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Transcript of Java Programming: From Problem Analysis to Program Design, 4e Chapter 5 Control Structures II:...
Java Programming: From Problem Analysis to Program Design, 4e
Chapter 5Control Structures II: Repetition
2Java Programming: From Problem Analysis to Program Design, 4e
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, 4e
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, 4e
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, 4e
The while Looping (Repetition) Structure (continued)
6Java Programming: From Problem Analysis to Program Design, 4e
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, 4e
The while Looping (Repetition) Structure (continued)
• Typically, while loops are written in the following form:
8Java Programming: From Problem Analysis to Program Design, 4e
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, 4e
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, 4e
Flag-Controlled while Loop
• Boolean value used to control loop
• General form:
11Java Programming: From Problem Analysis to Program Design, 4e
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, 4e
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, 4e
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, 4e
15Java Programming: From Problem Analysis to Program Design, 4e
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
16Java Programming: From Problem Analysis to Program Design, 4e
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
17Java Programming: From Problem Analysis to Program Design, 4e
The for Looping (Repetition) Structure
• Specialized form of while loop
• Simplifies the writing of count-controlled loops
• Syntaxfor (initial expression; logical expression;
update expression)
statement
18Java Programming: From Problem Analysis to Program Design, 4e
The for Looping (Repetition) Structure (continued)
• 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
19Java Programming: From Problem Analysis to Program Design, 4e
The for Looping (Repetition) Structure (continued)
20Java Programming: From Problem Analysis to Program Design, 4e
The for Looping (Repetition) Structure (continued)
Example 5-9The following for loop prints the first 10 nonnegative integers:
for (i = 0; i < 10; i++)
System.out.print(i + " ");
System.out.println();
21Java Programming: From Problem Analysis to Program Design, 4e
The for Looping (Repetition) Structure (continued)
Example 5-101. 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
22Java Programming: From Problem Analysis to Program Design, 4e
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, 4e 23
Java Programming: From Problem Analysis to Program Design, 4e 24
25Java Programming: From Problem Analysis to Program Design, 4e
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, 4e
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, 4e
Programming Example: Classify Numbers (Solution) (continued)
• The switch statement in Step 3c can also be written as an if...else statement as follows:
28Java Programming: From Problem Analysis to Program Design, 4e
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
29Java Programming: From Problem Analysis to Program Design, 4e
do…while Loop (Post-test Loop)
30Java Programming: From Problem Analysis to Program Design, 4e
do…while Loop (Post-test Loop)
31Java Programming: From Problem Analysis to Program Design, 4e
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
32Java Programming: From Problem Analysis to Program Design, 4e
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
33Java Programming: From Problem Analysis to Program Design, 4e
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
34Java Programming: From Problem Analysis to Program Design, 4e
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
35Java Programming: From Problem Analysis to Program Design, 4e
Nested Control Structure (Example)
for (i = 1; i <= 5; i++){ for (j = 1; j <= i; j++) System.out.print(" *");
System.out.println();}• Output:***************
36Java Programming: From Problem Analysis to Program Design, 4e
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