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Newsgroups: comp.lang.ada
Subject: Re: Why does this work? (overloads)
From: Georg Bauhaus <bauhaus@futureapps.de>
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Date: Thu, 08 Feb 2007 12:34:54 +0100
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Date: 2007-02-08T12:34:40+01:00
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On Thu, 2007-02-08 at 02:40 -0800, Jerry wrote:
> Jeffrey R. Carter wrote:
> > Jerry wrote:
> >
> > > Yes, but  time series are usually indexed from 0. Sometimes, the FFT
> > > of a time series is indexed from 0 and sometimes -NN/2 .. NN/2-1. When
> > > one uses a time series in a vector or matrix, conundrums obviously
> > > arise, not only in computer code but in print. I once figured out how
> > > to make Pascal let me have it both ways by declaring both types, one
> > > type indexed 0..NN-1 and the other 1..NN, and then I set a pointer of
> > > one type to an array originally declared with another type. I'm
> > > guessing that Ada doesn't allow this, however.
> >
> > No pointers needed:
> >
> > A : Vector (0 .. N - 1);
> > B : Vector (1 .. A'Length);
> >
> > A := B;
> > B := A;
> >
> > This is called sliding.
> >
> Good point. However, it uses twice the memory, and worse, changes made
> in A are not reflected in B without repeating the assignment A := B
> (correct?) and vice versa. My Pascal trick had neither of these
> problems yet carried boundary checking using either name.

Not sure whether it is what you are after, but you could either use
pointers to a common unconstrained array type, or simply slice renaming
if possible:

with Ada.Text_IO; use Ada;
procedure sp is

    type Index is range 0 .. 666;

    type A is array (Index range <>) of Float;

    c: A(0 .. 3);
    x: A renames c(0 .. 2);
    y: A renames c(1 .. 3);

    function "*"(v, w: A) return Float;
      -- sum of products of pairwise matching components of `v` and `w`
      -- pre: v'length = w'length

    function "*"(v, w: A) return Float is
        result: Float := 0.0;
        j: Index := w'first;
    begin
        pragma assert(v'length = w'length);
        for k in v'range loop
            result := result + v(k) * w(j);
            j := j + 1;
        end loop;
        return result;
    end "*";

    package FIO is new Text_IO.Float_IO(Float);

begin
    FIO.Default_Aft := 2; FIO.Default_Exp := 0;
    c := (1.0, 3.2, 3.7, 2.4);
    FIO.Put(x * y); Text_IO.new_line;
    c(2) := 1.0;
    FIO.Put(x * y); Text_IO.new_line;
    x(2) := -1.3;
    FIO.Put(x * y); Text_IO.new_line;
    y(2) := +1.3;
    FIO.Put(x * y); Text_IO.new_line;
end sp;