
>>> import weave
>>> includes="""
... #include <math.h>
... """
>>> c_code="""
... return_val=pow(a,N);
... """
>>> a=42
>>> N=42
>>> ans=weave.inline(c_code, ['a','N'], support_code=includes)
file changed
None
cc1plus: warning: command line option "-Wstrict-prototypes" is valid
for Ada/C/ObjC but not for C++
>>> ans
1.5013093754529656e+68
>>> a**N
150130937545296572356771972164254457814047970568738777235893533016064L
>>> #now call the function again. no compilation this time, the result was cached!
>>> ans=weave.inline(c_code, ['a','N'], support_code=includes)
>>> ans
1.5013093754529656e+68
----------------------------------------------------------------------------------
Look up weave for Python (it is a part of scipy) for more examples...

Hope this helps,
Nick Vatamaniuc


jeremito wrote:
> I am extending python with C++ and need some help.  I would like to
> convert a string to a mathematical function and then make this a C++
> function.  My C++ code would then refer to this function to calculate
> what it needs.  For example I want to tell my function to calculate
> "x^2 + 3x +2", but later on I may want: "x + 3".  Does anybody know how
> I can get an arbitrary string in Python (but proper mathematical
> function) turned into a C++ function?  My one idea (although I don't
> know how to implement it, I'm still new at this) is to pass to C++ a
> pointer to a (Python) function.  Will this work?
> Thanks,
> Jeremy