On 2014-10-16, Volker Braun <vbraun.n...@gmail.com> wrote:
> It would be easy enough to use a matrix class instead of VLAs, e.g. slap on
>
>
> #include <boost/numeric/ublas/matrix.hpp>
> using namespace boost::numeric::ublas;
>
> and replace int a[n][n] with matrix<int> a(n, n) and a[i][j] with a(i,j).
>
> If you just want to pass in an array into your C99 code you can just lie to
> Cython, use something like
>
> void foo(int n, int a[n][n])
>
> in your C99 header but declare it as
>
> cdef extern from ...
> void foo(int n, int *a)
>
> in Cython.
yes, I do the latter, it works, but gives
a warning at the stage of compiling the cython-generated
C file.
Strange that Cython does not allow a proper cast, to (int (*)[])).
Dima
>
> On Thursday, October 16, 2014 2:29:14 PM UTC+1, Dima Pasechnik wrote:
>>
>> On 2014-10-16, Volker Braun <vbrau...@gmail.com <javascript:>> wrote:
>> > C99 VLA are not dynamically allocated, they exhaust the static stack
>> rather
>> > quickly. If you are writing the C part of the code yourself it is in the
>> > long run almost certainly better to put matrices on the heap. If you
>> have a
>> > maximal size for arrays you could use that as static bound. Or replace
>> them
>> > with a C++ matrix class that does the dynamic allocation (e.g.
>> > boost::numeric::ublas::matrix)
>>
>> it's something I wrote 20+ years ago, and uses a lot of a[i][j] syntax.
>> Allocation was done statically. So I changed it to VLAs, but I wouldn't
>> want to spend more time on it.
>>
>> It's not 100% clear how to use heap instead.
>> Apparently I can call a function
>>
>> void f(int n, int a[][n]){...}
>>
>> by doing (instead of `int b[n][n]; f(n,b);`)
>>
>> int *b;
>> b=(int *)malloc(n*n*sizeof(int));
>> f(n, b);
>>
>> but this gives scary warnings about types:
>>
>> warning: passing argument 2 of ‘f’ from incompatible pointer type
>> [enabled by default]
>> note: expected ‘int (*)[(sizetype)(n)]’ but argument is of type ‘int *’
>>
>> even though the code appears to be working.
>>
>>
>> >
>> >
>> >
>> >
>> > On Thursday, October 16, 2014 12:28:21 PM UTC+1, Dima Pasechnik wrote:
>> >>
>> >> In C99 it's possible to do dynamic allocation of multidimensional
>> arrays:
>> >>
>> >> void blah(int n) {
>> >> int a[n][n];
>> >> ...}
>> >>
>> >> but this does not seem to be supported in Cython. If I try
>> >> sage -cython on
>> >>
>> >> def t(int n):
>> >> cdef int a[n][n]
>> >> cdef int i, j
>> >> for i in range(n):
>> >> for j in range(n):
>> >> a[i][j]=i+j
>> >> return a[0][n-1]
>> >>
>> >> I get
>> >>
>> >> Error compiling Cython file:
>> >> ------------------------------------------------------------
>> >> ...
>> >> def t(int n):
>> >> cdef int i, j
>> >> cdef int a[n][n]
>> >> ^
>> >> ------------------------------------------------------------
>> >>
>> >> m.pyx:3:18: Not allowed in a constant expression
>> >>
>> >> As I have C99 code I'd like to hook up to Sage, I'd really
>> >> like to know how to get around this limitation (with as little overhead
>> >> as possible)
>> >>
>> >> Thanks,
>> >> Dima
>> >>
>> >>
>> >>
>> >
>>
>>
>
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