Re: How to use associative arrays in Ada 2005?

[Matthew Heaney <matthewjheaney@earthlink.net>]

"snoopysalive" <matthias.kistler@gmx.de> writes:

> Can anybody explain me how to use associative arrays in Ada 2005? I
> mean hashes like in Perl. 

There are actually two kinds of "associative arrays" in Ada05: ordered maps and
hashed maps.  Each of these container types accept both a key and element as
generic formal types.  (There are also sets, that accept just an element type.)

If both the key and element are "definite" types (types that don't require a
constraint to specify the size), then you can use:

ada.containers.hashed_maps
ada.containers.ordered_maps

If either the key or element is an "indefinite" type (such as String), then you
can use:

ada.containers.indefinite_hashed_maps
ada.containers.indefinite_ordered_maps

If you use the hashed version, you'll need a hash function for your key type.
The library comes with a hash function for type String (and Wide_String, etc);
see Ada.Strings.Hash.

If you use the ordered version, you'll need a relation operation ("<") for your
key type.


> In my opinion, C++ is cool but Ada's syntax is
> better to avoid dirty code.

If you're familiar with the STL, then you should have no trouble with the Ada
container library.


> So, when I say "hash", I mean something Perl-like like
> "$hashname{key1}{key2} = 'something' ". Or something C++-like like "map
> <string,int> hashname; hashname["key1"] = 42;".

Right.  People often say "hash" but in reality that's just one way of
implementing an associative container.  In Ada05 you can use either the hashed
version or the ordered version.

Your example above would be:

package String_Integer_Maps is
  new Ada.Containers.Indefinite_Hashed_Maps
    (String,
     Integer,
     Ada.Strings.Hash,
     Equivalent_Keys => "=");

Note that there's no index operator (operator[]()) in Ada, so you say:

  procedure Op (M : in out String_Integer_Maps.Map) is
  begin
    M.Include ("key1", 42);
  end;

Note that this is not exactly equivalent to the C++ operation, since in Ada the
key is replaced too (if it already exists).  That's unnecessary here so a
slightly more efficient technique might be:

  procedure Op (M : in out String_Integer_Maps.Map) is
    C : Cursor;
    B : Boolean;
  begin
    M.Insert ("key1", 42, C, B);
    if not B then
      M.Replace_Element (C, 42);
    end if;
  end Op;


> Is there anything similar in Ada 2005, too? I bought the book
> "Programming in Ada 2005" by John Barnes but I don't understand the
> author's way of explaining a programming language (for me, the book is
> more confusing than explaining).

That book has a chapter (17?) on the container library.  If you have a specific
question just post here or send me some email.


> Or does anybody know a good tutorial similar to the lots of C- and
> Perl-tutorials? I found many Ada-tutorials but most of them just list
> the contents of the built-in packages and don't explain how to use
> them. As a beginner this is confusing.

If just gave a tutorial on the Ada05 container library.  I can give you the
power-point slides or send you a pdf version.  Drop me a line if you're
interested.

But as a said above, the Ada standard container library is very similar to the
C++ STL, so if you already know the latter than the former shouldn't be much of
a stretch.

-Matt