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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Concerning subscript bounds checks
Date: 1996/07/01
Message-ID: <Dtv6A6.B5K@world.std.com>#1/1
X-Deja-AN: 163084332
references: <4qdj3e$btf@goanna.cs.rmit.EDU.AU>
 <4ql9eq$hdt@goanna.cs.rmit.EDU.AU> <4r1aep$7ga@natasha.rmii.com>
 <4r7r65$nj@goanna.cs.rmit.EDU.AU>
organization: The World Public Access UNIX, Brookline, MA
keywords: subscripts
newsgroups: comp.lang.ada
Date: 1996-07-01T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <4r7r65$nj@goanna.cs.rmit.EDU.AU>,
Richard A. O'Keefe <ok@goanna.cs.rmit.EDU.AU> wrote:
>	type Coord is (X,Y,Z);
>	type Point is array (Coord) of Float;
>So Simplex_Range has to be some other type.  The obvious thing was
>	subtype Simplex_Range is Natural range 0 .. Point'Length;
>(note that not only is the first value of I in this case not zero,
>it isn't even a number).  The next obvious thing is
>
>	for I in Point'Range loop
>	    let J be Simplex_Range'Val(
>		Point'Range'Pos(I) - Point'Range'Pos(Point'Range'First))
>	    P(J) := ...
>	
>However, I couldn't figure out any way to code that conversion from I to J.
>(The immediate problem is that Point'Range is a _range_, not a _(sub)type_,
>so Point'Range'Pos is inexpressible.)

What's wrong with "Coord'Pos(I) - Coord'Pos(Point'First)"?

By the way, if you have an array with unknown bounds, you can construct
a subtype for those bounds like this:

    subtype The_Range is The_Index_Type range The_Array'Range;

But there's no need to do that to use the 'Pos attribute.

Note that for T'Pos, the bounds of subtype T are ignored -- all that
matters is the type of that subtype.  For example:

    subtype S is Character range 'a'..'b';
    X: Integer := S'Pos('*'); -- No problem here.

S'Pos('*') is exactly the same as Character'Pos('*').

- Bob