Today s Lecture. SYS-1A4Y Programming - Languages and Software Construction. Conditional Execution. Conditional Execution.

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1 SYS-1A4Y Programming - Languages and Software Construction Dr Gavin Cawley Lecture 9 - Selection Today s Lecture In the previous lectures: Using objects Strings NumberFormat In this lecture: Relational operators and expressions The if statement The if- statement The switch statement Lecture 9 : Slide 2 Conditional Execution Sometimes we need to be able to execute a statement or block of statements only of some condition is met, for example: if student s grade is greater or equal to than 50% then print Passed This is known as conditional execution or selection Lecture 9 : Slide 3 Conditional Execution In Java we would write: relational expression (must evaluate to a boolean value) brackets are required here System.out.println("Passed"); only executed if relational expression evaluates as Lecture 9 : Slide 4 Flowchart Syntax Diagram Relational if ( ) Expression grade >= 0.5 Print Passed Lecture 9 : Slide 5 Lecture 9 : Slide 6 1

2 Blocks A block is a sequence of statements, local class declarations and local variable declaration statements within braces [1, 14.2]. A block acts as a single statement. Always use a block in selection and repetition statements, even if it contains only one statement. Lecture 9 : Slide 7 Relational Expressions Java provides several relational operators that can be used to compare variables and constants: == equal to!= not equal to > greater than < less than >= greater than or <= less than or equal to equal to A relational expression returns a result of type boolean, which is if the condition is met and otherwise Lecture 9 : Slide 8 Combining Relational Expressions Java provides operators allowing us to combine simple relational expressions:! which means not which means or && which means and Example: Note that the precedence and associativity rules apply here as well, but parentheses often make the code easier to read. if ((a >= 0) && (a < 42)) // do if a is greater than or equal // to zero and less than 42 Lecture 9 : Slide 9 if- Selection Sometimes we want to specify an action to take when the condition is as well as when it is, for example: if student s grade is greater or equal to than 50% print Passed print Failed Lecture 9 : Slide 10 In Java Flowchart System.out.println("Passed"); System.out.println("Failed"); Executed if condition is Executed if condition is met Print Passed grade >= 0.5 Print Failed not met Lecture 9 : Slide 11 Lecture 9 : Slide 12 2

3 Syntax Diagram Relational if ( ) Expression Block Block Note alignment of braces under if and keywords Programming Style System.out.println("Passed"); Indent by three spaces for each pair or braces System.out.println("Failed"); Lecture 9 : Slide 13 Lecture 9 : Slide 14 Multi-Way Selection if- statements can be combined to form a multi-way selection statement, e.g.: if students grade is greater than or equal to 70% print Grade A ; if students grade is greater than or equal to 60% print Grade B ; if students grade is greater than or equal to 50% print Grade C ; if students grade is greater than or equal to 40% print Grade D ; print Grade E Lecture 9 : Slide 15 In Java if (grade >= 0.7) System.out.println("Grade A"); if (grade >= 0.6) System.out.println("Grade B"); System.out.println("Grade C"); if (grade >= 0.4) System.out.println("Grade D"); System.out.println("Grade E"); Do if grade 0.7 Do if 0.7 > grade 0.6 Do if 0.6 > grade 0.5 Do if 0.5 > grade 0.4 Do if grade < 0.4 Lecture 9 : Slide 16 How it works if (grade >= 0.7) System.out.println("Grade A"); if (grade >= 0.6) System.out.println("Grade B"); System.out.println("Grade C"); if (grade >= 0.4) System.out.println("Grade D"); System.out.println("Grade E"); All we have done is use another if- statement in the clause of the previous if- statement Lecture 9 : Slide 17 Programming Style if (grade >= 0.7) System.out.println("Grade A"); if (grade >= 0.6) System.out.println("Grade B"); System.out.println("Grade C"); if (grade >= 0.4) System.out.println("Grade D"); System.out.println("Grade E"); It is the convention to indent a multi-way if- statement like this, to save horizontal space. Lecture 9 : Slide 18 3

4 switch Selection s grade is a variable of type char System.out.println("jolly"); Lecture 9 : Slide 19 switch Selection s break causes the remainder of the System.out.println("jolly"); switch statement to be skipped Lecture 9 : Slide 20 switch Selection s switch Selection s Fall through System.out.println("jolly"); Lecture 9 : Slide 21 default action in case none of the cases match System.out.println("jolly"); Lecture 9 : Slide 22 case A case B Flowchart Case A action Case B action break fall through Syntax Diagram switch ( Expression ) Switch Switch Case case Z Case Z action break Literal case : Constant Block Default action default Lecture 9 : Slide 23 Switch Case Lecture 9 : Slide 24 4

5 Braces are not really needed here but it does make the code easier to read. Remember to indent by three spaces for each pair of braces Programming Style case a : // some case a code case b : // some case b code // some default code Note alignment of opening and closing braces underneath switch and case keywords Always provide a default action if only for error checking Lecture 9 : Slide 25 final int undernuclearattack = 1; final int safe = 0; int status = safe; // watch for nuclear attack! if (status = undernuclearattack) Lecture 9 : Slide 26 Assignment = instead of equality == javac says: Assignment Error : Incompatible type for if. Can't convert int to boolean. if (status = undernuclearattack) if (status = undernuclearattack) In C/C++ this code is always executed as undernuclearattack is non-zero Lecture 9 : Slide 27 Java s strong type checking prevents errors! Lecture 9 : Slide 28 Semi-colon marks the end of the if statement ; ; This block is outside the if statement and is always executed! Lecture 9 : Slide 29 Lecture 9 : Slide 30 5

6 ; dontworrybehappy(); Javac says: Error : '' without 'if'. ; Semi-colon marks the end of the statement dontworrybehappy() Lecture 9 : Slide 31 Lecture 9 : Slide 32 panic(); panic(); Lecture 9 : Slide 33 This statement is not part of the if statement and is always exectuted. Safer to use braces to show what is inside the if statement and what isn t! Lecture 9 : Slide 34 Why? Why have so many selection statements? Any selection statement can be implemented using only if statements. We choose to use the statement that most neatly expresses our ideas. Neat programs are easier to understand and debug. Further Reading John Lewis and William Loftus, Java Software Solutions: Foundations of Program Design (third edition), Addison- Wesley, ISBN , Sections David Flanagan, Java in a Nutshell - a Desktop Quick Reference (third edition), O Reilly, ISBN , Pages Lecture 9 : Slide 35 Lecture 9 : Slide 36 6

7 Conclusions In this lecture we have seen: Conditional execution (selection) The if statement The if- statement The switch statement In the next lecture: Repetition do-while loops while loops for loops References [1] James Gosling, Bill Joy, Guy Steele and Gilad Bracha, The Java Language Specification (second edition), Addison Wesley, 2000, ISBN Lecture 9 : Slide 37 Lecture 9 : Slide 38 7