Section 5.2 - Case Statements

Ada's mechanism is a case statement. A case statement computes the value of an expression, and then compares it to lists of values to determine what to execute next. Here's an example:

  case A is                      -- Execute something depending on A's value:
    when 1          => Fly;      -- if A=1, execute Fly.
    when 3 .. 10    => Put(A);   -- if A is 3 through 10, put it to the screen.
    when 11 | 14    => null;     -- if A is 11 or 14, do nothing.
    when 2 | 20..30 => Swim;     -- if A is 2 or 20 through 30, execute Swim.
    when others     => Complain; -- if A is anything else, execute Complain.
  end case;

Note that after each "when" is a list of one or more possible values. A "when others" clause matches anything that hasn't matched anything else. A case statement chooses one and only one alternative; the alternatives must be exhaustive (cover all possible cases) and exclusive (you can't have two "when 5 =>" clauses). An Ada compiler will detect missing or duplicate cases at compile-time.

Here's the full BNF for the case statement:

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

case_statement_alternative ::=
   "when" discrete_choice_list "=>" sequence_of_statements

discrete_choice_list ::= discrete_choice { | discrete_choice }

discrete_choice ::= expression | discrete_range | "others"

The Ada case statement is functionally identical to C's "switch" statement. Unlike C, Ada supports ranges of values and does not require a "break" statement at the end of each sequence (a common C error).

Case statements are useful, but a large number of case statements may indicate that you should be using a more object-oriented approach. Ada's object-oriented features will be discussed in a later lesson.

Quiz:

In the sample case statement above, if A contained 12, what would be executed?

1. Fly.
2. Put(A).
3. null (do nothing).
4. Swim.
5. Complain.

Go back to the previous section	Skip to the next section	Go up to lesson 5 outline