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From: Phil Clayton <phil.clayton@lineone.net>
Newsgroups: comp.lang.ada
Subject: Re: on using array index, vectorized operation
Date: Sun, 27 Mar 2011 15:09:04 -0700 (PDT)
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Date: 2011-03-27T15:09:04-07:00
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On Mar 27, 9:44=A0pm, "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):=3Du(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)?


> Using your solution, I would have to write
>
> u(u'first+1..u'last-1) :=3Du(u'first+1..u'last-1) -
> =A0 =A0 =A0 =A0 =A0(a/2.0)* (u(u'first+2..u'last) - u(u'first..u'last-2))=
+
> =A0 =A0 =A0 =A0 =A0(a**2)/2.0* =A0( u(u'first..u'last-2)-2.0*u(u'first+1.=
.u'last-1) +
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0u(u'first+2..u'last)) =
;

Do we now have vectorized arithmetic operations, e.g. + on array
types?

Phil