Chapter 4 Loops. Lecture notes for computer programming 1 Faculty of Engineering and Information Technology Prepared by: Iyad Albayouk

Transcription

1 Chapter 4 Loops Lecture notes for computer programming 1 Faculty of Engineering and Information Technology Prepared by: Iyad Albayouk ١

2 while Loop Flow Chart while (loop-continuation-condition) { // loop-body; Statement(s); int count = 0; while (count < 100) { System.out.println("Welcome to Java!"); count++; count = 0; Loop Continuation Condition? false (count < 100)? false true Statement(s) (loop body) tru System.out.println("Welcome to Java!"); (A) (B) ٢

3 Example while Loop intcount = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; ٣

4 Caution Don t use floating-point values for equality checking in a loop control. Since floating-point values are approximations, using them could result in imprecise counter values and inaccurate results. This example uses int value for data. If a floatingpoint type value is used for data, (data!= 0) may be true even though data is 0. // data should be zero double data = Math.pow(Math.sqrt(2), 2) - 2; if (data == 0) System.out.println("data is zero"); else System.out.println("data is not zero"); ٤

5 do-while Loop Statement(s) (loop body) do { // Loop body; Statement(s); true Loop Continuation Condition? false while (loop-continuation continuation-condition); condition); ٥

6 for Loops for (initial-action; action; loop- continuation-condition; condition; action-after after-each-iteration) { // loop body; Statement(s); I n it ia l-a c tio n int i; for (i = 0; i < 100; i++) { System.out.println( "Welcome to Java!"); i = 0 L o o p C o n tin u a t io n C o n d itio n? fa ls e ( i < )? fa ls e t r u e S t a t e m e n t(s) (lo o p b o d y ) tr u e S y s te m.o u t.p r in t ln ( " W e lc o m e t o A c t io n -A fte r -E a c h -Ite r a t io n i+ + (A ) ( B ) ٦

7 Example for Loop int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); ٧

8 Note The initial-action in a for loop can be a list of zero or more comma-separated expressions. The action-after-eachiteration in a for loop can be a list of zero or more commaseparated 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 (inti = 0, j = 0; (i + j < 10); i++, j++) { // Do something ٨

9 Note If the loop-continuation-condition in a for loop is omitted, it is implicitly true. Thus the statement given below in (a), which is an infinite loop, is correct. Nevertheless, it is better to use the equivalent loop in (b) to avoid confusion: for ( ; ; ) { // Do something (a) Equivalent while (true) { // Do something (b) ٩

10 Example: Using for Loops Problem: Write a program that sums a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: and so on. public class TestSum { public static void main(string[] args) { // Initialize sum float sum = 0; // Add 0.01, 0.02,..., 0.99, 1 to sum for (float i = 0.01f; i <= 1.0f; i = i f) sum += i; // Display result System.out.println("The sum is " + sum); ١٠

11 Nested Loops Problem: Write a program that uses nested for loops to print a multiplication table. Next slide has a program to print a multiplication table. ١١

12 import javax.swing.joptionpane; public class TestMultiplicationTable { public static void main(string[] args) ) { String output = " Multiplication Table\n"; output += "------" \n"; output += " "; for (int( intj j = 1; j <= 9; j++) output += " " + j; output += "\n";" for (int( inti i = 1; i <= 9; i++) { output += i + " "; for (int( intj j = 1; j <= 9; j++) { if (i * j < 10) output += " " + i * j; else output += " " + i * j; output += "\n";" JOptionPane.showMessageDialog(null,, output); ١٢

13 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) Equivalent initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; (b) ١٣

14 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. ١٤

15 Caution Adding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below: Logic Error for (int( i=0; i<10; i++); { System.out.println("i is " + i); ١٥

16 Caution, cont. Similarly, the following loop is also wrong: inti=0; while (i < 10); { Logic Error System.out.println("i is " + i); i++; In the case of the do loop, the following semicolon is needed to end the loop. inti=0; do { System.out.println("i is " + i); i++; Correct while (i<10); ١٦

17 Using break Examples for using the break keyword: public class TestBreak { public static void main(string[] args) { intsum = 0; intnumber = 0; while (number < 20) { number++; sum += number; if (sum >= 100) break; System.out.println("The number is " + number); System.out.println("The sum is " + sum); ١٧

18 Using continue Examples for using the continue keyword: public class TestContinue { public static void main(string[] args) { intsum = 0; intnumber = 0; while (number < 20) { number++; if (number == 10 number == 11) continue; sum += number; System.out.println("The sum is " + sum); ١٨