Case Statements
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 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.

#### Syntax

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case_statement ::=
case expression is
case_statement_alternative
{case_statement_alternative}
end case;
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case_statement_alternative ::=
when discrete_choice_list =>
sequence_of_statements

#### Name Resolution Rules

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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

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

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- 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.

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

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

#### Dynamic Semantics

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For the execution of a case_statement the expression is first evaluated.
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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.
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Otherwise (the value is not covered by any discrete_choice_list, perhaps due to being outside the base range), Constraint_Error is raised.
NOTES
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4 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

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Examples of case statements:
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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;
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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;
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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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