Re: on using array index, vectorized operation

["Nasser M. Abbasi" <nma@12000.org>]

On 3/27/2011 3:09 PM, Phil Clayton wrote:
> On Mar 27, 9:44 pm, "Nasser M. Abbasi"<n...@12000.org>  wrote:
>> Let me explain. Here is lax-wendroff scheme for 1-D advection pde, where
>> 'u' below is the solution:
>>
>> u(i):=u(i)-(a/2.0)*(u(i+1)-u(i-1))+(a**2)/2.0*(u(i-1)-2.0*u(i)+u(i+1));
>>
>> Where in the above, 'i' is the index range that I set up myself before.
>>
>> In Ada, I put the above line inside a loop over 'i'.
>

> I notice that the rhs mentions u(i+1) and u(i-1) so depending on
> whether you're decrementing or incrementing i, one of these will be
> overwritten on a previous loop iteration.  Presumably that isn't
> intended (looking at your vectorized code)?
>

Yes, sorry, ofcourse in the actuall code I have

LOOP over i
    u_new(i) = u(i)- .....
END LOOP

But when I posted it to this newsgroup, I was editing things by hand
to show.

Here is the actual code

http://12000.org/my_notes/solve_advection_1D_in_Ada/source/lax_wendroff_pkg.adb

>
>> Using your solution, I would have to write
>>
>> u(u'first+1..u'last-1) :=u(u'first+1..u'last-1) -
>>           (a/2.0)* (u(u'first+2..u'last) - u(u'first..u'last-2))+
>>           (a**2)/2.0*  ( u(u'first..u'last-2)-2.0*u(u'first+1..u'last-1) +
>>                           u(u'first+2..u'last)) ;
>
> Do we now have vectorized arithmetic operations, e.g. + on array
> types?
>
> Phil

--Nasser