Concepts of Programming Languages

358 Chapter 8 Statement-Level Control Structures

evaluated. The only version of Ruby’s case expressions that is described here is

semantically similar to a list of nested if statements:

case

when Boolean_expression then expression

...
when Boolean_expression then expression
[else expression]
end

The semantics of this case expression is that the Boolean expressions are

evaluated one at a time, top to bottom. The value of the case expression is the

value of the first then expression whose Boolean expression is true. The else

represents true in this statement, and the else clause is optional. For

example,^4

leap = case

when year % 400 == 0 then true

when year % 100 == 0 then false

else year % 4 == 0

end

This case expression evaluates to true if year is a leap year.

The other Ruby case expression form is similar to the switch of Java. Perl,

Python, and Lua do not have multiple-selection statements.

8.2.2.3 Implementing Multiple Selection Structures

A multiple selection statement is essentially an n-way branch to segments of

code, where n is the number of selectable segments. Implementing such a state-

ment must be done with multiple conditional branch instructions. Consider

again the general form of the C switch statement, with breaks:

switch (expression) {

case constant_expression 1 : statement 1 ;

break;

...
case constantn: statementn;
break;
[default: statementn+ 1 ]
}
4. This example is from Thomas et al. (2005).