Design by contracts

Price Range

Goal: use predicates to indicate the correct range of prices.

Steps:

  1. Complete the Prices package.

    1. Rewrite the type declaration of Price.

Requirements:

  1. Type Price must use a predicate instead of a range.

Remarks:

  1. As discussed in the course, ranges are a form of contract.

    1. For example, the subtype Price below indicates that a value of this subtype must always be positive:

      subtype Price is Amount range 0.0 .. Amount'Last;
      
    2. Interestingly, you can replace ranges by predicates, which is the goal of this exercise.

package Prices is type Amount is delta 10.0 ** (-2) digits 12; subtype Price is Amount range 0.0 .. Amount'Last; end Prices;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with System.Assertions; use System.Assertions; with Prices; use Prices; procedure Main is type Test_Case_Index is (Price_Range_Chk); procedure Check (TC : Test_Case_Index) is procedure Check_Range (A : Amount) is P : constant Price := A; begin Put_Line ("Price: " & Price'Image (P)); end Check_Range; begin case TC is when Price_Range_Chk => Check_Range (-2.0); end case; exception when Constraint_Error => Put_Line ("Constraint_Error detected (NOT as expected)."); when Assert_Failure => Put_Line ("Assert_Failure detected (as expected)."); 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;

Pythagorean Theorem: Predicate

Goal: use the Pythagorean theorem as a predicate.

Steps:

  1. Complete the Triangles package.

    1. Add a predicate to the Right_Triangle type.

Requirements:

  1. The Right_Triangle type must use the Pythagorean theorem as a predicate to ensure that its components are consistent.

Remarks:

  1. As you probably remember, the Pythagoras' theorem states that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides.

package Triangles is subtype Length is Integer; type Right_Triangle is record H : Length := 0; -- Hypotenuse C1, C2 : Length := 0; -- Catheti / legs end record; function Init (H, C1, C2 : Length) return Right_Triangle is ((H, C1, C2)); end Triangles;
package Triangles.IO is function Image (T : Right_Triangle) return String; end Triangles.IO;
package body Triangles.IO is function Image (T : Right_Triangle) return String is ("(" & Length'Image (T.H) & ", " & Length'Image (T.C1) & ", " & Length'Image (T.C2) & ")"); end Triangles.IO;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with System.Assertions; use System.Assertions; with Triangles; use Triangles; with Triangles.IO; use Triangles.IO; procedure Main is type Test_Case_Index is (Triangle_8_6_Pass_Chk, Triangle_8_6_Fail_Chk, Triangle_10_24_Pass_Chk, Triangle_10_24_Fail_Chk, Triangle_18_24_Pass_Chk, Triangle_18_24_Fail_Chk); procedure Check (TC : Test_Case_Index) is procedure Check_Triangle (H, C1, C2 : Length) is T : Right_Triangle; begin T := Init (H, C1, C2); Put_Line (Image (T)); exception when Constraint_Error => Put_Line ("Constraint_Error detected (NOT as expected)."); when Assert_Failure => Put_Line ("Assert_Failure detected (as expected)."); end Check_Triangle; begin case TC is when Triangle_8_6_Pass_Chk => Check_Triangle (10, 8, 6); when Triangle_8_6_Fail_Chk => Check_Triangle (12, 8, 6); when Triangle_10_24_Pass_Chk => Check_Triangle (26, 10, 24); when Triangle_10_24_Fail_Chk => Check_Triangle (12, 10, 24); when Triangle_18_24_Pass_Chk => Check_Triangle (30, 18, 24); when Triangle_18_24_Fail_Chk => Check_Triangle (32, 18, 24); 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;

Pythagorean Theorem: Precondition

Goal: use the Pythagorean theorem as a precondition.

Steps:

  1. Complete the Triangles package.

    1. Add a precondition to the Init function.

Requirements:

  1. The Init function must use the Pythagorean theorem as a precondition to ensure that the input values are consistent.

Remarks:

  1. In this exercise, you'll work again with the Right_Triangle type.

    1. This time, your job is to use a precondition instead of a predicate.

    2. The precondition is applied to the Init function, not to the Right_Triangle type.

package Triangles is subtype Length is Integer; type Right_Triangle is record H : Length := 0; -- Hypotenuse C1, C2 : Length := 0; -- Catheti / legs end record; function Init (H, C1, C2 : Length) return Right_Triangle is ((H, C1, C2)); end Triangles;
package Triangles.IO is function Image (T : Right_Triangle) return String; end Triangles.IO;
package body Triangles.IO is function Image (T : Right_Triangle) return String is ("(" & Length'Image (T.H) & ", " & Length'Image (T.C1) & ", " & Length'Image (T.C2) & ")"); end Triangles.IO;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with System.Assertions; use System.Assertions; with Triangles; use Triangles; with Triangles.IO; use Triangles.IO; procedure Main is type Test_Case_Index is (Triangle_8_6_Pass_Chk, Triangle_8_6_Fail_Chk, Triangle_10_24_Pass_Chk, Triangle_10_24_Fail_Chk, Triangle_18_24_Pass_Chk, Triangle_18_24_Fail_Chk); procedure Check (TC : Test_Case_Index) is procedure Check_Triangle (H, C1, C2 : Length) is T : Right_Triangle; begin T := Init (H, C1, C2); Put_Line (Image (T)); exception when Constraint_Error => Put_Line ("Constraint_Error detected (NOT as expected)."); when Assert_Failure => Put_Line ("Assert_Failure detected (as expected)."); end Check_Triangle; begin case TC is when Triangle_8_6_Pass_Chk => Check_Triangle (10, 8, 6); when Triangle_8_6_Fail_Chk => Check_Triangle (12, 8, 6); when Triangle_10_24_Pass_Chk => Check_Triangle (26, 10, 24); when Triangle_10_24_Fail_Chk => Check_Triangle (12, 10, 24); when Triangle_18_24_Pass_Chk => Check_Triangle (30, 18, 24); when Triangle_18_24_Fail_Chk => Check_Triangle (32, 18, 24); 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;

Pythagorean Theorem: Postcondition

Goal: use the Pythagorean theorem as a postcondition.

Steps:

  1. Complete the Triangles package.

    1. Add a postcondition to the Init function.

Requirements:

  1. The Init function must use the Pythagorean theorem as a postcondition to ensure that the returned object is consistent.

Remarks:

  1. In this exercise, you'll work again with the Triangles package.

    1. This time, your job is to apply a postcondition instead of a precondition to the Init function.

package Triangles is subtype Length is Integer; type Right_Triangle is record H : Length := 0; -- Hypotenuse C1, C2 : Length := 0; -- Catheti / legs end record; function Init (H, C1, C2 : Length) return Right_Triangle is ((H, C1, C2)); end Triangles;
package Triangles.IO is function Image (T : Right_Triangle) return String; end Triangles.IO;
package body Triangles.IO is function Image (T : Right_Triangle) return String is ("(" & Length'Image (T.H) & ", " & Length'Image (T.C1) & ", " & Length'Image (T.C2) & ")"); end Triangles.IO;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with System.Assertions; use System.Assertions; with Triangles; use Triangles; with Triangles.IO; use Triangles.IO; procedure Main is type Test_Case_Index is (Triangle_8_6_Pass_Chk, Triangle_8_6_Fail_Chk, Triangle_10_24_Pass_Chk, Triangle_10_24_Fail_Chk, Triangle_18_24_Pass_Chk, Triangle_18_24_Fail_Chk); procedure Check (TC : Test_Case_Index) is procedure Check_Triangle (H, C1, C2 : Length) is T : Right_Triangle; begin T := Init (H, C1, C2); Put_Line (Image (T)); exception when Constraint_Error => Put_Line ("Constraint_Error detected (NOT as expected)."); when Assert_Failure => Put_Line ("Assert_Failure detected (as expected)."); end Check_Triangle; begin case TC is when Triangle_8_6_Pass_Chk => Check_Triangle (10, 8, 6); when Triangle_8_6_Fail_Chk => Check_Triangle (12, 8, 6); when Triangle_10_24_Pass_Chk => Check_Triangle (26, 10, 24); when Triangle_10_24_Fail_Chk => Check_Triangle (12, 10, 24); when Triangle_18_24_Pass_Chk => Check_Triangle (30, 18, 24); when Triangle_18_24_Fail_Chk => Check_Triangle (32, 18, 24); 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;

Pythagorean Theorem: Type Invariant

Goal: use the Pythagorean theorem as a type invariant.

Steps:

  1. Complete the Triangles package.

    1. Add a type invariant to the Right_Triangle type.

Requirements:

  1. Right_Triangle is a private type.

    1. It must use the Pythagorean theorem as a type invariant to ensure that its encapsulated components are consistent.

Remarks:

  1. In this exercise, Right_Triangle is declared as a private type.

    1. In this case, we use a type invariant for Right_Triangle to check the Pythagorean theorem.

  2. As a bonus, after completing the exercise, you may analyze the effect that default values have on type invariants.

    1. For example, the declaration of Right_Triangle uses zero as the default values of the three triangle lengths.

    2. If you replace those default values with Length'Last, you'll get different results.

    3. Make sure you understand why this is happening.

package Triangles is subtype Length is Integer; type Right_Triangle is private; function Init (H, C1, C2 : Length) return Right_Triangle; private type Right_Triangle is record H : Length := 0; -- Hypotenuse C1, C2 : Length := 0; -- Catheti / legs end record; function Init (H, C1, C2 : Length) return Right_Triangle is ((H, C1, C2)); end Triangles;
package Triangles.IO is function Image (T : Right_Triangle) return String; end Triangles.IO;
package body Triangles.IO is function Image (T : Right_Triangle) return String is ("(" & Length'Image (T.H) & ", " & Length'Image (T.C1) & ", " & Length'Image (T.C2) & ")"); end Triangles.IO;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with System.Assertions; use System.Assertions; with Triangles; use Triangles; with Triangles.IO; use Triangles.IO; procedure Main is type Test_Case_Index is (Triangle_8_6_Pass_Chk, Triangle_8_6_Fail_Chk, Triangle_10_24_Pass_Chk, Triangle_10_24_Fail_Chk, Triangle_18_24_Pass_Chk, Triangle_18_24_Fail_Chk); procedure Check (TC : Test_Case_Index) is procedure Check_Triangle (H, C1, C2 : Length) is T : Right_Triangle; begin T := Init (H, C1, C2); Put_Line (Image (T)); exception when Constraint_Error => Put_Line ("Constraint_Error detected (NOT as expected)."); when Assert_Failure => Put_Line ("Assert_Failure detected (as expected)."); end Check_Triangle; begin case TC is when Triangle_8_6_Pass_Chk => Check_Triangle (10, 8, 6); when Triangle_8_6_Fail_Chk => Check_Triangle (12, 8, 6); when Triangle_10_24_Pass_Chk => Check_Triangle (26, 10, 24); when Triangle_10_24_Fail_Chk => Check_Triangle (12, 10, 24); when Triangle_18_24_Pass_Chk => Check_Triangle (30, 18, 24); when Triangle_18_24_Fail_Chk => Check_Triangle (32, 18, 24); 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;

Primary Color

Goal: extend a package for HTML colors so that it can handle primary colors.

Steps:

  1. Complete the Color_Types package.

    1. Declare the HTML_RGB_Color subtype.

    2. Implement the To_Int_Color function.

Requirements:

  1. The HTML_Color type is an enumeration that contains a list of HTML colors.

  2. The To_RGB_Lookup_Table array implements a lookup-table to convert the colors into a hexadecimal value using RGB color components (i.e. Red, Green and Blue)

  3. Function To_Int_Color extracts one of the RGB components of an HTML color and returns its hexadecimal value.

    1. The function has two parameters:

      • First parameter is the HTML color (HTML_Color type).

      • Second parameter indicates which RGB component is to be extracted from the HTML color (HTML_RGB_Color subtype).

    2. For example, if we call To_Int_Color (Salmon, Red), the function returns #FA,

      • This is the hexadecimal value of the red component of the Salmon color.

      • You can find further remarks below about this color as an example.

  4. The HTML_RGB_Color subtype is limited to the primary RGB colors components (i.e. Red, Green and Blue).

    1. This subtype is used to select the RGB component in calls to To_Int_Color.

    2. You must use a predicate in the type declaration.

Remarks:

  1. In this exercise, we reuse the code of the Colors: Lookup-Table exercise from the Arrays labs.

  2. These are the hexadecimal values of the colors that we used in the original exercise:

    Color

    Value

    Salmon

    #FA8072

    Firebrick

    #B22222

    Red

    #FF0000

    Darkred

    #8B0000

    Lime

    #00FF00

    Forestgreen

    #228B22

    Green

    #008000

    Darkgreen

    #006400

    Blue

    #0000FF

    Mediumblue

    #0000CD

    Darkblue

    #00008B

  3. You can extract the hexadecimal value of each primary color by splitting the values from the table above into three hexadecimal values with two digits each.

    • For example, the hexadecimal value of Salmon is #FA8072, where:

      • the first part of this hexadecimal value (#FA) corresponds to the red component,

      • the second part (#80) corresponds to the green component, and

      • the last part (#72) corresponds to the blue component.

package Color_Types is type HTML_Color is (Salmon, Firebrick, Red, Darkred, Lime, Forestgreen, Green, Darkgreen, Blue, Mediumblue, Darkblue); subtype Int_Color is Integer range 0 .. 255; function Image (I : Int_Color) return String; type RGB is record Red : Int_Color; Green : Int_Color; Blue : Int_Color; end record; function To_RGB (C : HTML_Color) return RGB; function Image (C : RGB) return String; type HTML_Color_RGB_Array is array (HTML_Color) of RGB; To_RGB_Lookup_Table : constant HTML_Color_RGB_Array := (Salmon => (16#FA#, 16#80#, 16#72#), Firebrick => (16#B2#, 16#22#, 16#22#), Red => (16#FF#, 16#00#, 16#00#), Darkred => (16#8B#, 16#00#, 16#00#), Lime => (16#00#, 16#FF#, 16#00#), Forestgreen => (16#22#, 16#8B#, 16#22#), Green => (16#00#, 16#80#, 16#00#), Darkgreen => (16#00#, 16#64#, 16#00#), Blue => (16#00#, 16#00#, 16#FF#), Mediumblue => (16#00#, 16#00#, 16#CD#), Darkblue => (16#00#, 16#00#, 16#8B#)); subtype HTML_RGB_Color is HTML_Color; function To_Int_Color (C : HTML_Color; S : HTML_RGB_Color) return Int_Color; -- Convert to hexadecimal value for the selected RGB component S end Color_Types;
with Ada.Integer_Text_IO; package body Color_Types is function To_RGB (C : HTML_Color) return RGB is begin return To_RGB_Lookup_Table (C); end To_RGB; function To_Int_Color (C : HTML_Color; S : HTML_RGB_Color) return Int_Color is begin -- Implement function! return 0; end To_Int_Color; function Image (I : Int_Color) return String is subtype Str_Range is Integer range 1 .. 10; S : String (Str_Range); begin Ada.Integer_Text_IO.Put (To => S, Item => I, Base => 16); return S; end Image; function Image (C : RGB) return String is begin return ("(Red => " & Image (C.Red) & ", Green => " & Image (C.Green) & ", Blue => " & Image (C.Blue) &")"); end Image; end Color_Types;
with Ada.Command_Line; use Ada.Command_Line; with Ada.Text_IO; use Ada.Text_IO; with Color_Types; use Color_Types; procedure Main is type Test_Case_Index is (HTML_Color_Red_Chk, HTML_Color_Green_Chk, HTML_Color_Blue_Chk); procedure Check (TC : Test_Case_Index) is procedure Check_HTML_Colors (S : HTML_RGB_Color) is begin Put_Line ("Selected: " & HTML_RGB_Color'Image (S)); for I in HTML_Color'Range loop Put_Line (HTML_Color'Image (I) & " => " & Image (To_Int_Color (I, S)) & "."); end loop; end Check_HTML_Colors; begin case TC is when HTML_Color_Red_Chk => Check_HTML_Colors (Red); when HTML_Color_Green_Chk => Check_HTML_Colors (Green); when HTML_Color_Blue_Chk => Check_HTML_Colors (Blue); 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;