[SciPy-User] Help of understanding C code in weave

[Zheng Ruan]

Thank you for the explanation! That makes more sense to me.

Zheng

On Thu, Jan 29, 2015 at 3:22 AM, Santosh Kushwaha <
santoshvkushwaha at gmail.com> wrote:

>  On 01/29/2015 04:04 AM, Zheng Ruan wrote:
>
> Hi Scipy users,
>
>  I am trying to read code that uses scipy.weave and I don't understand
> some of the code in C. To make it easy and clear, I'll just post the part
> that confused me.
>
>  I have a numpy array (a) with a shape of (2, 623, 3, 333). And another
> array numpy array (d) with a shape of (1, 623, 623).
>
>  In the C code part I have something like this:
>
>  code = """
> ...
> c = *(a + b);
> *(d+b) += 1;
> ...
> """
>
>  In the above code, b and c are float type in C. I just don't understand
> how the c value are calculated and what does "*(d+b) += 1;" do.
>
>  The code is very old and I saw some deprecated warnings when I compile
> it.
>
>  Thank you so much and any hints are welcome!!!
>
>  Zheng
>
>
> _______________________________________________
> SciPy-User mailing listSciPy-User at scipy.orghttp://mail.scipy.org/mailman/listinfo/scipy-user
>
>  In C language, the name of an array can be considered just as a pointer
> to the beginning of a memory block as big as the array. Only difference
> being that compiler keeps some extra info for the arrays to keep track of
> their storage requirements.
> you can check it by:
> sizeof(pointer) and sizeof(array);
> last one should print the sum of size of all element in the array
>  So, back to your question, as 'a' and 'd' are the arrays it could be
> written as simply a or d respectively.
> i.e.  array = pointer,
>
>  and when you put an asterisk  or indirection to your pointer variable or
> array variable as in your case it simply means that you are dereferencing
> the value of array at the beginning of the array. i.e. value at array[0];
> there are another forms to write it as well like,
> *array = array[index], where index = 0, both are same.
>
> in case you are retrieving the value of an array at a particular
> index/offset the it would be like:
> *(array+index) = array[index/offset],   both are same.
>
> c = *(a+b);
> in above code 'a' is the array, 'b' is the index of the array and *(a+b)
> is the value of the array at position 'b', So basically you are assigning
> the vale of *(a+b) to variable 'c'.
>
> *(d+b) += 1;
> value of *(d+b) is incremented by 1.
>
> And the 'b' and 'c' which are float, would be implicitly cast to int by
> numpy
> --
> Regards,
> Santosh Kushwaha
>
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>
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