Section 6.10 The switch
Multiway Selection Structure
Subsection 6.10.1 Switch
Another selection structure to add to our repertoire is the switch/break structure. It is meant to provide a shorthand way of coding the following type of multiway selection structure:
if (integralVar == integralValue1)
// some statements
else if (integralVar == integralValue2)
// some statements
else if (integralVar == integralValue3)
// some statements
else // some statements
Note that each of the conditions in this case involves the equality of an integral variable and an integral value. This type of structure occurs so frequently in programs that most languages contain statements specially designed to handle it. In Java, we use a combination of the switch
and break
statements to implement multiway selection.
The switch
is designed to select one of several actions depending on the value of some integral expression:
switch (integralExpression)
{ case integralValue1:
// some statements
case integralValue2:
// some statements
case integralValue3:
// some statements
default:
some statements
}
The integralExpression must evaluate to a primitive integral value of type byte
, short
, int
, char
, or boolean
. It may not be a long
, float
, double
, or a class type. The integralValues must be literals or final
variables. They serve as labels in the one or more case clauses that make up the switch
statement body. The default
clause is optional, but it is a good idea to include it.
A switch
statement is executed according to the following rules:
The integralExpression is evaluated.
Control passes to the statements following the
case
label whose value equals the integralExpression or, if no cases apply, to thedefault
clause.Beginning at the selected label or at the default, all of the statements up to the end of the
switch
are executed. \end{NL}
Consider the following example:
int m = 2;
switch (m)
{
case 1:
System.out.print(" m = 1");
case 2:
System.out.print(" m = 2");
case 3:
System.out.print(" m = 3");
default:
System.out.print(" default case");
}
In this case, because m equals 2, the following output would be produced:
m = 2 m = 3 default case
Obviously, this output does not match the following if-else multiway selection structure, which would output, simply, m = 2
:
int m = 2;
if (m == 1)
System.out.print(" m = 1");
else if (m == 2)
System.out.print(" m = 2");
else if (m == 3)
System.out.print(" m = 3");
else
System.out.print(" default case");
The reason for this disparity is that the switch
executes all statements following the label that matches the value of the integralExpression (see again Rule 3 above).
In order to use the switch as a multiway selection, you must force it to break
out of the case clause after executing that clause's statements:
int m = 2;
switch (m)
{ case 1:
System.out.print(" m = 1");
break;
case 2:
System.out.print(" m = 2");
break;
case 3:
System.out.print(" m = 3");
break;
default:
System.out.print(" default case");
}
In this example, the break
statement causes control to pass to the end of the switch
, with the effect being that one and only one case will be executed within the switch
. Thus, the output of this code segment will be simply m = 2 , matching exactly the behavior of the multiway if-else selection structure (Fig. FigureĀ 6.10.1).
Principle 6.10.2. PROGRAMMING TIP: Multiway Selection.
A typical use for the switch
statement is to use it together with break
to code a multiway selection structure.
Principle 6.10.3. break
.
The break
statement transfers control out of its enclosing block, where a block is any sequence of statements contained within curly brackets { and }.
Principle 6.10.4. DEBUGGING TIP: Switch without break
.
A common error in coding the switch-based multiway selection is forgetting to put a break
statement at the end of each clause. This may cause more than one case to be executed.
Exercises 6.10.2 Self-Study Exercises
1.
Identify and fix the errors in the following switch structure (if there is no error, specify the output).
2.
Identify and fix the errors in the following switch structure (if there is no error, specify the output).
3.
Identify and fix the errors in the following switch structure (if there is no error, specify the output).
4.
Write a switch
statement that checks an integer variable flavor
and prints out the name of the ice cream flavor (where 0 is vanilla, 1 is chocolate, and 2 is strawberry) or prints āErrorā in the default case. Test by changing the flavor variable's value. Then, modify your solution to use constants (final variables) to represent the ice cream flavors.