5.4 Case Statements

1. A case_statement selects for execution one of a number of alternative sequences_of_statements; the chosen alternative is defined by the value of an expression.
   Syntax
   

2. case_statement ::=
      case expression is
         case_statement_alternative
         {case_statement_alternative}
      end case;
   

3. case_statement_alternative ::=
      when discrete_choice_list =>
         sequence_of_statements
   

   Name Resolution Rules
   
4. The expression is expected to be of any discrete type. The expected type for each discrete_choice is the type of the expression.
   Legality Rules
   
5. The expressions and discrete_ranges given as discrete_choices of a case_statement shall be static. A discrete_choice others, if present, shall appear alone and in the last discrete_choice_list.
6. The possible values of the expression shall be covered as follows:
   1. If the expression is a name (including a type_conversion or a function_call) having a static and constrained nominal subtype, or is a qualified_expression whose subtype_mark denotes a static and constrained scalar subtype, then each non-others discrete_choice shall cover only values in that subtype, and each value of that subtype shall be covered by some discrete_choice (either explicitly or by others).
   2. If the type of the expression is root_integer, universal_integer, or a descendant of a formal scalar type, then the case_statement shall have an others discrete_choice.
   3. Otherwise, each value of the base range of the type of the expression shall be covered (either explicitly or by others).

1. Two distinct discrete_choices of a case_statement shall not cover the same value.
   Dynamic Semantics
   
2. For the execution of a case_statement the expression is first evaluated.
3. If the value of the expression is covered by the discrete_choice_list of some case_statement_alternative, then the sequence_of_statements of the _alternative is executed.
4. Otherwise (the value is not covered by any discrete_choice_list, perhaps due to being outside the base range), Constraint_Error is raised.
   

   NOTES
   

5. (5) The execution of a case_statement chooses one and only one alternative. Qualification of the expression of a case_statement by a static subtype can often be used to limit the number of choices that need be given explicitly.
   Examples
   
6. Examples of case statements:

7. case Sensor is
      when Elevation  => Record_Elevation(Sensor_Value);
      when Azimuth    => Record_Azimuth  (Sensor_Value);
      when Distance   => Record_Distance (Sensor_Value);
      when others     => null;
   end case;
   

8. case Today is
      when Mon        => Compute_Initial_Balance;
      when Fri        => Compute_Closing_Balance;
      when Tue .. Thu => Generate_Report(Today);
      when Sat .. Sun => null;
   end case;
   

9. case Bin_Number(Count) is
      when 1      => Update_Bin(1);
      when 2      => Update_Bin(2);
      when 3 | 4  =>
         Empty_Bin(1);
         Empty_Bin(2);
      when others => raise Error;
   end case;
   

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