Lecture 4 Objectives

Java Programming: From Problem Analysis to Program Design, Second Edition 1

Lecture 4 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.


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.


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.


The while Looping (Repetition) Structure

Example 5-1

i = 0; //Line 1while (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


The while Looping (Repetition) Structure

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)

.

.

.

}


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 programint counter = 0;while (counter < N){ . . . counter++; . . .}


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; . . .}


Flag-Controlled while Loop

Boolean value used to control loop. General form:

boolean found = false; while (!found){ .

. . if (expression) found = true; . . .}


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.


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}


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}


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.


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).


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.


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.


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


The for Looping (Repetition) Structure 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.



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();


The for Looping (Repetition) StructureExample 5-91. 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("*");



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.


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.


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


The do…while Loop (Repetition) Structure

Syntax:

do

statementwhile (expression);

Statements are executed first and then expression is evaluated.

Statements are executed at least once and then continued if expression is true.


do…while Loop (Post-Test Loop)


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.


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.


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.


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:***************


Lecture 4 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

Lecture 4 Objectives

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