P.S. here's an implementation of the index generating scheme you described:
#include <stdlib.h>
int * reverseOfTranspose(int n) {
int * r= malloc(n*n*sizeof n);
if (!r) return r;
for (int j= 0; j < n; j++)
for (int k= 0; k < n; k++)
r[j+n*k]= j*n+n-k;
return r;
}
I used index arithmetic instead of multi-dimensional indices because
that was easier.
I hope that mail does not mess up my indentation.
It's not tested, so I might have made a mistake, but I think this
should at least be close to what you want.
Nothing here is J programming, so I've sent this to chat.
FYI,
--
Raul
On Mon, Jan 28, 2013 at 1:30 PM, Raul Miller <[email protected]> wrote:
> The C approach to doing transpose typically involves swapping the
> indices when they are used, and not bothering to make a fresh copy (or
> updated copy) of the data.
>
> This of course yields a fair bit of complexity which gets pushed into
> the code (or into the objects) but that's how it's usually done.
>
> That said, you can achieve the same thing by explicitly copying the
> matrix using swapped indices. In a loop; b[j][k]= a[k][j]
>
> Or am I overlooking something important?
>
> (Also, perhaps this thread should be in chat?)
>
> Thanks,
>
> --
> Raul
>
> On Mon, Jan 28, 2013 at 12:58 PM, Brian Schott <[email protected]> wrote:
>> I am trying to use objective C and am using j to develop some
>> pieces. I have the verb `transposeSquare` which transposes a square matrix's
>> indices using indices and reproduces j's Transpose. In C code the indices
>> are named ii and are being generated in a for loop as follows, where width
>> and height are actually both 13 but my j examples use 3 instead of 13.
>>
>> for (int ii = 0 ; ii < width * height ; ++ii)
>>
>> transposeSquare =: (] <.@% %:@#)+#| %:@# *]
>> transposeSquare i. 9
>> 0 3 6 1 4 7 2 5 8
>> ,|:i. 3 3 NB. this is the "J way"
>> 0 3 6 1 4 7 2 5 8
>>
>> Now with C code for loop in mind I need a verb which first does what
>> transposeSquare does and then reverses the indices using the operators
>> available to C so that the result of the verb would be as follows.
>>
>> ,|.|:i. 3 3 NB. this is the "J way"
>> 2 5 8 1 4 7 0 3 6
>>
>> That is, I need a verb (say, reverseOfTranspose, rOT), which gives
>> the same result as ,|.|:i. 3 3 but with the scalar argument 3 and is limited
>> to C operators.
>>
>> (B=)
>> ----------------------------------------------------------------------
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