Case statements

• alternative to multiple if
• used when can explicitly list all alternatives for one selector

An example program shows the use of a case statement to classify characters. Note that all possible characters are handled, and that the selectors for each case are expressed in a variety of different ways.

A second example shows the use of a case statement to classify operators for a simple calculator.

Statement form

	statement_before;

	case selector is
		 when value_list_1 =>
			statement(s)_1;
		 when value_list_2 =>
			statement(s)_2;
			...
		 when others =>
			statement(s)_n;
	end case;

	statement_after;

Note preferred layout style:

• one level of indent for when value list
• second level of indent for statements associated with when alternatives

Statement semantics

Selectors

The selector value_list may be:

• a single constant value, eg. 'a'
• a series of alternatives, eg. 'a' | 'b' | 'c'
• a range of values eg. 'a' .. 'z'
• or any combination of the above

Any number of when alternatives may be used

Restrictions on Case Statements

All possible values of the selector must be supplied, either explicitly or using when others.

when others indicates the action when none of the listed when alternatives are matched

• it must be the last alternative
• to specify no action, use the "null;" statement.

Case vs Multiple if

For example, the first case statement example program, which classifies characters, can easily be rewritten using if statements.

Often the case implementation is shorter, cleaner, and more elegant.

Case

• table of values and actions
• easy to specify range of selector values
• easy to specify alternative selector values

Multiple if

• sequence of decisions and actions
• used when cannot specify range directly
  • selector is not discrete
  • choice depends on more than one selector

Use case whenever appropriate