©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Chapter 6 Repetition Statements

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Definition Repetition statements (loops) allow a block of code to be executed multiple times. Loop termination is controlled by a Boolean expression –Counting loops terminate the execution of the block after it is executed for a fixed number of times. –Sentinel loops terminate the execution of the block after a designated value called a sentinel is encountered. Java has three loop structures

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Pre-Test loop int sum = 0, number = 1 number <= 100 ? false sum = sum + number; number = number + 1; true

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter while ( number <= 100 ) { sum = sum + number; number = number + 1; } Syntax for the while Statement while ( ) Statement (loop body) Boolean Expression

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Some Examples Keeps adding the numbers 1, 2, 3, … until the sum becomes larger than 1,000,000. Computes the product of the first 20 odd integers. int sum = 0, number = 1; while ( sum <= ) { sum = sum + number; number = number + 1; } 1 int product = 1, number = 1, count = 20, lastNumber; lastNumber = 2 * count - 1; while (number <= lastNumber) { product = product * number; number = number + 2; } 2

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter String inputStr; int age; inputStr = JOptionPane.showInputDialog(null, "Your Age (between 0 and 130):"); age = Integer.parseInt(inputStr); while (age 130) { JOptionPane.showMessageDialog(null, "Invalid age : Please try again."); inputStr = JOptionPane.showInputDialog(null, "Your Age (between 0 and 130):"); age = Integer.parseInt(inputStr); } Example: Testing Input Data Priming Read

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Loop Pitfalls There are a number of problems that are commonly encountered when using loops –infinite loops occur when the control expression can never be false no change is made to the variables in the control expression the control expression is too restrictive –off-by-one errors occur when there are one too many or one too few iterations of the loop

Examples of Infinite loops Infinite Loops Both loops will not terminate because the boolean expressions will never become false. int count = 1; while ( count != 10 ) { count = count + 2; } 2 int product = 0; while ( product < ) { product = product * 5; } 1

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Overflow An infinite loop often results in an overflow error. An overflow error occurs when you attempt to assign a value larger than the maximum value the variable can hold. In Java, an overflow does not cause program termination. With types float and double, a value that represents infinity is assigned to the variable. With type int, the value “wraps around” and becomes a negative value.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Round-off Errors -> Infinite Loops Using Real Numbers Loop 2 terminates, but Loop 1 does not because only an approximation of a real number can be stored in a computer memory. float count = 0.0f; while ( count != 1.0f ) { count = count f; } //eight 3s 2 float count = 0.0f; while ( count != 1.0f ) { count = count f; }//seven 3s 1

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Round-off Errors -> Off-By-One Errors int result = 0; double cnt = 1.0; while (cnt <= 10.0){ cnt += 1.0; result++; } System.out.println(result); 1 int result = 0; double cnt = 0.0; while (cnt <= 1.0){ cnt += 0.1; result++; } System.out.println(result); 2 Using Real Numbers Loop 1 prints out 10, as expected, but Loop 2 prints out 11. The value 0.1 cannot be stored precisely in computer memory

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Off-by-One Errors Goal: Execute the loop body 10 times. count = 1; while ( count < 10 ){... count++; } 1 count = 0; while ( count <= 10 ){... count++; } 3 count = 1; while ( count <= 10 ){... count++; } 2 count = 0; while ( count < 10 ){... count++; } 4 13 andexhibit off-by-one error.

Post-Test Loop int sum = 0, number = 1 sum += number; number++; sum <= ? true false

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter do { sum += number; number++; } while ( sum <= ); Syntax for the do-while Statement do while ( ) ; Statement (loop body) Boolean Expression

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Counting Loop i = 0; false number =... ; sum += number; true i ++; i < 20 ?

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter for (i = 0; i < 20; i++ ) { number = scanner.nextInt(); sum += number; } Syntax for the for Statement for ( ; ; ) Initialization Boolean Expression Increment Statement (loop body)

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter for Loop Examples for (int i = 0; i < 100; i += 5) 1 i = 0, 5, 10, …, 95 for (int j = 2; j < 40; j *= 2) 2 j = 2, 4, 8, 16, 32 for (int k = 100; k > 0; k--) ) 3 k = 100, 99, 98, 97,..., 1

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Loops can be nested We can use nested loops to generate tables for (int i=1; i<=10; i++) { for (int j=1; j<=10; j++) System.out.print(i * j + "\t"); System.out.println(); } How many times does the loop execute? –The inner loop executes 10 times for each value of i

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Recursion Recursion is another technique that can be used for iteration A recursive method is one that calls itself n! = n * (n-1) * (n-2) * … * 2 * 1 int factorial ( int n) { if (n<=1) return 1; else return n * factorial( n-1); } Recursion is generally less efficient than loops so use it only when you need to

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Finding GCD