Chapters 4 and 5 Control Structures

Chapters 4 and 5: Control Structures • • Java has three kinds of control structures: – Sequence statement – Selection statements (three types) – Repetition statements (three types) Sequential statements – • Statements are normally executed one after the other in the order in which they are written. Selection Statements – Java has three types of selection statements: – if statement • – if…else statement • – Double-selection statement switch statement • • Single-selection statement Multiple-selection statement if statement – Execute an action if the specified condition is true – For example, the statement: if ( studentGrade >= 60 ) System.out.println( "Passed" ); If the condition is true, “Passed” is printed, and the next statement in order is performed. If the condition is false, the Print statement is ignored, and the next statement in order is performed. The indentation of the second line of this selection statement is optional, but recommended. 21 • if…else statement – Executes one action if the specified condition is true or a different action if the specified condition is false – For example, the statement: if ( grade >= 60 ) System.out.println( "Passed" ); else System.out.println( "Failed" ); prints “Passed” if the student’s grade is greater than or equal to 60, but prints “Failed” if it is less than 60. In either case, after printing occurs, the next statement in sequence is performed. • Conditional Operator ( ? : ) – Java provides the conditional operator (?:) that can be used in place of an if…else statement. – ?: is a ternary operator (takes three operands) – ? : and its three operands form a conditional expression – • The first operand (to the left of the ?) is a boolean expression. • The second operand (between the ? and :) is the value of the conditional expression if the boolean expression is true • The third operand (to the right of the :) is the value of the conditional expression if the boolean expression evaluates to false. For example, System.out.println( studentGrade >= 60 ? "Passed" : "Failed" ); The conditional expression in this statement evaluates to the string "Passed" if the boolean expression studentGrade >= 60 is true and evaluates to the string "Failed" if the boolean expression is false. 22 • Nested if…else statements – if…else statements can be put inside other if…else statements – For example, the following java code represents a nested if…else that prints A for exam grades greater than or equal to 90, B for grades in the range 80 to 89, C for grades in the range 70 to 79, D for grades in the range 60 to 69 and F for all other grades: if ( studentGrade >= 90 ) System.out.println( "A" ); else if ( studentGrade >= 80 ) System.out.println( "B" ); else if ( studentGrade >= 70 ) System.out.println( "C" ); else if ( studentGrade >= 60 ) System.out.println( "D" ); else System.out.println( "F" ); - Most Java programmers prefer to write the preceding if…else statement as if ( studentGrade >= 90 ) System.out.println( "A" ); else if ( studentGrade >= 80 ) System.out.println( "B" ); else if ( studentGrade >= 70 ) System.out.println( "C" ); else if ( studentGrade >= 60 ) System.out.println( "D" ); else System.out.println( "F" ); The two forms are identical except for the spacing and indentation, which the compiler ignores. 23 • Dangling-else problem – The Java compiler always associates an else with the immediately preceding if unless told to do otherwise by the placement of braces ({ and }). This behavior can lead to what is referred to as the dangling-else problem. For example: if ( x > 5 ) if ( y > 5 ) System.out.println( "x and y are > 5" ); else System.out.println( "x is < = 5" ); appears to indicate that if x is greater than 5, the nested if statement determines whether y is also greater than 5. If so, the string "x and y are > 5" is output. Otherwise, it appears that if x is not greater than 5, the else part of the if…else outputs the string "x is < = 5". Beware! This nested if…else statement does not execute as it appears. The compiler actually interprets the statement as: if ( x > 5 ) if ( y > 5 ) System.out.println( "x and y are > 5" ); else System.out.println( "x is < = 5" ); To force the nested if…else statement to execute as it was originally intended, we must write it as follows: if ( x > 5 ) { if ( y > 5 ) System.out.println( "x and y are > 5" ); } else System.out.println( "x is < = 5" ); The braces ({}) indicate to the compiler that the second if statement is in the body of the first if and that the else is associated with the first if. 24 • Blocks – Braces { } associate statements into blocks – To include several statements in the body of an if , enclose the statements in braces ({ and }). – The following example includes a block in the else-part of an if…else statement: if ( grade >= 60 ) System.out.println( "Passed" ); else { System.out.println( "Failed" ); System.out.println("You must take this course again"); } In this case, if grade is less than 60, the program executes both statements in the body of the else and prints Failed You must take this course again • Logical operators - Allows for forming more complex conditions (Combines simple conditions) - Some Java logical operators are: o && (conditional AND) o || (conditional OR) o & (boolean logical AND) o | (boolean logical inclusive OR) o ! (logical NOT) 25 • Conditional AND (&&) Operator expression1 expression2 expression1 && expression2 false false true true – false true false true False False False True Consider the following if statement if ( gender == “FEMALE” && age >= 65 ) ++count; • Conditional OR (||) Operator – expression1 expression2 expression1 || expression2 false false true true false true false true false true true true Consider the following if statement if ( ( semesterAverage >= 90 ) || ( finalExam >= 90 ) System.out.println( “Student grade is A” ); • Short-Circuit Evaluation of Complex Conditions – Parts of an expression containing && or || operators are evaluated only until it is known whether the condition is true or false – E.g., ( gender == “FEMALE” ) && ( age >= 65 ) • • Boolean Logical AND (&) Operator – • Works identically to && BUT & always evaluates both operands Boolean Logical OR (|) Operator – • Stops immediately if gender is not equal to FEMALE Works identically to || BUT | always evaluate both operands Logical Negation (!) Operator – Unary operator 26 • The precedence of the Java operators. The operators are shown from top to bottom in decreasing order of precedence. Operators Associativity Type ++ Left to right Increment and decrement operators Left to right Multiplicative + - Left to right Additive < < = > >= Left to right Relational == != Left to right Equality & Left to right Boolean logical AND | Left to right Boolean logical inclusive OR && Left to right Conditional AND || Left to right Conditional OR ?: Right to left Conditional = += -= *= /= %= Right to left Assignment -- * - / % switch statement - Used for multiple selections - The switch statement performs one of many different actions depending on the value of an expression. - The general format of the switch statement is: switch(Expression){ case value1: Statement; break; case value1: Statement; break; . . default: Statement; } 27 - Although each case and the default case in a switch can occur in any order, place the default case last. When the default case is listed last, the break for that case is not required. Some programmers include this break for clarity and symmetry with other cases - Expression in each case can be: – Constant integral expression • – Character constant • – Combination of integer constants that evaluates to a constant integer value (byte, short, int or char, but not long) E.g., ‘A’, ‘7’ or ‘$’ Constant variable • Declared with keyword final - The switch statement differs from other control statements in that it does not require braces around multiple statements in a case. - Without break statements, each time a match occurs in the switch, the statements for that case and subsequent cases execute until a break statement or the end of the switch is encountered. - If no match occurs between the controlling expression’s value and a case label, the default case executes. If no match occurs and the switch does not contain a default case, program control simply continues with the first statement after the switch. - Example: int i=1; switch (i) { case 0: System.out.println("zero"); break; case 1: System.out.println("one"); case 2: System.out.println("two"); default: System.out.println("default"); } The Output is: one two default 28 • break/continue statements – • Alter flow of control break statement – Causes immediate exit from control structure • – 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Used in while, for, do…while or switch statements Example: // Fig. 5.12: BreakTest.java // break statement exiting a for statement. public class BreakTest { public static void main( String args[] ) { int count; // control variable also used after loop terminates for ( count = 1; count < = 10; count++ ) // loop 10 times { if ( count == 5 ) // if count is 5, break; // terminate loop System.out.printf( "%d ", count ); } // end for System.out.printf( "\nBroke out of loop at count = %d\n", count ); } // end main } // end class BreakTest 1 2 3 4 Broke out of loop at count = 5 29 • continue statement – Skips remaining statements in loop body – Proceeds to next iteration • Used in while, for or do…while statements 1 2 3 // Fig. 5.13: ContinueTest.java // continue statement terminating an iteration of a for statement. public class ContinueTest 4 5 { public static void main( String args[] ) { 6 7 8 for ( int count = 1; count < = 10; count++ ) // loop 10 times { 9 if ( count == 5 ) // if count is 5, 10 continue; // skip remaining code in loop 11 12 System.out.printf( "%d ", count ); 13 } // end for 14 15 System.out.println( "\nUsed continue to skip printing 5" ); 16 } // end main 17 } // end class ContinueTest 1 2 3 4 6 7 8 9 10 Used continue to skip printing 5 • Repetition statements ( looping statements) – Repeatedly performs an action while its loop-continuation condition remains true – Java provides three repetition statements: • while statement • • do…while statement • • Performs the actions in its body zero or more times Performs the actions in its body one or more times for statement • Performs the actions in its body zero or more times 30 • while statement – Repeats an action while its loop-continuation condition remains true – The general form of while statement is: initialization; while ( loopContinuationCondition ) { statement; increment; } – Not providing, in the body of a while statement, an action that eventually causes the condition in the while to become false normally results in a logic error called an infinite loop, in which the loop never terminates – Counter-controlled repetition requires: – • Control variable (loop counter) • Initial value of the control variable • Increment/decrement of control variable through each loop • Loop-continuation condition that tests for the final value of the control variable Example 1: 1 // Fig. 5.1: WhileCounter.java 2 // Counter-controlled repetition with the while repetition statement. 3 4 public class WhileCounter 5 { 6 public static void main( String args[] ) 7 { int counter = 1; // declare and initialize control variable 8 9 10 while ( counter < = 10 ) // loop-continuation condition 11 { 12 System.out.printf( "%d 13 ++counter; // increment control variable by 1 ", counter ); } // end while 14 15 System.out.println(); // output a newline 16 } // end main 17 18 } // end class WhileCounter 1 2 3 4 5 6 7 8 9 10 31 - Note that the program performs the body of this while even when the control variable is 10. The loop terminates when the control variable exceeds 10 (i.e., counter becomes 11) - The program in Fig. 5.1 can be made more concise by initializing counter to 0 in line 8 and preincrementing counter in the while condition as follows: while ( ++counter < = 10 ) // loop-continuation condition System.out.printf( "%d ", counter ); - This code saves a statement (and eliminates the need for braces around the loop’s body), because the while condition performs the increment before testing the condition. - Example 2: 32 The output: Example 3: • int x = 0,y=0; while (x < = 0) { if (x < 0){ y = x; x =x+4; } else{ y = -x; x = x-3; } } System.out.printf("y=%d",y); Output: y= -3 do…while Repetition Statement – Similar to while structure – The do…while statement tests the loop-continuation condition after executing the loop’s body; therefore, the body always executes at least once. – The general format of the do/while statement is: do { statements } while ( condition ); 33 – Example1: 1 // Fig. 5.7: DoWhileTest.java 2 3 // do...while repetition statement. 4 5 6 7 8 9 public class DoWhileTest { public static void main( String args[] ) { int counter = 1; // initialize counter 10 11 do { 12 13 14 15 System.out.printf( "%d ", counter ); ++counter; } while ( counter < = 10 ); // end do...while System.out.println(); // outputs a newline 16 } // end main 17 18 } // end class DoWhileTest 1 2 - 3 4 5 6 7 8 9 10 Example2: int x=1; do{ x = x*2; System.out.print(" " +x); } while(x< =8); The output is: 2 4 6 8 16 • for Repetition Statement - Java provides the for repetition statement, which specifies the counter-controlledrepetition details in a single line of code. - The general format of the for statement is: for (initialization statement; - ; loopContinuationCondition where : 34 ; increment ) - - the initialization expression names the loop’s control variable and optionally provides its initial value, loopContinuationCondition is the condition that determines whether the loop should continue executing and increment modifies the control variable’s value (possibly an increment or decrement), so that the loop-continuation condition eventually becomes false. - The two semicolons in the for header are required The for statement can be represented with an equivalent while statement as follows: initialization; while ( loopContinuationCondition ) { statement; increment; } 1 2 3 Example: // Fig. 5.2: ForCounter.java // Counter-controlled repetition with the for repetition statement. 4 public class ForCounter 5 { 6 public static void main( String args[] ) 7 { 8 // for statement header includes initialization, 9 // loop-continuation condition and increment 10 for ( int counter = 1; counter < = 10; counter++ ) 11 System.out.printf( "%d ", counter ); 12 13 System.out.println(); // output a newline 14 } // end main 15 } // end class ForCounter 1 2 3 4 5 6 7 8 9 10 - Note: When a for statement’s control variable is declared in the initialization section of the for’s header, using the control variable after the for’s body is a compilation error. - Placing a semicolon immediately to the right of the right parenthesis of a for header makes that for’s body an empty statement. This is normally a logic error. - Infinite loops occur when the loop-continuation condition in a repetition statement never becomes false. To prevent this situation in a counter-controlled loop, ensure that the control variable is incremented (or decremented) during each iteration of the loop. In a sentinelcontrolled loop, ensure that the sentinel value is eventually input. 35 - Examples Using the for Statement • Varying control variable in for statement – Vary control variable from 1 to 100 in increments of 1 • – Vary control variable from 100 to 1 in increments of –1 • – for ( int i = 2; i < = 20; i += 3 ) Vary control variable over the sequence: 99, 88, 77, 66, 55, 44, 33, 22, 11, 0 • – for ( int i = 20; i >= 2; i -= 2 ) Vary control variable over the sequence: 2, 5, 8, 11, 14, 17, 20 • – for ( int i = 7; i < = 77; i += 7 ) Vary control variable from 20 to 2 in decrements of 2 • – for ( int i = 100; i >= 1; i-- ) Vary control variable from 7 to 77 in increments of 7 • – for ( int i = 1; i < = 100; i++ ) for ( int i = 99; i >= 0; i -= 11 ) Example: Summing the Even Integers from 2 to 20 1 2 3 // Fig. 5.5: Sum.java – // Summing integers with the for statement. 4 5 6 public class Sum { public static void main( String args[] ) 7 8 9 10 { int total = 0; // initialize total // total even integers from 2 through 20 11 for ( int number = 2; number < = 20; number += 2 ) 12 total += number; 13 14 System.out.printf( "Sum is %d\n", total ); // display results 15 } // end main 16 } // end class Sum Sum is 110 36 - All three expressions in a for header are optional. If the loopContinuationCondition is omitted, Java assumes that the loop-continuation condition is always true, thus creating an infinite loop. - You might omit the initialization expression if the program initializes the control variable before the loop. You might omit the increment expression if the program calculates the increment with statements in the loop’s body or if no increment is needed. - The initialization, loop-continuation condition and increment portions of a for statement can contain arithmetic expressions. For example, assume that x = 2 and y = 10. If x and y are not modified in the body of the loop, the statement for ( int j = x; j < = 4 * x * y; j += y / x ) is equivalent to the statement for ( int j = 2; j < = 80; j += 5 ) - If the loop-continuation condition is initially false, the program does not execute the for statement’s body. - Example: int a =0; int b = 4; for (a =1; a< b;a++) System.out.println(a++); The output is: • 1 3 Nested Loops: - - The body of a loop can contain itself a loop, called a nested loop. It is possible to nest an arbitrary number of loops. If a loop is nested the inner loop will execute all of its iterations for each time the outer loop executes once. Example: for (int i =1 ; i< =3;i++){ for (int j =4; j< =5; j++) System.out.print(i+" "+j+" " ); System.out.println(); } The output is: 1 4 1 5 2 4 2 5 3 4 3 5 37