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5.4 Case Statements

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

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

       case_statement_alternative ::=
          when discrete_choice_list =>
             sequence_of_statements

Name Resolution Rules

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

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.

The possible values of the expression shall be covered as follows:

• 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).

• 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.

• Otherwise, each value of the base range of the type of the expression shall be covered (either explicitly or by others).

Two distinct discrete_choices of a case_statement shall not cover the same value.

Dynamic Semantics

For the execution of a case_statement the expression is first evaluated.

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.

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) 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

Examples of case statements:

    
       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;

       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;

       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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