Lec . 03: Control statements

1. Lec. 03: Control statements Java Programming

2. Content • Using System.in object to input data • Definition of Java statements • Categories of flow control statements • Condition constructs: if and switch • Iteration constructs: for, while and do-while • Nested constructs • Special control instructions: break and continue Java Programming

3. A simple way for inputting data • System.in is an object representing the input stream associated with the console input, i.e. the keyboard. • When it receives the read() message, it waits until the user presses a key and then returns the result to the message sender. • Browsing the methods defined in java.io.InputStream class! • The character returned from read() is an int, so it must be cast into a char to assign it to a char variable. Java Programming

4. This declaration will be discussed later! DO NOT remove it, otherwise, you will get error message. Example of using System.in // Read a character from the keyboard. class KbIn { public static void main(String args[]) throws java.io.IOException{ char ch; for(;;) { System.out.print("Press a key followed by ENTER: "); ch = (char) System.in.read(); // get a char System.out.println("Your key is: " + ch); if(ch == ‘z’) break; } } } Java Programming

5. Java statements • A Java statementforms a complete unit of execution. • Each statement is terminated by a semicolon (;). • Types of statements • Expression statements • Declaration statements • Control flow statements (the major topic of this lecture) Java Programming

6. Expression statements • A expression statement is one of the following expressions. • Assignment expressions • Any use of ++ or -- • Method invocations • Object creation expressions • Examples • val = 3.2; • ++x; • System.out.println(); • new Integer(3); Java Programming

7. Declaration statements • A declaration statement declares a variable. • Example • Double obj; • double dVal; Java Programming

8. Categories of flow control • Sequential • Sequential is the flow type where instructions are executed one after another, from top to bottom. • Selection • Selection is the flow type where one path out of a number of possibilities is taken. • Iteration • Iteration is the flow type where one or more instructions is executed repeatedly if certain conditions are fulfilled. • Transfer • Transfer is the flow type where the point of execution jumps to a different point in the program. • Using transfer is considered as a poor programming style and makes the code maintenance difficult. • Java only supports the forward transfer, which transfer the execution point to a point beyond the current execution point. • Sometimes forward transfer makes the code less complex. Java Programming

9. if-construct • Syntax form if(<condition>) statement else statement • ifand else parts control a single statement. • If more than one statement needed to be controlled, use code block. • The else part is optional. • The else part must be associated with anif-part if it exists. • The condition expression must be a boolean expression. • If the conditional expression is true, the target of theif will be executed; otherwise, if it exists, the target of the else will be executed. Java Programming if( i > 0 ) a++; // this if statement is terminated here b++; // since the target of if statement is a single statement // if( i > 0 ) statement is terminated here else c++; // no if part to match, so it’s illegal

10. Flowchart for if-construct <condition> false true Java Programming Statement Statement Next statement

11. A guessing game // Guess the letter game. class Guess { public static void main(String args[]) throws java.io.IOException { char ch, answer = 'K'; System.out.println("I'm thinking of a letter between A and Z."); System.out.print("Can you guess it: "); // read a char from the keyboard ch = (char) System.in.read(); if(ch == answer) System.out.println("** Right **"); else System.out.println("...Sorry, you're wrong."); } } Java Programming

12. Nested if-construct • A nested if-construct is an if statement that is the target of another if or else. • An else statement always refers to the nearest if statement that is within the same block as the else and not already associated with an else. Java Programming

13. Examples of nested if-construct • Example 1 • Example 2 if(i == 10) { if(j < 20) a = b; if(k > 100) c = d; else a = c; // this else refers to if(k > 100) } else a = d; // this else refers to if(i == 10) Java Programming Java’s syntax is free of position! Using proper indentation makes programs clearer! if(i == 10) { if(j < 20) { a = b; if(k > 100) c = d; } else a = c; // this else refers to if(j<20) } else a = d; // this else refers to if(i == 10)

14. switch-construct • The switch-construct enables a program to select among several alternatives. • Syntax form switch(<expression>) { case <constant1>: statement sequence break; case <constant2>: statement sequence break; case <constant3>: statement sequence break; … default: statement sequence } Java Programming

15. Executing switch-construct • Evaluating <expression> andcomparing the result sequentially, from top to bottom, with constants following case-clauses. • When a match is found, the statements associated with that case are executed until the break is encountered or, in the case of default or the last case, until the end of the switch is reached. • Thedefaultstatement sequence, if it exists, is executed if no case constant matches the expression. • The default-clause is optional. • If default-clause is not present, no action takes place if all matches fail. Java Programming

16. Syntax rules for switch-construct • Prior to JDK 7, the <expression> controlling the switch must be of type byte, short, int, char, or enum. • Beginning with JDK 7, <expression>can also be of type String. • Each value specified in the case clauses must be a uniqueconstant expression. • A constant expression is a expression which can be evaluated during the compiling time. • The type of each value specified in the case clauses must be compatible with the type of <expression>. Java Programming

17. Demo segment for switch-construct switch (month) { case 1: case 3: case 5: case 7: case 8: case 10: case 12 : System.out.println(“There are 31 days"); break; case 2: System.out.println(“There are 28 or 29 days"); break; case 4: case 6: case 9: case 11: System.out.println(“There are 30 days"); break; default: System.out.println("Invalid month."); } Java Programming

18. looping • Java provides four types of looping (iteration) statements • Traditional for • while • do-while • Enhanced for • Supported after Java 2 version 1.5 • All looping have 4 parts • Initialization • Iteration condition • Body • Termination condition Java Programming

19. Traditional for-construct • Syntax of while-statement for ( <init_exp> ; <test_exp> ; <post_exp> ) single-statement; init-_exp Example int sum = 0; for(int count=0; count<=n; ++count ) { sum += count; } // count does not exist after this point Test_ exp Java Programming post-_exp false true statement Next statement

20. More on for-construct • The type of control variable can be any numeric types and char. • The loop control variable can be modified by any amount in <post-exp>. • It is a pre-condition looping. • The <post-exp> is tested at the beginning of each loop. • The code inside the loop may not be executed at all. • The <init-exp> and <post-exp> may consist of more than one expression separated by comma. Java Programming inti, j; for(i=0, j=i+10; i < j; i++, j--) System.out.println("i and j: " + i + " " + j);

21. More on for-construct • The <test-exp> must be a boolean expression. • It does not need to involve the loop control variable. • All of the <init-exp> , <test-exp> and <post-exp> are option, but all three semicolons are not. • Missing <test-exp> means the result of <test-exp> is always true. • The simplest for-construct for(;;); will perform nothing for ever. • When you declare a variable inside a for loop, the scope of that variable ends when the for statement does. Java Programming for( inti = 0; i < 100; i++) sum += i; System.out.print(sum / i); // illegal since i is not declared

22. while-construct • Syntax of while statement while ( <condition> ) single-statement; Example int sum = 0; int count = 0; while ( count <= n ) { sum += count; count++; } <condition> Java Programming false true statement The while statement is a pre-test looping. Next statement

23. do-while-construct • Syntax of do-while statement do single-statement; while ( <condition> ); statement Example int sum = 0; int count = 0; if ( count <= n ) do { sum += count; count++; } while ( count <= n ); Java Programming false <condition> true The do-while statement is a post-test looping. Next statement

24. break statement • The break statement is used to force an immediate exit from a loop enclosing the statement, bypassing any remaining code in the body of the loop and the loop’s conditional test. • The break statement can be only used within a loop body or the body of switch-construct. Java Programming

25. Demo program for break statement // Using break with nested loops. class Break3 { public static void main(String args[]) { for(inti=0; i<3; i++) { System.out.println("Outer loop count: " + i); System.out.print(" Inner loop count: "); int t = 0; while(t < 100) { if(t == 10) break;//terminate while loop if t is 10 System.out.print(t + " "); t++; } System.out.println(); } System.out.println("Loops complete."); } } Java Programming

26. break statement with label • The break statement with label can be used to terminate the execution of a code block. • The code block terminated by using a break statement with label does not need to be the body of loop or switch. • Syntax form for break statement with label break <label>; • A label is the identifier used to identify a code block or a statement. • Syntax form for labeling code block <label>: { <statement sequences> } • When this form of break executes, control is transferred out of the named code block or statement. • The labeled code block or statement must enclose the breakstatement, but it does not need to be the immediately enclosing block. • This means that you can use a labeled break statement to exit from a set of nested blocks. Java Programming

27. Demo program for break statement with label // Using break with a label. class Break4 { public static void main(String args[]) { inti; for(i=1; i<4; i++) { one: { two: { three: { System.out.println("\ni is " + i); if(i==1) break one; if(i==2) break two; if(i==3) break three; System.out.println("won't print"); // this is never reached } System.out.println("After block three."); } System.out.println("After block two."); } System.out.println("After block one."); } System.out.println("After for."); } } Java Programming

28. Try this … class Break6 { public static void main(String args[]) { int x=0, y=0; // here, put label before for statement. stop1: for(x=0; x < 5; x++) { for(y = 0; y < 5; y++) { if(y == 2) break stop1; // stop the for loop System.out.println("x and y: " + x + " " + y); } } System.out.println(); // now, put label immediately before { for(x=0; x < 5; x++){ stop2: { for(y = 0; y < 5; y++) { if(y == 2) break stop2; // stop the body of for, not for System.out.println("x and y: " + x + " " + y); } } } } } Java Programming

29. continue-construct • The continuestatement forces the next iteration of the loop to take place, skipping any code between itself and the conditional expression that controls the loop. • In while and do-while loops, a continue statement will cause control to continues the loop by directly evaluating the <test-exp>. • In the case of the for, the <post-exp> of the loop is evaluated, then the loop continues by evaluating the <test-exp>. Java Programming

30. Demo program for continue-construct // Use continue. class ContDemo { public static void main(String args[]) { inti; // print even numbers between 0 and 100 for(i = 0; i<=100; i++) { if((i%2) != 0) continue; // skipping the println and // continuing the next loop System.out.println(i); } } } Java Programming The boolean expression is true for odd number.

31. continue statement with label • The continue may specify a label to describe which enclosing loop to continue. • The statement labeled by a label used with continue statement must be a loop statement. Java Programming

32. Demo program for continue statement with label // Use continue with a label. class ContToLabel { public static void main(String args[]) { outerloop: for(int i=1; i < 10; i++) { System.out.print("\nOuter loop pass " + i + ", Inner loop: "); for(int j = 1; j < 10; j++) { if(j == 5) continue outerloop; // continue outer loop System.out.print(j); } } } } Java Programming