8.2 Selection Statements 355selector needs an expression with only two possible values. Another issue is
whether single statements, compound statements, or statement sequences may
be selected. Next, there is the question of whether only a single selectable seg-
ment can be executed when the statement is executed. This is not an issue for
two-way selectors, because they always allow only one of the clauses to be on a
control path during one execution. As we shall see, the resolution of this issue
for multiple selectors is a trade-off between reliability and flexibility. Another
issue is the form of the case value specifications. Finally, there is the issue of
what should result from the selector expression evaluating to a value that does
not select one of the segments. (Such a value would be unrepresented among the
selectable segments.) The choice here is between simply disallowing the situa-
tion from arising and having the statement do nothing at all when it does arise.
The following is a summary of these design issues:
- What is the form and type of the expression that controls the selection?
- How are the selectable segments specified?
- Is execution flow through the structure restricted to include just a single
selectable segment? - How are the case values specified?
- How should unrepresented selector expression values be handled, if at all?
8.2.2.2 Examples of Multiple Selectors
The C multiple-selector statement, switch, which is also part of C++, Java,
and JavaScript, is a relatively primitive design. Its general form is
switch (expression) {
case constant_expression 1 :statement 1 ;
...
case constantn: statement_n;
[default: statementn+ 1 ]
}
where the control expression and the constant expressions are some discrete
type. This includes integer types, as well as characters and enumeration types.
The selectable statements can be statement sequences, compound statements,
or blocks. The optional default segment is for unrepresented values of the
control expression. If the value of the control expression is not represented and
no default segment is present, then the statement does nothing.
The switch statement does not provide implicit branches at the end of its
code segments. This allows control to flow through more than one selectable
code segment on a single execution. Consider the following example:
switch (index) {
case 1: