Re: How to use associative arrays in Ada 2005?

[Matthew Heaney <matthewjheaney@earthlink.net>]

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

> And now, my next question: How to handle "hashes of hashes" in Ada?

You have to make two distinct instantiations.


> procedure book is
>     package Str_Int_Maps is
>         new Ada.Containers.Indefinite_Hashed_Maps
>             (String,
>              Integer,
>              Ada.Strings.Hash,
>              "=");
>     use Str_Int_Maps;
>     package Str_Map_Maps is
>         new Ada.Containers.Indefinite_Hashed_Maps
>             (String,
>              Str_Int_Maps.Map,
>              Ada.Strings.Hash,
>              "=");
> 
>     Ages : Str_Map_Maps.Map; -- That's the "hash of a hash"
> 
> begin
>     Ages.Insert("family name",Insert("name",23));
> end book;
> -- Here, the code ends

You have to make two distinct insertions.  The first uses the form of Insert
that inserts a key and a default element value (the element here is itself
another map), and the second uses an in-place update to insert a key/elem pair
into the secondary map.  Something like:

declare
   C : Str_Map_Maps.Cursor;
   B : Boolean;
begin
   -- this is the insert that inserts an element with its
   -- "default value" into the map:
   Ages.Insert ("family name", C, B);  -- first insert

   -- C now designates the map we care about (note that
   -- it doesn't matter what value B has)

   declare
     procedure Update 
      (S : String;  -- the family name
       M : in out Str_Int_Maps.Map) is
     begin
       M.Insert ("name", 23);  -- second insert
     end;
   begin
     Ages.Update_Element (C, Update'Access);
   end;
end;
   

> The statement "Ages.Insert("family name",Insert("name",23));" doesn't
> work. So, how is it possible to do something like this in C++:
> "...
> map<string, map<string,int>> ages;
> ages["family name"]["name"] = 23;
> ..."

Ada doesn't have an index operator, so it's not going to be as concise as the
C++.  If you're doing this a lot then you can always refactor the code above
into its own stand-alone operation.  Something like:

procedure Insert
  (M : in out Str_Map_Maps.Map;
   Family_Name : String;
   Name : String;
   Age  : Integer) is ... -- as above