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Java Programming: From Problem
Analysis to Program Design, 5e
Chapter 5
Control Structures II: Repetition

2
Java Programming: From Problem Analysis to Program Design, 5e
Chapter Objectives
• Learn about repetition (looping) control structures
• Explore how to construct and use count-
controlled, sentinel-controlled, flag-controlled,
and EOF-controlled repetition structures
• Examine break and continue statements
• Discover how to form and use nested control
structures

3
Why Is Repetition Needed?
• There are many situations in which the
same statements need to be executed
several times
• Example
– Formulas used to find average grades for
students in a class

4
The while Looping (Repetition)
Structure
• Syntax
while (expression)
statement
• Expression is always true in an infinite loop
• Statements must change value of expression to
false

5
Structure (continued)

6
Example 5-1
i = 0; //Line 1
while (i <= 20) //Line 2
{
System.out.print(i + " "); //Line 3
i = i + 5; //Line 4
}
System.out.println(); //Line 5
Output:
0 5 10 15 20

7
• Typically, while loops are written in the
following form:

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Counter-Controlled while Loop
• Used when exact number of data or entry
pieces is known
• General form:

9

10

11

12

13
Sentinel-Controlled while Loop
• Used when exact number of entry pieces is
unknown but last entry (special/sentinel value) is
known
• General form:

14

15

16
Flag-Controlled while Loop
• Boolean value used to control loop
• General form:

17
EOF (End of File)-Controlled while
Loop
• Used when input is from files
• Sentinel value is not always appropriate
• In an EOF-controlled while loop that uses
the Scanner object console to input
data, console acts at the loop control
variable
• The method hasNext, of the class
Scanner, returns true if there is an input
in the input stream; otherwise it returns
false

18
EOF (End of File)-Controlled
while Loop (continued)
• The expression console.hasNext() evaluates
to true if there is an input in the input stream;
otherwise it returns false
• Expressions such as console.nextInt() act
as the loop condition
• A general form of the EOF-controlled while loop
that uses the Scanner object console to input
data is of the form:

19
EOF (End of File)-Controlled
while Loop (continued)
• Suppose that inFile is a Scanner object
initialized to the input file; in this case, the EOF-
controlled while loop takes the following form:

20

21
Programming Example:
Fibonacci Number
• Fibonacci formula for any Fibonacci sequence:
an
= an-1
+ an-2
• Input: first two Fibonacci numbers in sequence,
position in sequence of desired Fibonacci number
(n)
– int previous1 = Fibonacci number 1
– int previous2 = Fibonacci number 2
– int nthFibonacci = position of nth Fibonacci number
• Output: nth Fibonacci number

22
Programming Example: Fibonacci
Number (Solution)
if (nthFibonacci == 1)
current = previous1;
else if (nthFibonacci == 2)
current = previous2;
else
{
counter = 3;
while (counter <= nthFibonacci)
{
current = previous2 + previous1;
previous1 = previous2;
previous2 = current;
counter++;
}
}
• Final result found in last value of current

23
The for Looping (Repetition)
Structure
• Specialized form of while loop
• Simplifies the writing of count-controlled loops
• Syntax
for (initial expression; logical expression;
update expression)
statement

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• Execution
– Initial statement executes
– The loop condition is evaluated
– If loop condition evaluates to true, execute
for loop statement and execute update
statement
– Repeat until loop condition is false

25

26
Example 5-9
The following for loop prints the first 10 nonnegative
integers:
for (i = 0; i < 10; i++)
System.out.print(i + " ");
System.out.println();

27
Example 5-10
• The following for loop outputs the word Hello and a star (on separate
lines) five times:
for (i = 1; i <= 5; i++)
{
System.out.println("Hello");
System.out.println("*");
}
2.
for (i = 1; i <= 5; i++)
System.out.println("Hello");
System.out.println("*");
This loop outputs the word Hello five times and the star only once

28
• Does not execute if initial condition is
false
• Update expression changes value of loop
control variable, eventually making it false
• If logical expression is always true, result
is an infinite loop
• Infinite loop can be specified by omitting all
three control statements

• If logical expression is omitted, it is
assumed to be true
• for statement ending in semicolon is
empty
Java Programming: From Problem Analysis to Program Design, 5e 29

32
Programming Example: Classify
Numbers
• Input: N integers (positive, negative, and zeros)
int N = 20; //N easily modified
• Output: number of 0s, number of even integers,
number of odd integers

33
Numbers (Solution)
for (counter = 1; counter <= N; counter++)
{
number = console.nextInt();
System.out.print(number + " ");
switch (number % 2)
{
case 0:
evens++;
if (number == 0)
zeros++;
break;
case 1:
case -1:
odds++;
} //end switch
} //end for loop

34
Numbers (Solution) (continued)
• The switch statement in Step 3c can also be written
as an if...else statement as follows:

35
The do…while Loop
(Repetition) Structure
• Syntax
• Statements executed first, and then logical
expression evaluated
• Statement(s) executed at least once and then
continued if logical expression is true

36
do…while Loop (Post-test Loop)

37
(continued)

38
(continued)
• A do...while loop can be used for input validation.
• Suppose that a program prompts a user to enter a test score, which
must be greater than or equal to 0 and less than or equal to 50.
• If the user enters a score less than 0 or greater than 50, the user
should be prompted to re-enter the score.
• The following do...while loop can be used to accomplish this
objective:

39
Choosing the Right Looping Structure
• All three loops have a place in Java
• If you know or the program can determine in advance the number
of repetitions needed, the for loop is the correct choice
• If you do not know and the program cannot determine in advance
the number of repetitions needed, and it could be zero, the while
loop is the right choice
• If you do not know and the program cannot determine in advance
the number of repetitions needed, and it is at least one, the
do...while loop is the right choice

40
break Statements
• Used to exit early from a loop
• Used to skip remainder of switch
structure
• Can be placed within if statement of a
loop
– If condition is met, loop exited immediately

41
continue Statements
• Used in while, for, and do...while
structures
• When executed in a loop, the remaining
statements in the loop are skipped; proceeds
with the next iteration of the loop
• When executed in a while/do…while
structure, expression evaluated immediately
after continue statement
• In a for structure, the update expression is
executed after the continue statement;
then the loop condition executes

42
The objective is to find the sum of the numbers in each line. For each
line, output the numbers together with their sum. Let us consider the
following program:

43

44

45

46
• The Sample Run shows that there is a bug in the program because
after correctly producing the three lines of output, the program
crashes with error messages.
• This is an example of a run (execution) time error. That is, the
program compiled correctly, but crashed during execution.
• The last line of output shows that the error is in line 23 of the code,
which is the put statement: num = infile.nextInt();.
• The program is trying to read data from the input file, but there is
no more input in the file.
• At this point the value of the outer loop variable i is 4. Clearly,
there is a bug in the program and we must fix the code. Some
programmers, especially some beginners, address the symptom of
the problem by adding a software patch.
• A beginning programmer might fix the code by adding a software
patch as shown in the following modified program:

47

48

49

50
• The programmer merely observed the symptom and addressed the
problem by adding a software patch.
• Not only will the program execute extra statements, it is also an
example of a partially understood concept.
• It appears that the programmer does not have a good grasp of why
the earlier program produced four lines rather than three. Adding
a patch eliminated the symptom, but it is a poor programming
practice.
• The programmer must resolve why the program produced four
lines.
• Looking at the program closely, we can see that the four lines are
produced because the outer loop executes four times.

51
• The values assigned to loop control variable i are 1, 2, 3, and 4.
• This is an example of the classic ‘‘off by one’’ problem. (In the
‘‘off by one’’ problem, either the loop executes one too many or
one too few times.)
• We can eliminate this problem by correctly setting the values of
the loop control variable. For example, we can rewrite the loops
as follows:

52

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• If there are syntax errors, the compiler will identify them.
• If there are logical errors, we must carefully look at the code or
even maybe at the design to try to find the errors.
• To increase the reliability of the program, errors must be
discovered and fixed before the program is released to the users.
• Once an algorithm is written, the next step is to verify that it
works properly.
• If the algorithm is a simple sequential flow or contains a branch, it
can be hand traced or you can use the debugger, if any, provided
by the IDE.

54
• Typically, loops are harder to debug. The correctness of a loop
can be verified by using loop invariants.
• A loop invariant is a set of statements that remains true each time
the loop body is executed. Let p be a loop invariant and q be the
(logical) expression in a loop statement.
• Then p && q remains true before each iteration of the loop and p
&& not(q) is true after the loop terminates.

55
• The most common error associated with loops is off by one.
• If a loop turns out to be an infinite loop, the error is most likely in
the logical expression that controls the execution of the loop.
• Check the logical expression carefully and see if you have reversed
an inequality, an assignment statement symbol appears in place of
the equality operator, or && appears in place of ||.
• If the loop changes the values of variables, you can print the values
of the variables before and/or after each iteration or you can use
your IDE’s debugger, if any, and watch the values of variables
during each iteration.
• Debugging can be a tiresome process.
• If your program is very bad, do not debug; throw it away and start
over.

56
Nested Control Structure
• Provides new power, subtlety, and
complexity
• if, if…else, and switch structures can
be placed within while loops
• for loops can be found within other for
loops

57
Nested Control Structure (Example)
for (i = 1; i <= 5; i++)
{
for (j = 1; j <= i; j++)
System.out.print(" *");
System.out.println();
}
• Output:
*
**
***
****
*****

58
Chapter Summary
• Looping mechanisms
– Counter-controlled while loop
– Sentinel-controlled while loop
– Flag-controlled while loop
– EOF-controlled while loop
– for loop
– do…while loop
• break statements
• continue statements
• Nested control structures

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