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Newsgroups: comp.lang.ada
Date: Fri, 12 Mar 2021 13:22:32 -0800 (PST)
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Subject: Re: array from static predicate on enumerated type
From: Egil H H <ehh.public@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:61504
List-Id: <comp.lang.ada>

On Friday, March 12, 2021 at 9:49:29 PM UTC+1, Matt Borchers wrote:
> Say, for example, I define a static predicate on a sub-type of an enumera=
ted type, like:=20
>=20
> type LETTERS is ( A, B, C, D, E, F, G, H, I , J, K );=20
>=20
> subtype CURVED is LETTERS=20
> with Static_Predicate CURVED in B | C | D | G | J;=20
>=20
> What I want is an array over CURVED (using CURVED as the index), but sinc=
e attributes 'First and 'Last (and thus 'Range) is not allowed, this cannot=
 be done.=20
>=20

You should read what the Rationale says about predicates (http://www.ada-au=
th.org/standards/12rat/html/Rat12-2-5.html),
especially the part about 'First_Valid and 'Last_Valid:

"These rules can also be illustrated by considering the dartboard. We might=
 like to accumulate a count of the number of times each particular score ha=
s been achieved. So we might like to declare
type Hit_Count is array (Score) of Integer;   -- illegal
but sadly this would result in an array with holes and so is forbidden. How=
ever, we could declare an array from 1 to 60 and then initialize it with 0 =
for those components used for hits and =E2=80=931 for the unused components=
. Of course, we ought not to repeat literals such as 1 and 60 because of po=
tential maintenance problems. But, we can use new attributes First_Valid an=
d Last_Valid thus
type Hit_Count is array (Score'First_Valid .. Score'Last_Valid) of Integer =
:=3D
       (Score =3D> 0, others =3D> =E2=80=931);
which uses Score to indicate the used components. The attributes First_Vali=
d and Last_Valid can be applied to any static subtype but are particularly =
useful with static predicates.
In detail, First_Valid returns the smallest valid value of the subtype. It =
takes any range and/or predicate into account whereas First only takes the =
range into account. Similarly Last_Valid returns the largest value. Inciden=
tally, they are illegal on null subtypes (because null subtypes have no val=
id values at all)."