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From: "Peter C. Chapin" <pcc482719@gmail.com>
Subject: Copying rows in a two dimensional array.
Newsgroups: comp.lang.ada
Date: Sun, 31 Jan 2010 21:11:12 -0500
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Date: 2010-01-31T21:11:12-05:00
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This is something of a newbie question..

I'm working with a two dimensional array of floating point values. Lets call
it A. The array has type

type Matrix is array(Positive range <>, Positive range <>) of Floating_Type;

I need to exchange two rows in this array. What I'd like to do is something
along these lines:

   Temp_Array      := A(I, 1 .. Size);
   A(I, 1 .. Size) := A(K, 1 .. Size);
   A(K, 1 .. Size) := Temp_Array;

The compiler (GNAT GPL 2009) has a problem with this syntax and, after looking
into it some, I think that's because slicing only works for one dimensional
arrays. Fair enough.

So I thought, "Perhaps A needs to be an array of arrays."

type Matrix is array(Positive range <>) of WHAT_EXACTLY?

Apparently the component type of an array needs to be fully constrained (which
again makes sense) yet I don't know the size I'll want to use at the point
where this type is declared.

So now I'm thinking that I'll have to write a procedure to explicity swap each
row element one at a time. Of course this is not a terrible thing, but I'm
wondering if there is a more elegant way that I'm missing. I have some
confidence that the compiler can optimize slice operations reasonably well.
I'm less confident about its ability to optimize element by element
operations (maybe I'm overly pessimistic).

Peter