Switch and Looping

THE SWITCH STATEMENT
AND
INTRODUCTION TO
LOOPING
Review
• You can test a series of condition with the…
• if-else if statement
• Nesting is…
• enclosing one structure inside of another.
• Binary logical operators combine…
• two boolean expressions into one.
• Binary logical operators:
• &&
• ||
• Unary logical operator:

• !
Review
• What does the == operator compare when the operands are strings?
• Their references
• What should you use instead of logical operators when comparing
strings?
• equals
• compareTo
• A variable has block-level scope if…

• It is declared inside of a block
• A variable that is declared inside of a block has scope beginning at…
• It’s declaration
• A variable that is declared inside of a block has scope ending at…
• The end of the block in which it was declared
The
•
switch Statement
It is often the case that you want the value of a single variable decide which
branch a program should take:
if (x == 1)
statement or block 1
if else (x == 2)
if else (x == 3)
else
• This is tedious and not very aesthetically pleasing.
• Java provides a structure that lets the value of a variable or expression decide
which branch to take
• This structure is called a switch statement.
The switch Statement
• General form of a switch statement:
switch (SwitchExpression) {
case CaseExpression1:
//One or more statements
break;
break;
default:
}
• switch – keyword that begins a switch statement
• SwitchExpression – a variable or expression that has to be either char, byte, short, or int.
• case – keyword that begins a case statement (there can be any number of case statements)
• CaseExpression1 – a literal or final variable that is of the same type as SwitchExpression .
break;
break;
default:
}
• Inside a case statement, one or more valid programming statements may appear.
• After the statement(s) inside of a case statement’s block, often the keyword break appears.
• After all of the case statements, there is the default case, which begins with the keyword default.
break;
break;
default:
}
• What this does is compare the value of SwitchExpression to each CaseExpressions.
• If they are equal, the statements after the matching case statement are executed.
• Once the break keyword is reached, the statements after the switch statement’s block are executed.
• break is a keyword that breaks the control of the program out of the current block.
• If none of the CaseExpressions are equal to SwitchExpression, then the statements below the default case
are executed.
if (x == 1)
y = 4;
if else (x == 2)
y = 9;
else
y = 22;
Is the same as…
switch (x) {
case 1:
y = 4;
break;
case 2:
y = 9;
break;
default:
y = 22;
}
switch Statement Example
• New Topics:
• The switch Statement
The switch Statement Notes
• The CaseExpression of each case statement must be unique.
• The default section is optional.
• Again, the SwitchExpression and all of the
CaseExpressions must be either char, byte,

short, or int.
• Without the break; at the end of the statements associated with a
case statement, the program “falls through” to the next case
statement’s statements, and executes them.
• If this is what you actually want, then leave out the break;, but often it isn’t.
• Why doesn’t default statement have a break; at the end?
Loops
• So far we have used decision structures to determine which statements are
executed and which are not depending on a condition.
• We used this for:
• Validation
• General control flow
• More specifically, we’ve used decision structures to execute statements that follow the
condition one or zero times.
• What if we want the user to keep trying to put in valid input until she succeeds?
• How would we do this with decision structures?
• Can we?
• Answer: No
• Solution: Loops
• A loop is a control structure that causes a statement or group of statements to
repeat.
• We will discuss three (possibly four) looping control structures.
• They differ in how they control the repitition.
The while Loop
• The first looping control structure we will discuss is the while loop.
• General Form:
while (BooleanExpression)
Statement or Block
• First, the BooleanExpression is tested
• If it is true, the Statement or Block is executed
• After the Statement or Block is done executing, the BooleanExpression
is tested again
• If it is still true, the Statement or Block is executed again
• This continues until the test of the BooleanExpression results in false.
• Note: the programming style rules that apply to decision statements

also apply.
The while Loop Flowchart
Boolean True
Statement or Block
Expression
False
while Loop Example
• New Topics:
• while Loop
number True
Print “Hello!” number++
<= 5
False
• Here, number is called a loop control variable.

• A loop control variable determines how many times a loop repeats.
• Each repetition of a loop is called an iteration.
• The a while loop is known as a pretest loop, because it tests the boolean expression before it executes the statements in its body.
• Note: This implies that if the boolean expression is not initially true, the body is never executed.
Infinite Loops
• In all but rare cases, loops must contain a way to terminate within
themselves.
• In the previous example number was incremented so that eventually
number <= 5 would be false.
• If a loop does not have a way of terminating it’s iteration, it is said
to be an infinite loop, because it will iterate indefinitely.
• This is a bad logic error!
• ...often, but always in this class
• If we removed number++ from the previous example, it would be an
infinite loop.
• Can also be created by putting a semicolon after the loop header or not
using brackets properly.
The do-while Loop
• while loops are considered pretest, but Java also provides a posttest loop called the do-
while loop:
do
Statement or Block
while (BooleanExpression);
• Here, the Statement or Block is executed first
• Next, the BooleanExpression is tested
• If true, the Statement or Block is executed
• Then the BooleanExpression is tested
• This continues until the BooleanExpression is false.
• Again, this is a posttest loop, meaning the BooleanExpression is tested at

the end.
• Note that this means the Statement or Block will ALWAYS be executed at least once.
• Also not the semicolon at the end of the last line.
The do-while Loop Flowchart
Statement or Block
True
Boolean
Expression
False
do-while Loop Example
• New Topic:
• do-while Loop

Switch and Looping

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THE SWITCH STATEMENT

• Unary logical operator:

• A variable has block-level scope if…

• The default section is optional.

• Again, the SwitchExpression and all of the

CaseExpressions must be either char, byte,

• Note: the programming style rules that apply to decision statements

• Here, number is called a loop control variable.

• Again, this is a posttest loop, meaning the BooleanExpression is tested at

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