10 Control and Iteration

Transcription

1 10 Control and Iteration 10.1 Introduction This chapter introduces control and iteration in Java. In Java, as in many other languages, the mainstay of the control and iteration processes are the if and swi tch statements and the for and while loops Control Structures The if Statement The basic format of an if statement in Java is the same as that in C. A test is performed and, depending on the result of the test, a statement is performed. A set of statements to be executed can be grouped together in curly brackets {}. For example: if (a == 5) System.out.println("true") System.out.println("false"); if (a == 5) System.out.print("a = 5"); System.out.println("The answer is therefore true"); System.out.print("a!= 5"); System.out.println("The answer is therefore false"); Of course, the if statement need not include the optional e 1 s e construct: if (a == 5) System.out.print("a = 5"); System.out.println("The answer is therefore true"); J. Hunt, Java for Practitioners Springer-Verlag London Limited

2 96 Java for Practitioners You must have a boolean in a condition expression, so you cannot make the same equality mistake as in C. The following code always generates a compile-time error (as "=" indicates assignment): if (a = 1) Unfortunately, assigning a boolean to a variable results in a boolean (all expressions return a result) and thus the following code is legal, but does not result in the intended behaviour (the string "Hello" is always printed on the console): public class Test { public static void main (String args []) boolean a = false; if (a = true) System.out.println("Hello") ; You can construct nested if statements, as in any other language: if (count < 100) if (index < 10) {... } {... } {... } However, it is easy to get confused. Java does not provide an explicit if-thene 1 s e i f - e 1 s e type of structure. In some languages, you can write: if (n < 10) print ("less than 10"); if (n < 100) print ("greater than 10 but less than 100"); if (n < 1000) print ("greater than 100 but less then 1000"); print ("greater than 1000"); This code is intended to be read as laid out above. However if we write it in Java, it should be laid out as below: if (n < 10) print (" less than 10"); if (n < 100) print ("greater than 10 but less than 100"); if (n < 1000) print ("> than 100 but < 1000");

3 Control and Iteration 97 print ("> than 1000"); This code is comprised of three staements rather than one and has a very different meaning (although it may have the same effect). This can lead to the infamous "dangling e 1 sen problem. A much better solution is the switch statement The Conditional Operator Java has inherited the conditional operator from C. This has both good and bad points. It is good because it is a very concise and efficient way of performing a test and carrying out one of two operations. It is bad because its terse nature is not clear to non-c programmers. However, it is a part of the language and all Java programmers must understand it. The Java conditional operator has three operands which are separated by two symbols in the following format: test expresslon? true expression : false expression The boolean expression determines whether the true or false expression is evaluated. For example, the following expression prints the maximum of two numbers: m >= n? System.out.println(m) : System.out.println(n); The conditional operator, unlike an if statement, returns a value. It can therefore be used in an assignment statement. For example, we can assign the larger of two numbers to a third variable: x = m >= n? m : n; Notice that this is becoming less readable (unless you are an experienced C programmer, in which case you would argue that it is obvious!) The swi tch Statement The conditional operator is not the only control statement that Java inherits from C; it also inherits the (flawed) C swi tch statement. This is a multi-way selection statement (similar to the case or select statements of some other programming languages). The structure of the swi tch statement is basically: switch (expression) case labell break; case labe12 break; default:

4 98 Java for Practitioners The expression returns an integer value and the case labels represent the possible values produced by the expression. Each case label is followed byone or more statements which are executed until a break (or return) statement is encountered. A s wit ch statement may include a default statement which is executed if none of the case labels match the integer in the expression. The swi tch statement has two major flaws. The first flaw is the need to "break" out of each case block. This is a major problem which has led to many software bugs. For example, in December 1989 the long-distance telephone service in the USA was disrupted by a software problem in the AT&T electronic switching systems. The problem was allegedly traced to the misuse of a break statement in a C program. The inclusion of such a feature has serious implications for the construction of high integrity software. The second major flaw is the inability of the swi tch statement to deal with anything other than integer comparisons. There are many situations in which it would be far easier to write the following code: switch (student.getmark() case> Instead, you must convert the tested value into an integer and then test the integers explicitly: public class Grades { public static void main (String argv[]) Grades g = new Grades(); g.classify(60) ; public void classify(int anumber) int temp = 0; System.out.println("The grade mark is II + anumber) ; temp anumber / 10; switch (temp) case 4 System.out.println("Pass"); break; case 5 System.out.println("2.2"); break; case 6 System.out.println("2.1"); break; case 7 case 8: case 9: case 10 : System.out.println("lst"); break; default : System. out. println ("Fail") ;

5 Control and Iteration 99 The result of running this application is illustrated below: C:>java Grades The grade mark is Iteration Iteration in Java is accomplished using the for, whi Ie and do-whi Ie statements. Just like their counterparts in other languages, these statements repeat a sequence of instructions a given number of times for Loops A for loop in Java is very similar to a for loop in C. It is used to step a variable through a series of values until a given test is false. Many languages have a very simple for loop, for example: for i = 1 to 10 do endfor; In this construct, you do not need to specify the end condition, nor how the variable i is incremented; in Java, you must specify both: for (initial-expression; test; increment-expression) statement This has the disadvantage of making the for construct more complicated, but it does offer a great deal of control. One point to note with this for loop is that the boolean t est expression indicates the condition that must hold while the loop is repeated. That is, it is a while true loop, rather than an until true loop. An example for loop is presented below: for (n = 1; n <= 10; n = n + 1) System.out.println(n); This loop assigns n the initial value 1. While n <= 10, it executes the println method and increments the value of n. We can repeat more than one statement if we enclose them in curly brackets { }. As in C, you can use a comma-delimited list to initialize and increment (decrement) several variables in a for loop. The expressions separated by commas are evaluated from left to right: for (i = 0, j = 10; i < 10; i++, j--) System.out.println (i + " : " + j);

6 100 Java for Practitioners while Loops The w hi I e loop exists in almost all programming languages. In most cases, it has a basic form such as: while (test expression) statement This is also true for Java. The while expression controls the execution of one or more statements. If more than one statement is to be executed then the statements must be enclosed in curly brackets { }: n = 1; while (n <= 10) System.out.println(n); n++; The above loop tests to see whether the value of n is less than or equal to 10, and then prints the current value of n before incrementing it by one. This is repeated until the test expression returns false (i.e. n > 11). You must assign n an initial value before the condition expression. If you do not provide an initial value for n, it defaults to null and the comparison with a numeric value raises an exception. It is interesting, and potentially useful, to note that "whi le (test) { }"isexactlyequivalentto"for (; test;) { }" do Loops In some cases, we want to execute the body of statements at least once; you can accomplish this with the do loop construct: do statement while (test expression); This loop is guaranteed to execute at least once, as the test is only performed after the statement has been evaluated. As with the whi Ie loop, the do loop repeats until the condition is false. You can repeat more than one statement by bracketing a series of statements into a block using curly brackets { }: n = 10; do { System.out.println(n); n--; while (n > 0); The above do loop prints the numbers from 10 down to 1 and terminates when n o.

7 Control and Iteration An Example of Loops As a concrete example of the for and w hi 1 e loops, consider the following class. It possesses a method which prints numbers from 0 to 1 less than the Ma x Va 1 u e class variable: public class Counter { II A class variable public static int MaxValue = 10; public static void main (String argv[]) Counter c = new Counter(); c. count () ; public void count() int i; System.out.println(n----- For n); for (i = 0; i < MaxValue; ++i) System.out.print(n " + i); System.out.println(" "); System.out.println("----- While "); i = 0; while (i < MaxValue) System.out.print(" " + i); ++i; System.out.println(" "); System.out.println(" ") ; The result of running this application should be: >java Counter For o While Recursion Recursion is a very powerful programming idiom found in many languages. Java is no exception. The following class illustrates how to use recursion to generate the factorial of a number. A factorial is a number calculated as a repeated series of

8 102 Java for Practitioners multiplications. The factorial is the number (x) times itself for (x) iterations. A factorial number is written as the number of the factorial followed by an exclamation mark. For example, 4! is equal to lx2x3x4 = 24. public class Factorial { public static void main (String argv[]) Factorial f = new Factorial(); System.out.println(f.factorial(5»; public int factorial (int anumber) System.out.println(aNumber); if (anumber == l)return 1; return anumber * factorial(--anumber); The result of running this application is illustrated below: C:>java Factorial Exercise: Factorial The aim of this exercise is to get you to use the loop facilities in Java. Above, an example of generating a factorial number using recursion is presented. This exercise requires that you implement a similar program, but using iteration loops rather than recursion What You Should Do Create a Java application that will print the factorials of 2, 4, 6 and Notes 1. Remember System.out.println() takes a string as an argument and outputs that string to standard out followed by a new line. 2. Concatenating a string with anything produces a string which is the combination of the original string and the string representation of the object concatenated. For example, the following code results in the string "4": "" + 4

9 Control and Iteration Summary You now know the basics of iteration and control in Java. You are now ready to consider a much larger application in Java. This will involve you considering inheritance and reuse, encapsulation, data structures and the collection API in Java. These issues are covered in subsequent chapters.