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Transcript of Chap05
Java Programming: From Problem Analysis to Program Design, 3e
Chapter 5
Control Structures II: Repetition
2Java Programming: From Problem Analysis to Program Design, 3e
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
3Java Programming: From Problem Analysis to Program Design, 3e
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
4Java Programming: From Problem Analysis to Program Design, 3e
The while Looping (Repetition) Structure
• Syntaxwhile (expression)
statement
• Expression is always true in an infinite loop
• Statements must change value of expression to false
5Java Programming: From Problem Analysis to Program Design, 3e
The while Looping (Repetition) Structure (continued)
6Java Programming: From Problem Analysis to Program Design, 3e
The while Looping (Repetition) Structure (continued)
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
7Java Programming: From Problem Analysis to Program Design, 3e
The while Looping (Repetition) Structure (continued)
• Typically, while loops are written in the following form:
8Java Programming: From Problem Analysis to Program Design, 3e
Counter-Controlled while Loop• Used when exact number of data or entry pieces is
known• General form:
9Java Programming: From Problem Analysis to Program Design, 3e
Sentinel-Controlled while Loop
• Used when exact number of entry pieces is unknown but last entry (special/sentinel value) is known
• General form:
10Java Programming: From Problem Analysis to Program Design, 3e
Flag-Controlled while Loop
• Boolean value used to control loop
• General form:
11Java Programming: From Problem Analysis to Program Design, 3e
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
12Java Programming: From Problem Analysis to Program Design, 3e
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:
13Java Programming: From Problem Analysis to Program Design, 3e
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:
14Java Programming: From Problem Analysis to Program Design, 3e
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
15Java Programming: From Problem Analysis to Program Design, 3e
Programming Example: Checking Account Balance (continued)
• Solution – Read data– EOF-controlled loop– switch structure of transaction types – Determine action (add to balance or subtract
from balance depending on transaction type)
16Java Programming: From Problem Analysis to Program Design, 3e
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
17Java Programming: From Problem Analysis to Program Design, 3e
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
18Java Programming: From Problem Analysis to Program Design, 3e
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
19Java Programming: From Problem Analysis to Program Design, 3e
The for Looping (Repetition) Structure (continued)
• Execution– Initial statement executes – The loop condition evaluated– If loop condition evaluates to true, execute for loop statement and execute update statement
– Repeat until loop condition is false
20Java Programming: From Problem Analysis to Program Design, 3e
The for Looping (Repetition) Structure (continued)
21Java Programming: From Problem Analysis to Program Design, 3e
The for Looping (Repetition) Structure (continued)
Example 5-8The following for loop prints the first 10 nonnegative integers:
for (i = 0; i < 10; i++)
System.out.print(i + " ");
System.out.println();
22Java Programming: From Problem Analysis to Program Design, 3e
The for Looping (Repetition) Structure (continued)
Example 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.
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
23Java Programming: From Problem Analysis to Program Design, 3e
The for Looping (Repetition) Structure (continued)
• 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
The for Looping (Repetition) Structure (continued)
• 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, 3e 24
25Java Programming: From Problem Analysis to Program Design, 3e
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
26Java Programming: From Problem Analysis to Program Design, 3e
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
27Java Programming: From Problem Analysis to Program Design, 3e
The do…while Loop (Repetition) Structure
• Syntax:
• Statements executed first, then logical expression evaluated
• Statement(s) executed at least once then continued if logical expression is true
28Java Programming: From Problem Analysis to Program Design, 3e
do…while Loop (Post-test Loop)
29Java Programming: From Problem Analysis to Program Design, 3e
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
30Java Programming: From Problem Analysis to Program Design, 3e
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
31Java Programming: From Problem Analysis to Program Design, 3e
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
32Java Programming: From Problem Analysis to Program Design, 3e
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:***************
33Java Programming: From Problem Analysis to Program Design, 3e
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