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.
On Thursday, October 16, 2014 2:29:14 PM UTC+1, Dima Pasechnik wrote:
>
> On 2014-10-16, Volker Braun <[email protected] <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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