Re: Copying rows in a two dimensional array.

["Randy Brukardt" <randy@rrsoftware.com>]

"(see below)" <yaldnif.w@blueyonder.co.uk> wrote in message 
news:C7964E2E.135979%yaldnif.w@blueyonder.co.uk...
> On 08/02/2010 21:20, in article wccd40fpgpu.fsf@shell01.TheWorld.com,
> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote:
>
>> It's trivial if you only want slices as R-values.
>> And anyway, slices as L-values don't really work:
>
> I can't agree. I have this code:
>
> procedure FFT_to_HWT (FFTCs    : in  complex_array; ...
>                      HWT_tree : out complex_array; ...) is
> ...
>   iFFTCs : complex_array (1..f(FFTCs'length));
> begin
> ...
>   iFFTCs(1..nr_bins) := FFTCs(first_bin..last_bin);
>   iFFTCs(nr_bins+1..iFFT_size) := (others => (0.0,0.0));
>   inverse_FFT(iFFTCs(1..iFFT_size));
> ...
>   HWT_tree(next_HWTC..next_HWTC+iFFT_size-1) := iFFTCs(1..iFFT_size);
> ...
> end FFT_to_HWT;
>
> I think that using slices as L-values as well as R-values helps to make 
> this
> a lot clearer than it otherwise would be, and probably faster as well.

It's cases like this that make both the pro and con for slices. Expressions 
like these are probably easier to read and probably faster than the 
equivalent non-slice code.

But on the flip side, it's very hard to get the bounds right (I think I get 
these sorts of slices wrong 25% of the time). So you end up spending a lot 
of time debugging (at least Ada detects this). Moreover, it's hard to figure 
out from the code whether or not the bounds are right -- I'll often end 
drawing a picture and plug in various values to see if it makes sense. Case 
in point is the first statement; it fails unless last_bin - first_bin + 1 = 
nr_bins. So it's not clear how that is saving anything.

                                              Randy.