Re: How to use associative arrays in Ada 2005?

[Matthew Heaney <matthewjheaney@earthlink.net>]

Georg Bauhaus <bauhaus@futureapps.de> writes:

> I think that in this case the Ada.Containers requirement
> of being minimal building blocks applies. 

Right, the library is designed to be composable.  Here's a case where a
container needs to contain another container.


> If the elements in a map are containers themselves (or are
> otherwise big), you may want to use Update_Element.

Right.  Use Insert to create the element in the outer container, and then use
Update_Element to manipulate the inner container.


> That is in order to manipulate one of the 2nd level maps,
> you could either:

Ouch!  A lot of copying here...

> Or,
> 
> - Get a cursor for the element at key "family name".
> - Define a subprogram that manipulates the 2nd level map
>   (the one containing "name" as key).
> - Call Update_Element with the cursor and the subprogram.

Right, this is the correct approach.


>     Ages : Str_Map_Maps.Map; -- That's the "hash of a hash"
>     Named_Age: Str_Int_Maps.Map;

You don't need the Named_Age map.  There's a version of Insert that accepts
just a key (which is different from the other versions, which accept both key
and element).  


>     procedure Set_Age(name: String; value: in out Str_Int_Maps.Map) is
>     begin
>         Named_Age.Include("name", Age);  -- Note how `Age` is used here

Here you can just say:
          Value.Include ("name", Age);

>     end Set_Age;

> 
> begin
>     Ages.Insert("family name", Named_Age);

It's probably better to use the insert that returns a cursor, and then reuse
that cursor value, since that will obviate the need for a separate Find.  

You can also eliminate the Name_Age map object, if you use the Insert that
accepts a key only.  (If the key doesn't already exists, then it inserts an
element with a default value.  Here that would mean an empty map, which is just
what we want.)


>         Ages.Update_Element(position => Ages.Find("family name"),
>                             process => Set_Age'access); -- in situ

But note that Ages.Find duplicates the work of Ages.Insert (since Insert must
perform an internal search).  If you use an Insert that returns a cursor, then
you can just reuse the cursor, instead of Find'ing it again.  Your method works
but it's less efficient.


>     end loop;
> end book2;