Privacy

Directions

Goal: create a package that handles directions and geometric angles using a previous implementation.

Steps:

  1. Fix the implementation of the Test_Directions procedure.

Requirements:

  1. The implementation of the Test_Directions procedure must compile correctly.

Remarks:

  1. This exercise is based on the Directions exercise from the Records labs.

    1. In this version, however, Ext_Angle is a private type.

  2. In the implementation of the Test_Directions procedure below, the Ada developer tried to initialize All_Directions — an array of Ext_Angle type — with aggregates.

    1. Since we now have a private type, the compiler complains about this initialization.

  3. To fix the implementation of the Test_Directions procedure, you should use the appropriate function from the Directions package.

  4. The initialization of All_Directions in the code below contains a consistency error where the angle doesn't match the assessed direction.

    1. See if you can spot this error!

    2. This kind of errors can happen when record components that have correlated information are initialized individually without consistency checks — using private types helps to avoid the problem by requiring initialization routines that can enforce consistency.

package Directions is type Angle_Mod is mod 360; type Direction is (North, Northwest, West, Southwest, South, Southeast, East); function To_Direction (N : Angle_Mod) return Direction; type Ext_Angle is private; function To_Ext_Angle (N : Angle_Mod) return Ext_Angle; procedure Display (N : Ext_Angle); private type Ext_Angle is record Angle_Elem : Angle_Mod; Direction_Elem : Direction; end record; end Directions;
with Ada.Text_IO; use Ada.Text_IO; package body Directions is procedure Display (N : Ext_Angle) is begin Put_Line ("Angle: " & Angle_Mod'Image (N.Angle_Elem) & " => " & Direction'Image (N.Direction_Elem) & "."); end Display; function To_Direction (N : Angle_Mod) return Direction is begin case N is when 0 => return East; when 1 .. 89 => return Northwest; when 90 => return North; when 91 .. 179 => return Northwest; when 180 => return West; when 181 .. 269 => return Southwest; when 270 => return South; when 271 .. 359 => return Southeast; end case; end To_Direction; function To_Ext_Angle (N : Angle_Mod) return Ext_Angle is begin return (Angle_Elem => N, Direction_Elem => To_Direction (N)); end To_Ext_Angle; end Directions;
with Directions; use Directions; procedure Test_Directions is type Ext_Angle_Array is array (Positive range <>) of Ext_Angle; All_Directions : constant Ext_Angle_Array (1 .. 6) := ((0, East), (45, Northwest), (90, North), (91, North), (180, West), (270, South)); begin for I in All_Directions'Range loop Display (All_Directions (I)); end loop; end Test_Directions;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with Test_Directions; procedure Main is type Test_Case_Index is (Direction_Chk); procedure Check (TC : Test_Case_Index) is begin case TC is when Direction_Chk => Test_Directions; end case; end Check; begin if Argument_Count < 1 then Put_Line ("ERROR: missing arguments! Exiting..."); return; elsif Argument_Count > 1 then Put_Line ("Ignoring additional arguments..."); end if; Check (Test_Case_Index'Value (Argument (1))); end Main;

Limited Strings

Goal: work with limited private types.

Steps:

  1. Implement the Limited_Strings package.

    1. Implement the Copy function.

    2. Implement the = operator.

Requirements:

  1. For both Copy and =, the two parameters may refer to strings with different lengths. We'll limit the implementation to just take the minimum length:

    1. In case of copying the string "Hello World" to a string with 5 characters, the copied string is "Hello":

         S1 : constant Lim_String := Init ("Hello World");
         S2 :          Lim_String := Init (5);
      begin
         Copy (From => S1, To => S2);
         Put_Line (S2);     --  This displays "Hello".
      
    2. When comparing "Hello World" to "Hello", the = operator indicates that these strings are equivalent:

         S1 : constant Lim_String := Init ("Hello World");
         S2 : constant Lim_String := Init ("Hello");
      begin
         if S1 = S2 then
            --  True => This branch gets selected.
      
  2. When copying from a short string to a longer string, the remaining characters of the longer string must be initialized with underscores (_). For example:

       S1 : constant Lim_String := Init ("Hello");
       S2 :          Lim_String := Init (10);
    begin
       Copy (From => S1, To => S2);
       Put_Line (S2);     --  This displays "Hello_____".
    

Remarks:

  1. As we've discussed in the course:

    1. Variables of limited types have the following limitations:

      • they cannot be assigned to;

      • they don't have an equality operator (=).

    2. We can, however, define our own, custom subprograms to circumvent these limitations:

      • In order to copy instances of a limited type, we can define a custom Copy procedure.

      • In order to compare instances of a limited type, we can define an = operator.

  2. You can use the Min_Last constant — which is already declared in the implementation of these subprograms — in the code you write.

  3. Some details about the Limited_Strings package:

    1. The Lim_String type acts as a container for strings.

      1. In the the private part, Lim_String is declared as an access type to a String.

    2. There are two versions of the Init function that initializes an object of Lim_String type:

      1. The first one takes another string.

      2. The second one receives the number of characters for a string container.

    3. Procedure Put_Line displays object of Lim_String type.

    4. The design and implementation of the Limited_Strings package is very simplistic.

      1. A good design would have better handling of access types, for example.

package Limited_Strings is type Lim_String is limited private; function Init (S : String) return Lim_String; function Init (Max : Positive) return Lim_String; procedure Put_Line (LS : Lim_String); procedure Copy (From : Lim_String; To : in out Lim_String); function "=" (Ref, Dut : Lim_String) return Boolean; private type Lim_String is access String; end Limited_Strings;
with Ada.Text_IO; package body Limited_Strings is function Init (S : String) return Lim_String is LS : constant Lim_String := new String'(S); begin return Ls; end Init; function Init (Max : Positive) return Lim_String is LS : constant Lim_String := new String (1 .. Max); begin LS.all := (others => '_'); return LS; end Init; procedure Put_Line (LS : Lim_String) is begin Ada.Text_IO.Put_Line (LS.all); end Put_Line; function Get_Min_Last (A, B : Lim_String) return Positive is begin return Positive'Min (A'Last, B'Last); end Get_Min_Last; procedure Copy (From : Lim_String; To : in out Lim_String) is Min_Last : constant Positive := Get_Min_Last (From, To); begin -- Complete the implementation! null; end; function "=" (Ref, Dut : Lim_String) return Boolean is Min_Last : constant Positive := Get_Min_Last (Ref, Dut); begin -- Complete the implementation! return True; end; end Limited_Strings;
with Ada.Text_IO; use Ada.Text_IO; with Limited_Strings; use Limited_Strings; procedure Check_Lim_String is S : constant String := "----------"; S1 : constant Lim_String := Init ("Hello World"); S2 : constant Lim_String := Init (30); S3 : Lim_String := Init (5); S4 : Lim_String := Init (S & S & S); begin Put ("S1 => "); Put_Line (S1); Put ("S2 => "); Put_Line (S2); if S1 = S2 then Put_Line ("S1 is equal to S2."); else Put_Line ("S1 isn't equal to S2."); end if; Copy (From => S1, To => S3); Put ("S3 => "); Put_Line (S3); if S1 = S3 then Put_Line ("S1 is equal to S3."); else Put_Line ("S1 isn't equal to S3."); end if; Copy (From => S1, To => S4); Put ("S4 => "); Put_Line (S4); if S1 = S4 then Put_Line ("S1 is equal to S4."); else Put_Line ("S1 isn't equal to S4."); end if; end Check_Lim_String;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with Check_Lim_String; procedure Main is type Test_Case_Index is (Lim_String_Chk); procedure Check (TC : Test_Case_Index) is begin case TC is when Lim_String_Chk => Check_Lim_String; end case; end Check; begin if Argument_Count < 1 then Put_Line ("ERROR: missing arguments! Exiting..."); return; elsif Argument_Count > 1 then Put_Line ("Ignoring additional arguments..."); end if; Check (Test_Case_Index'Value (Argument (1))); end Main;

Bonus exercise

In previous labs, we had many source-code snippets containing records that could be declared private. The source-code for the exercise above (Directions) is an example: we've modified the type declaration of Ext_Angle, so that the record is now private. Encapsulating the record components — by declaring record components in the private part — makes the code safer. Also, because many of the code snippets weren't making use of record components directly (but handling record types via the API instead), they continue to work fine after these modifications.

This exercise doesn't contain any source-code. In fact, the goal here is to modify previous labs, so that the record declarations are made private. You can look into those labs, modify the type declarations, and recompile the code. The corresponding test-cases must still pass.

If no other changes are needed apart from changes in the declaration, then that indicates we have used good programming techniques in the original code. On the other hand, if further changes are needed, then you should investigate why this is the case.

Also note that, in some cases, you can move support types into the private part of the specification without affecting its compilation. This is the case, for example, for the People_Array type of the List of Names lab mentioned below. You should, in fact, keep only relevant types and subprograms in the public part and move all support declarations to the private part of the specification whenever possible.

Below, you find the selected labs that you can work on, including changes that you should make. In case you don't have a working version of the source-code of previous labs, you can look into the corresponding solutions.

Colors

Chapter: Records

Steps:

  1. Change declaration of RGB type to private.

Requirements:

  1. Implementation must compile correctly and test cases must pass.

List of Names

Chapter: Arrays

Steps:

  1. Change declaration of Person and People types to limited private.

  2. Move type declaration of People_Array to private part.

Requirements:

  1. Implementation must compile correctly and test cases must pass.

Price List

Chapter: More About Types

Steps:

  1. Change declaration of Price_List type to limited private.

Requirements:

  1. Implementation must compile correctly and test cases must pass.