Re: [igraph] igraph_matrix_copy_to()

[Gábor Csárdi]

Tamas,

you are right, this is one thing. But even if it is igraph_real_t the
compiler gives a warning (gcc) or error (g++), because
igraph_matrix_copy_to expects a pointer to a real, and not a pointer
to an array. See e.g.
http://www.lysator.liu.se/c/c-faq/c-2.html#2-10

Fernando, consider using a pointer to the first element of the array
instead of a pointer to an array. I think this can be done by a simple
casting, although I might miss something here, pointers to arrays are
tricky. Also, I don't know whether using g++ instead of gcc changes
anything here, except for getting an error instead of a warning.

But actually, why copying the thing anyway? What is wrong with keeping
it in an igraph_matrix_t?

Best,
Gabor

On Mon, Jun 28, 2010 at 8:22 PM, Tamas Nepusz <address@hidden> wrote:
> Hi Fernando,
>
> igraph matrices store real numbers, not integers, therefore long is not a 
> suitable data type for igraph_matrix_copy_to. Use an array of igraph_real_t 
> instead:
>
> igraph_real_t distances[numberofnodes][numberofnodes];
>
> --
> Tamas
>
> On 2010.06.28., at 18:44, Fernando Martinez wrote:
>
>> Dear All,
>>
>> I'm having problems when using the function igraph_matrix_copy_to()
>>
>> As I understand it, this function allows to copy an igraph matrix into a 
>> normal C matrix. However, I get an error when compiling.
>>
>> My code is the following:
>>
>> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
>>
>>
>>
>> #include <igraph.h>
>>
>> int main()
>> {
>>
>>
>>       igraph_matrix_t matriz;
>>       igraph_vector_t vec;
>>       igraph_vs_t from;
>>       igraph_neimode_t mode = IGRAPH_ALL;
>>
>>       long numberofnodes=512;
>>
>>       long distances[numberofnodes][numberofnodes];
>>
>>       igraph_vs_all(&from);
>>
>>       igraph_shortest_paths(&graph, &matriz, from, mode);
>>
>>       igraph_matrix_copy_to(&matriz, distances);
>>
>>       return(0);
>>
>> }
>>
>>
>> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
>>
>> I compile in Unbuntu 9.04
>>
>> g++ -I/igraph -L/lib w1024.cpp -ligraph -lgsl -lgslcblas -o w1024
>>
>> and gives error for the line of "igraph_matrix_copy_to(&matriz, distances);":
>>
>> error: cannot convert `long int (*)[((long unsigned int)((long 
>> int)numberofnodes))]' to `igraph_real_t*' for argument `2' to `int 
>> igraph_matrix_copy_to(const igraph_matrix_t*, igraph_real_t*)'
>>
>> Any ideas on how to pass the data from the igraph matrix to a C matrix?
>>
>>
>> Cheers,
>> Fernando
>>
>>
>>
>>
>>
>>
>>
>>
>> _______________________________________________
>> igraph-help mailing list
>> address@hidden
>> http://lists.nongnu.org/mailman/listinfo/igraph-help
>
>
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-- 
Gabor Csardi <address@hidden>     UNIL DGM