04slide_2007 - Chapter 4 Loops EE3206/EE5805 Java...

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Unformatted text preview: Chapter 4 Loops EE3206/EE5805 Java Programming & Application EE3206/EE5805 1 Objectives To use while, do-while, and for loop statements to control the To repetition of statements. To understand the flow of control in loop statements. To To use Boolean expressions to control loop statements. To To write nested loops. To To know the similarities and differences of three types of loops. To To implement program control with break and continue. To EE3206/EE5805 Java Programming & Application EE3206/EE5805 2 while Loop Flow Chart while while (loop-continuationcondition) { // loop-body; Statement(s); } } count = 0; int count = 0; while (count < 100) { System.out.println("Welcome to Java!"); count++; Loop Continuation Condition? true Statement(s) (loop body) false (count < 100)? false true System.out.println("Welcome to Java!"); count++; (A) (B) EE3206/EE5805 Java Programming & Application EE3206/EE5805 3 animation Trace while Loop int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } EE3206/EE5805 Java Programming & Application EE3206/EE5805 4 Initialize count (count < 2) is true animation Trace while Loop, cont. int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } EE3206/EE5805 Java Programming & Application EE3206/EE5805 5 Print Welcome to Java Increase count by 1 count is 1 now animation Trace while Loop, cont. int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } EE3206/EE5805 Java Programming & Application EE3206/EE5805 6 (count < 2) is still true since count is 1 Print Welcome to Java animation Trace while Loop, cont. int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } EE3206/EE5805 Java Programming & Application EE3206/EE5805 7 Increase count by 1 count is 2 now (count < 2) is false since count is 2 now animation Trace while Loop int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } The loop exits. Execute the next statement after the loop. EE3206/EE5805 Java Programming & Application EE3206/EE5805 8 Example: An Advanced Math Learning Tool The Math subtraction learning tool program generates just one question for each run. You can use a loop to generate questions repeatedly. This example gives a program that generates ten questions and reports the number of the correct answers after a student answers all ten questions. SubtractionTutorLoop Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 9 Example: Ending a Loop with a Sentinel Value Often the number of times a loop is executed is not predetermined. You may use an input value to signify the end of the loop. Such a value is known as a sentinel value. sentinel Write a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input. SentinelValue Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 10 do-while Loop Do once first, i.e. the loop body will be performed at least 1 time. Statement(s) (loop body) true Loop Continuation Condition? false do { // Loop body; Statement(s); } while (loop-continuation-condition); EE3206/EE5805 Java Programming & Application EE3206/EE5805 11 for Loops for (initial-action; loop-continuation-condition; action-after-each-iteration) { // loop body; Statement(s); } int i; for (i = 0; i < 100; i++) { System.out.println( "Welcome to Java!"); } Initial-Action i=0 Loop Continuation Condition? true Statement(s) (loop body) false (i < 100)? true System.out.println( "Welcome to Java"); i++ false Action-After-Each-Iteration (A) EE3206/EE5805 Java Programming & Application EE3206/EE5805 (B) 12 animation Trace for Loop Declare i int i; for (i = 0; i < 2; i++) { System.out.println( "Welcome to Java!"); } int i; for (i = 0; i < 2; i++) { System.out.println( "Welcome to Java!"); } Execute initializer i is now 0 EE3206/EE5805 Java Programming & Application EE3206/EE5805 13 animation Trace for Loop, cont. int i; for (i = 0; i < 2; i++) { System.out.println( "Welcome to Java!"); } int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } (i < 2) is true since i is 0 Print Welcome to Java EE3206/EE5805 Java Programming & Application EE3206/EE5805 14 animation Trace for Loop, cont. int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Execute adjustment statement i now is 1 (i < 2) is still true since i is 1 EE3206/EE5805 Java Programming & Application EE3206/EE5805 15 animation Trace for Loop, cont. Print Welcome to Java int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Execute adjustment statement i now is 2 EE3206/EE5805 Java Programming & Application EE3206/EE5805 16 animation Trace for Loop, cont. int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } (i < 2) is false since i is 2 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Exit the loop. Execute the next statement after the loop EE3206/EE5805 Java Programming & Application EE3206/EE5805 17 Note •DO NOT use floating-point values for equality checking in a loop control as floating-point values are approximations. •The initial-action and action-after-each-iteration in a for loop can be a comma-separated statements. Therefore, the following two for loops are correct. They are rarely used in practice, however. for (int i = 1; i < 100; System.out.println(i++)); for (int i = 0, j = 0; (i + j < 10); i++, j++) { // Do something } •If the loop-continuation-condition in a for loop is omitted, it is implicitly true and equivalent to a while loop. for ( ; ; ) { // Do something } (a) Equivalent EE3206/EE5805 Java Programming & Application EE3206/EE5805 while (true) { // Do something } (b) 18 Example: Using for Loops Problem: Write a program that sums a series that starts Problem: with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03 and so on. TestSum Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 19 Nested Loops Problem: Write a program that uses nested for loops to Problem: print a multiplication table. TestMultiplicationTable Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 20 Using break and continue break continue •break •Terminate a loop and return to the flow of an upper control •continue •Skip the following loop-body statement(s) and perform next iteration TestBreak TestContinue Run Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 21 Which Loop to Use? The three forms of loop statements, while, do-while, and for, are expressively equivalent; that is, you can write a loop in any of these three forms. For example, a while loop in (a) in the following figure can always be converted into the following for loop in (b): while (loop-continuation-condition) { // Loop body } (a) Equivalent for ( ; loop-continuation-condition; ) // Loop body } (b) A for loop in (a) in the following figure can generally be converted into the following while loop in (b) except in certain special cases (see Review Question 3.19 for one of them): for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; } (a) EE3206/EE5805 Java Programming & Application EE3206/EE5805 Equivalent initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; } (b) 22 Recommendations •Use the one that is most intuitive and comfortable for you. •In general, a for loop may be used if the number of repetitions is known, as, for example, when you need to print a message 100 times. •A while loop may be used if the number of repetitions is not known, as in the case of reading the numbers until the input is 0. •A do-while loop can be used to replace a while loop if the loop body has to be executed before testing the continuation condition. EE3206/EE5805 Java Programming & Application EE3206/EE5805 23 Common Error Adding a semicolon at the end of the for and while clause before the loop body is a common mistake, as shown below: for (int i=0; i<10; i++); { System.out.println("i is " + i); } int i=0; while (i < 10); { System.out.println("i is " + i); i++; } Logic Error In the case of the do loop, the following semicolon is needed to end the loop. Correct int i=0; do { System.out.println("i is " + i); i++; } while (i<10); EE3206/EE5805 Java Programming & Application EE3206/EE5805 24 More Examples EE3206/EE5805 Java Programming & Application EE3206/EE5805 25 Example: Finding the Greatest Common Divisor Problem: Write a program that prompts the user to enter two positive integers and finds their greatest common divisor. Solution: Suppose you enter two integers 4 and 2, their greatest common divisor is 2. Suppose you enter two integers 16 and 24, their greatest common divisor is 8. So, how do you find the greatest common divisor? Let the two input integers be n1 and n2. You know number 1 is a common divisor, but it may not be the greatest commons divisor. So you can check whether k (for k = 2, 3, 4, and so on) is a common divisor for n1 and n2, until k is greater than n1 or n2. GreatestCommonDivisor Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 26 Example: Finding the Sales Amount Problem: You have just started a sales job in a department store. Your pay consists of a base salary and a commission. The base salary is $5,000. The scheme shown below is used to determine the commission rate. Sales Amount Commission Rate $0.01–$5,000 8 percent $5,000.01–$10,000 10 percent $10,000.01 and above 12 percent Your goal is to earn $30,000 in a year. Write a program that will find out the minimum amount of sales you have to generate in order to make $30,000. FindSalesAmount Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 27 Example: Displaying Prime Numbers Problem: Write a program that displays the first 50 prime numbers in five lines, each of which contains 10 numbers. An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not. Solution: The problem can be broken into the following tasks: •For number = 2, 3, 4, 5, 6, ..., test whether the number is prime. •Determine whether a given number is prime. •Count the prime numbers. •Print each prime number, and print 10 numbers per line. PrimeNumber Run EE3206/EE5805 Java Programming & Application EE3206/EE5805 28 ...
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This note was uploaded on 01/11/2011 for the course EE 3206 taught by Professor Cwting during the Spring '07 term at City University of Hong Kong.

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