Numpy only does machine datatypes and python objects. You can do a numpy array of python wrappers of mpfr reals but that would not be a good solution here.
If the library is under your own control, I would recommend to use C++ instead of C and then design the interface around std::vector<double>. This is easier than C arrays, and more easily wrapped in Cython as well. On Thursday, September 26, 2013 2:14:29 PM UTC+1, Dima Pasechnik wrote: > > On 2013-09-25, Volker Braun <vbrau...@gmail.com <javascript:>> wrote: > > A pointer to a dynamically malloc'ed array doesn't know how long the > array > > is. Your C library function must returns its size, too. You can then > read > > it out one-by-one but you can't call list(c_array) since the latter > doesn't > > even know how long it is. > > I read somewhere that it is convenient to use numpy arrays in such > a situation (they are stored intrenally as C arrays, and there are extra > Python hooks kept by numpy). > Not sure I can reproduce details here. > > > > > > > > > On Wednesday, September 25, 2013 5:02:55 PM UTC+1, mmarco wrote: > >> > >> I see, thanks. > >> > >> So, if i understand it correctly, i import my_c_function and then, to > call > >> it, i create the memory space for the array, copy the data into it and > pass > >> the array to the function. > >> > >> I guess the result will be another c array that i can access from > python > >> in a transparent way, right? > >> > >> I mean, if i write: > >> > >> res=my_c_function(c_values) > >> > >> Then i can just use > >> > >> list(res) > >> > >> to get a list of floats? > >> > >> > >> El miércoles, 25 de septiembre de 2013 13:22:51 UTC+2, Volker Braun > >> escribió: > >>> > >>> Definitely use Cython. > >>> > >>> For array of doubles, say, you just need a sage/libs/my_library.pyx > with > >>> > >>> include "stdsage.pxi" > >>> > >>> cdef extern from "my_library.h" > >>> my_c_function(double*) > >>> > >>> def my_python_function(values): > >>> cdef double * c_values = <double*> > >>> sage_malloc(sizeof(double)*len(values)) > >>> for i,v in enumerate(values): > >>> c_values[i] = values[i] > >>> my_c_function(c_values) > >>> > >>> > >>> > >>> > >>> > >>> > >>> On Wednesday, September 25, 2013 10:08:24 AM UTC+1, mmarco wrote: > >>>> > >>>> We are working on a c library to do homotoy continuation of > polynomial > >>>> roots using interval arithmetic. Our idea is to make a spkg with it, > and > >>>> write some functions in the sage library that would use it (in > particular, > >>>> to compute the fundamental group of the complement of an algebraic > curve). > >>>> so i have a question: > >>>> > >>>> how should we pass the data to the library, and retrieve it back? > Both > >>>> the input and output can be seen as an array of mpfr reals (or, > depending > >>>> on the version, floats or doubles). The length of the arrays is not > known a > >>>> priori. > >>>> > >>>> Which should be the best way to go? Write our interface in cython? or > >>>> use ctypes? And in any case, is there some easy tutorial that we > could > >>>> follow? > >>>> > >>>> Thanks in advance. > >>>> > >>> > > > > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/groups/opt_out.
