Lecture 7.1 Selection - the if Instruction

© 2006 Pearson Addison-Wesley. All rights reserved An algorithm often needs to make choices… choose to print a special warning if the bank balance is negative choose whether to move a bouncing ball up or down choose how many dots to place on the dice face Selection instructions allow code to make choices. Java selection instructions:

© 2006 Pearson Addison-Wesley. All rights reserved Syntax if (some_condition) { then_clause } Notes some_condition must be a valid boolean expression. then_clause is a sequence of zero or more instructions. then_clause should be indented as shown The {... } enclosure can be eliminated if then_clause is a single instruction. Semantics some_condition is evaluated and then_clause is executed if and only if some_condition is true Example (Assume that pay, wage, hours and bonus are double instance variables.) pay = wage * hours; if (hours > 40) { pay = pay + (hours - 40) * wage * 0.5; } pay = pay + bonus;

© 2006 Pearson Addison-Wesley. All rights reserved Syntax if (some_condition) { then_clause } else { else_clause } Notes some_condition must be a valid boolean expression. then_clause and else_clause are a sequences of zero or more instructions. then_clause and else_clause should be indented as shown The {... } enclosures can be eliminated for single instruction clauses. Semantics some_condition is evaluated and then_clause is executed if and only if some_condition is true, otherwise else_clause is executed Example (Assume score, exam, homework are int instance variables and grade is a char variable.) score = exam * 25 + homework; if (score > 59) { grade = 'P'; } else { grade = 'F'; } System.out.println( “Grade is “ + grade );

© 2006 Pearson Addison-Wesley. All rights reserved Finding the maximum of two int variables Only create a rectangle if width and height are positive if (theWidth > 0 && theHeight > 0) { theRect = new Rectangle(10, 10, theWidth, theHeight); } Color an oval green 30% of the time and red 70% if ( ) { theOval.setBackground( Color.green ); } else { theOval.setBackground( Color.red ); }

© 2006 Pearson Addison-Wesley. All rights reserved Any two primitive expressions of compatible type can be compared using a relational operator, and the result is a boolean (true or false) value. Operators ==equal to (warning: don’t confuse with =) !=not equal to <less than <=less than or equal to >greater than >=greater than or equal to Example if ( count != 0 ) { average = total / count; }

© 2006 Pearson Addison-Wesley. All rights reserved boolean is a primitive data type for storing logical values. Infix Operators and (warning: don’t confuse with &) logical (inclusive) or Constants true false Prefix Operator not (logical negation) Operator Precedence (highest to lowest) ! - (unary negation) * / % + - >= == != && || =

© 2006 Pearson Addison-Wesley. All rights reserved Some declarations private int int1, int2; private double dbl1, dbl2; private char letter; Valid boolean expressions int1 < true && (letter == ‘Z’) ! (dbl1 == int2) (0 < int1) && (int1 < 100) letter==‘a’ || letter==‘e’ || letter==‘i’ || letter==‘o’ || letter == ‘u’ dbl2 != 0.0 && dbl1/dbl2 < Invalid boolean expressions int1 < = int2 ‘a’ < letter < ‘z’ int1 = int2

© 2006 Pearson Addison-Wesley. All rights reserved Way Selection Both if-then and if-then-else forms of the if instruction exhibit 2- way selection, because there are two options at execution time. Example if (examScore > 90) { letterGrade = ‘A’; } else if (examScore 80) { letterGrade = ‘B’; } else if (examScore 70) { letterGrade = ‘C’; } else if (examScore 60) { letterGrade = ‘D’; } else { letterGrade = ‘F’; } Multi-Way Selection Java includes a switch instruction that provides for limited multi- way selection, but using nested if instructions is the more general solution

© 2006 Pearson Addison-Wesley. All rights reserved An else line applies to the most recent if instruction that is not terminated. Example (Don’t write code this way...) if (someInt < 10) { System.out.println(“small”); if (secondInt < 10) { System.out.println(“small again”); } else { System.out.println(“big”); } The Moral... Indentation of code is important. In general, clauses should be indented and right parentheses aligned with the notation from which they began.