Control structures ii

Chapter 5: Control StructuresChapter 5: Control Structures
IIII
JJavaava PProgramming:rogramming:
From Problem Analysis to Program Design,From Problem Analysis to Program Design,
Second EditionSecond Edition

2
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
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

6
Typically, while loops are written in the following form:
//initialize the loop control variable(s)
while (expression) //expression tests the LCV
{
.
.
.
//update the loop control variable(s)
.
.
.
}

7
Counter-Controlled while Loop
 Used when exact number of data or entry pieces is
known.
 General form:
int N = //value input by user or specified
//in program
int counter = 0;
while (counter < N)
{
.
.
.
counter++;
.
.
.
}

8
Sentinel-Controlled while Loop
 Used when exact number of entry pieces is unknown, but
last entry (special/sentinel value) is known.
 General form:
Input the first data item into variable;
while (variable != sentinel)
{
.
.
.
input a data item into variable;
.
.
.
}

9
Flag-Controlled while Loop
 Boolean value used to control loop.
 General form:
boolean found = false;
while (!found)
{
.
.
.
if (expression)
found = true;
.
.
.
}

10
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.
 The expression console.hasNext() acts as the
loop condition.
 Expressions such as console.nextInt() update the
value of the loop condition.

11
EOF-Controlled while Loop
 A general form of the EOF-controlled while loop
that uses the Scanner object console to input
data is:
while (console.hasNext())
{
//Get the next input and store in an
//appropriate variable
//Process data
}

12
EOF-Controlled while Loop
 Suppose that inFile is a Scanner object
initialized to the input file. In this case, the EOF-
controlled while loop takes the following form:
while (inFile.hasNext())
{
//Get the next input and store in an
//appropriate variable
//Process data
}

13
Programming Example: Checking
Account Balance
 Input file: Customer’s account number, account
balance at beginning of month, transaction type
(withdrawal, deposit, interest), transaction amount.
 Output: Account number, beginning balance, ending
balance, total interest paid, total amount deposited,
number of deposits, total amount withdrawn,
number of withdrawals.

14
Programming Example: Checking
Account Balance
 Solution:
 Read data.
 EOF-controlled loop.
 switch structure of transaction types.
 Determine action (add to balance or subtract
from balance depending on transaction type).

15
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.

16
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++;
}
}

17
The for Looping (Repetition) Structure
 Specialized form of while loop.
 Simplifies the writing of count-controlled loops.
 Syntax:
for (initial statement; loop condition;
update statement)
statement

18
 Execution:
 Initial statement executes.
 Loop condition is evaluated.
 If loop condition evaluates to true, execute for
loop statement and execute update statement.
 Repeat until loop condition is false.

19
Example 5-8
The following for loop prints the first 10
nonnegative integers:
for (i = 0; i < 10; i++)
System.out.print(i + " ");
System.out.println();

20
Example 5-9
1. 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. The following for loop outputs the word Hello five times and the
star only once:
for (i = 1; i <= 5; i++)
System.out.println("Hello");
System.out.println("*");

21
 Does not execute if initial condition is false.
 Update expression changes value of loop control
variable, eventually making it false.
 If loop condition is always true, result is an infinite
loop.
 Infinite loop can be specified by omitting all three
control statements.
 If loop condition is omitted, it is assumed to be
true.
 for statement ending in semicolon is empty.

22
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.

23
Programming Example: Classify 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

24
The do…while Loop (Repetition)
Structure
 Syntax:
do
statement
while (expression);
 Statements are executed first and then expression is
evaluated.
 Statements are executed at least once and then
continued if expression is true.

25
do…while Loop (Post-Test Loop)

26
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 is exited immediately.

27
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 is evaluated immediately after continue
statement.
 In a for structure, the update statement is executed
after the continue statement; the loop condition
then executes.

28
Nested Control Structures
 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.

29
Nested Control Structures (Example)
for (int i = 1; i <= 5; i++)
{
for (int j = 1; j <= i; j++)
System.out.print(" *");
System.out.println();
}
Output:
*
**
***
****
*****

30
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

Control structures ii

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Control structures ii