5.4 Case Statements

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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.


case_statement ::=
    case expression is
    end case;

case_statement_alternative ::=
    when discrete_choice_list =>

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.


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 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    =>    
    when others   => raise Error;
end case;

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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual