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Java Programming: From Problem Analysis to Program Design, Second Edition 1
Chapter 5:
Repetition Control Structures
Repetition (looping) control structures.
Count-controlled, sentinel-controlled, flag-
controlled, and EOF-controlled repetition
structures.
Break and continue statements.
nested control structures.
Java Programming: From Problem Analysis to Program Design, Second Edition 2
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 3
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 4
The while Looping (Repetition) Structure
Example 5- 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
Java Programming: From Problem Analysis to Program Design, Second Edition 7
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; . . . }
Java Programming: From Problem Analysis to Program Design, Second Edition 8
Flag-Controlled while Loop
Boolean value used to control loop. General form: boolean found = false; while (!found) { . . . if (expression) found = true; . . . }
Java Programming: From Problem Analysis to Program Design, Second Edition 9
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 10
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 }
Java Programming: From Problem Analysis to Program Design, Second Edition 13
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).
Java Programming: From Problem Analysis to Program Design, Second Edition 14
Programming Example:
Fibonacci Number
Fibonacci formula for any Fibonacci sequence:
a (^) n = a (^) n -1 + a (^) n -
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 15
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 16
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
Java Programming: From Problem Analysis to Program Design, Second Edition 19
The for Looping (Repetition) Structure
Example 5-
- 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("*"); }
- 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("*");
Java Programming: From Problem Analysis to Program Design, Second Edition 20
The for Looping (Repetition) Structure
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 21
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 22
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
Java Programming: From Problem Analysis to Program Design, Second Edition 25
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 26
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 27
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 28
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:
**
