Re: Newbie - HashMap!

[Matthew Heaney <matthewjheaney@earthlink.net>]

fphsml@gmail.com writes:

> What is the equivalent of the following C++ STL code?
> 
> hash_map<string, string> hm;
> hm["key"] = "value";

Ada doesn't have a user-definable index operator, so to find the Ada
analog of the C++ code fragment above you have to unwind it a bit.  The
code above is equivalent to:

   typedef hash_map<string, string> hm_t;

   hm_t hm;

   typedef hm_t::iterator iter_t;
   typedef pair<iter_t, bool> status_t;

   const hm_t::value_type value(string("key"), string());

   const status_t status = hm.insert(value);

   status.first->second = "value";

The Ada equivalent is:

declare
   package Hashed_Map_Types is
      new Ada.Containers.Indefinite_Hashed_Maps
        (Key_Type     => String,
         Element_Type => String,
         Hash         => Ada.Strings.Hash,
         Equivalent_Keys => "=");

   use Hashed_Map_Types;

   HM : Map;

   C  : Cursor;
   B  : Boolean;
begin
   Insert (HM, "Key", "", C, B);
   Replace_Element (C, By => "Value");
end;

Note that in both the C++ and Ada cases, an element with a default value
is allocated, and then immediately assigned a new value.  It's probably
easier to say instead:

  Include (HM, "Key", "Value");

This inserts value "Value" if "Key" doesn't exist, and replaces the
existing value with "Value" if "Key" does exist.

The operations Insert, Include, and Replace all insert elements, but
vary by their exception behavior.


> I just need simple key lookup. Which library should I use?

You can use a reference implementation of the Ada 2005 standard
container library:

http://charles.tigris.org/source/browse/charles/src/ai302/

You want the indefinite hashed map: a-cihama.ad[sb] .

However, this library is written in Ada 2005, so it might not compile
for you.  (Check with your vendor.)

You could try using the Charles library, which is based on the STL, and
is written in Ada95:

http://charles.tigris.org/source/browse/charles/src/

The file is: charles-maps-hashed-strings-unbounded.ads .

However, the generic formal Element_Type is definite, so you'd probably
have to use an unbounded string:

package Hashed_Map_Types is
  new Charles.Maps.Hashed.Strings.Unbounded 
    (Unbounded_String);  --default everthing else

declare
   HM : Hashed_Map_Types.Container_Type;
begin
   Replace (HM, "Key", To_Unbounded_String ("Value"));
begin



-Matt