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Date: Mon, 01 Feb 2010 08:55:01 +0200
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Organization: Tidorum Ltd
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Newsgroups: comp.lang.ada
Subject: Re: Copying rows in a two dimensional array.
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Date: 2010-02-01T08:55:01+02:00
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Peter C. Chapin wrote:
> 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.

One solution -- which may not match your design in other respects -- is 
to make the Matrix an array of accesses to rows:

    type Row is array (Positive range <>) of Float;
    type Row_Ref is access Row;
    type Matrix is array (Positive range <>) of Row_Ref;

Exchanging two rows is then very quick:

    Temp_Ref := A(I);
    A(I)     := A(K);
    A(K)     := Temp_Ref;

However, you now have to allocate the rows using "new Row (1 .. Size)" 
and perhaps later deallocate them using Unchecked_Deallocation.

Another solution -- which perhaps you have considered -- is to use a 
generic package to define row and matrix types:

    generic Size : Positive;
    package Matrices
    is
       type Row is array (1 .. Size) of Float;
       type Matrix is array (1 .. Size) of Row;
    end Matrices;

However, this will probably force some other parts of your code to 
become generic, too, parametrized either by a Size, or by an instance of 
Matrices.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .