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From: "Chad  R. Meiners" <chad.rmeiners@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: initialize an array (1-D) at elaboration using an expression
 based on the index?
Date: Thu, 21 Oct 2010 06:44:47 -0700 (PDT)
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Date: 2010-10-21T06:44:47-07:00
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On Oct 15, 7:03=A0pm, "Nasser M. Abbasi" <n...@12000.org> wrote:
> Ada experts:
>
> I am not able to find an example on this.
>
> I was wondering if in Ada I can initialize some array, where I can fill
> the content of each entry in the array based on some function of the inde=
x?
>
> For example, in Fortran one can do this:
> -----------------------
> program main
> =A0 =A0integer, parameter :: N =3D 10
> =A0 =A0real, dimension(N) :: a=3D(/(j**2,j=3D1,N)/)
>
> =A0 =A0print*, a
> end program
> ------------------------
> $ gfortran t3.f90
> $ ./a.exe
> =A0 =A0 1.0000000 =A0 =A0 =A0 4.0000000 =A0 =A0 =A0 9.0000000 =A0 =A0 =A0=
 16.000000 etc....
>
> In Ada, what would I need to do in the initialization below?
> ---------------------------
> procedure t2 is
> =A0 =A0 N: constant integer :=3D 10;
> =A0 =A0 a: array(1..N) of float :=3D (others=3D>0.0); --??
> begin
> =A0 =A0 null;
> end t2;
> -----------------------------
>
> I know ofcourse I can initialize it in the body by a loop, just wanted
> to see if it is possible to do it in the elaboration.
>
> thanks
> --Nasser

We can borrow a page from either ocaml and F# and set up a generic
initialize function.

I don't have access to a compiler and I haven't programmed in Ada for
years, but below is the general idea.

generic
  type T is array (I) of E;
  with function expression(Index: I) return E;
function init return T is
  val : T;
begin
  for index in I'Range loop
    val(index) :=3D expression(I);
  end loop;
  return val;
end function;

procedure t2 is
  N : constant Integer :=3D 10;

  type Foo is array(1..N) of float;
  function exp(j : Integer) return float is return float(j**2); end
function;
  function initFoo is new init(Foo,exp);

  a : Foo :=3D initFoo;
begin
  null;
end t2;

for those interested how it would be done in F# (which has a type
generic init function for arrays):

let t2 () =3D
  let N =3D 10 in
  let a =3D Array.init N (fun j -> let j =3D j+1 in j*j |> float)
  in ()