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Subject: Re: [hwloc-devel] What does this C99 notation mean?
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2011-02-14 08:14:58


On Feb 11, 2011, at 10:51 PM, Samuel Thibault wrote:

> Jeff Squyres, le Sat 12 Feb 2011 04:34:36 +0100, a écrit :
>> static void
>> hwloc_setup_groups_from_distances(struct hwloc_topology *topology,
>> unsigned nbobjs,
>> struct hwloc_obj **objs,
>> float *_distances)
>> {
>> float (*distances)[nbobjs][nbobjs] = (float (*)[nbobjs][nbobjs])_distances;
>>
>> As far as I can tell, _distances is a 1D array of floats, but it is of size (nbobjs*nbobjs) -- i.e., you could think of it as a 2D array, and index the i'th column in the j'th row at _distances[i * nbobjs + j]. From how its used in the code, distances is treated as a 2D array -- i.e., distances[i][j] is accessed.
>
> That's exactly it. It permits to keep code less clumsly to read.

Does it silently make a (temporary) 2nd array of pointers for the 2nd dimension indexing, or does the compiler translate all distances[i][j] to _distances[i*nbojbs + j] ?

-- 
Jeff Squyres
jsquyres_at_[hidden]
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