Ranges Constraints and Subtypes

Range Constraints

Range Constraints

• Variable declarations can constrain the set of legal values:

   procedure whatever is
      x: Integer;
      exam_score: Integer range 0 .. 100;
      exam_average: Float range 0.0 .. 100.0;
   begin
      exam_score := 95;  -- Okay
      --  exam_score := 195;  -- Error

      exam_score := exam_score + 10;  -- might cause error
      --  exam_score := exam_score + 100;  -- will cause error

      get(exam_score);  -- Could cause error

      get(x);
      exam_score := x;  -- Could cause error
   

Subtypes

Subtype Declarations

• Set of legal values can be named:

   procedure whatever is
      x: Integer;

      subtype Possible_Exam_Score is Integer range 0 .. 100;

      exam_score1: Possible_Exam_Score;
      exam_score2: Possible_Exam_Score;
   begin
      exam_score := 95;  -- Okay
      --  exam_score := 195;  -- Error

      exam_score := exam_score + 10;  -- might cause error
      --  exam_score := exam_score + 100;  -- will cause error

      get(exam_score);  -- Could cause error

      get(x);
      exam_score := x;  -- Could cause error
   

• Types and subtypes can be freely mixed in assignment, calls, and expressions

What is a Type

• A type is :
• A set of values, and
• A set of operations


• Example: Integers
• A set of values: All possible integers
• A set of operations: Arithmetic ops, IO, declaration, assignment


• Example: class Pair
• A set of values: All possible pairs
• A set of operations: Methods, including inherited

Subtypes and Type Checking

• A subtype is a subset of a type (ie subset of the values):
• A parent type, a subtype, or a sibling type can be used in a context where any are expected
• Example - all of the assignments below compile:

      i: Integer;
      n: Natural;
      p: Positive;
   begin
      i := p;  -- Expect integer, found positive
      p := i;  -- Expect positive, found integer
      n := p;  -- Expect natural, found positive
      p := i;  -- Expect positive, found integer
   

What is a Subtype

• A subtype is a subset of a type (ie subset of the values):

Anonymous Subtype

• An integer range is an anonymous subtype (ie it does not have a name)

Subtype can be used to Rename

• A subtype can be the whole set

   subtype NewName is Integer;