Announcements Program 1 due Wed @ noon Lab 1 due Wed @ noon Michele Weigle - COMP 14 - Spr 04 Announcements Program 1 due Wed @ noon Lab 1 due Wed @ noon 1

Questions? Last lecture covered conditionals

Today in COMP 110 The type boolean switch statements Enumerations

Review If-Statements Syntax Used to make decisions or check conditions in a program E.g. check the value of a variable Syntax if(Boolean_Expression) Statement_1 else Statement_2 If Boolean_Expression is true, Statement_1 is executed; otherwise Statement_2 is executed

If Statement Exercise What is the output? x Output 4 C 5 6 AB 9 10 A if(x > 5) { System.out.println(‘A’); if(x < 10) System.out.println(‘B’); } else System.out.println(‘C’); x Output 4 C 5 6 AB 9 10 A 11

Boolean Expressions An expression that evaluates to either true or false Consider ((x > 10) || (x < 100)) Why is this probably not what the programmer intended? It’s true for any x Consider ((2 < 5) && (x < 100)) It’s the same as (x < 100)

The Type boolean boolean is a primitive type in Java We can declare variables of type boolean just like we declare an int, double, etc. boolean ready; boolean error;

Booleans Using booleans can make your programs easier to understand //a bit difficult to read if(temp <= 100 && thrust >= 12000 && cabinPressure > 30) System.out.println(“Launch”); else System.out.println(“Abort”);

Booleans Booleans can be used inside boolean expressions boolean tempSafe = temp <= 100; boolean thrustSafe = thrust >= 12000; boolean pressureSafe = cabinPressure > 30; boolean systemsGo = tempSafe && thrustSafe && pressureSafe; //much easier to read if(systemsGo) System.out.println(“Launch”); else System.out.println(“Abort”);

True and False The words true and false are also reserved words in java We can use them to initialize boolean variables boolean ready = false; boolean initialized = true;

Booleans There’s no need to write if(systemsGo == true) System.out.println(“Launch”); The more concise and equivalent way is if(systemsGo)

Naming Booleans Choose names for boolean variables that sound true when the value of the variable is true boolean ready; //are we ready? boolean readingInput; //are we reading input? boolean errorEncountered; //have we encountered //an error?

Precedence Java uses precedence rules when evaluating boolean expressions Example score >= 80 && score < 90 The expressions to the left and right of && are evaluated first

Operator Precedence Highest Precedence Lowest Precedence First: the unary operators +, -, ++, --, ! Second: the binary operators *, /, % Third: the binary operators +, - Fourth: the boolean operators <, >, >=, <= Fifth: the boolean operators ==, != Sixth: the boolean operator && Seventh: the boolean operator || Lowest Precedence

Boolean Precedence Example 4 < 3 / 2 – 10 || 4 * -2 > 9 4 < 1 – 10 || 4 * -2 > 9 4 < 1 – 10 || -8 > 9 4 < -9 || -8 > 9 false || -8 > 9 false || false false

Style It’s usually best to indicate precedence in boolean expressions explicitly with parentheses (score < ((min / 2) – 10)) || (score > 90)

Short-Circuit Evaluation In some cases, the result of a boolean expression can be determined before all subparts of the expression are evaluated Example true || (x >= 60) This expression is true regardless of the value of x

Short-Circuit Evaluation Java uses what’s called short-circuit evaluation when evaluating boolean expression If at any point in the evaluation of a boolean expression the outcome is determined, any remaining subparts are not evaluated

Short-Circuit Evaluation Example We’re computing an average homework score Print good work if the average is above 60 if(numAssignments > 0 && ((total / numAssignments) > 60) System.out.println(“Good work!”); Without short-circuit evaluation, we would divide by zero if numAssignments == 0

Reading in Booleans We can read in booleans from the keyboard just like any other variable boolean bVar; Scanner keyboard = new Scanner(System.in); bVar = keyboard.nextBoolean();

Switch Statements If-statements with many branches can be difficult to read The switch statement can be used as an alternative to a multi-branch if-statement in certain cases

Switch Statement A switch statement begins like this switch(Controlling_Expression) { … } Controlling_Expression must have type byte, short, int or char

Switch Statements Inside the body of a switch statement, a number of case labels will appear The different cases are separated with a break statement case Case_Label_1: Statements_1 break; … case Case_Label_n: Statements_n

Switch Statement Example int year; … switch(year) { case 1: System.out.println(“You are a freshman”); break; case 2: System.out.println(“You are a sophomore”); case 3: System.out.println(“You are a junior”); case 4: System.out.println(“You are a senior”); default: System.out.println(“This is a default case”); }

Switch Statement Syntax switch(Controlling_Expression) { case Case_Label: Statements break; case Case_Label : … default: }

Switch Statements If no break statement is specified, for a case, execution will continue down to the next case int n; … switch(n) { case 1: System.out.println(“one”); case 2: System.out.println(“two”); break; } If n == 1, this code prints both one and two

Switch Statement Example char input; … switch(input) { case 'y': case 'Y': System.out.println(“You entered yes”); break; case 'n': case 'N': System.out.println(“You entered no”); default: System.out.println(“Invalid input”); }

Default Case When using a switch statement you should always provide a default case This catches any conditions you may not have checked for, such as errors default: System.out.println(“This is a default case”); break;

Conversion to If The previous example can also be written equivalently with an if-statement if(input == 'y' || input == 'Y') System.out.println(“You entered yes”); else if(input == 'n' || input == 'N') System.out.println(“You entered no”); else System.out.println(“Invalid input”);

If/Switch How to know whether to use a switch or an if-statement? Switch statement Can only be used with type char or byte, short, int When you want to choose between many, specific values such as 5,6, 'y' etc. If statement Can only be used with boolean expressions When the number of choices is relatively small When you want to check a range of possibilities, e.g. x > 5, y <= 1000

Enumerations Suppose you wanted to write a computer program that stores different flavors of ice cream Vanilla, Chocolate, Strawberry, etc. How would we store them in a computer program?

Enumerations We can give the flavors an underlying numeric representation Vanilla = 0, Chocolate = 1, Strawberry = 2 We could declare a variable integer to store our flavor, but this is error-prone int flavor = 0; //vanilla flavor = 1; //chocolate

Enumerations An enumeration allows us to give unique numeric values to a list of items enum Flavor {Vanilla, Chocolate, Strawberry} This statement assigns a unique numeric value to each of {Vanilla, Chocolate, Strawberry}

Switch/Enum Example enum Flavor {Vanilla, Chocolate, Strawberry} Flavor flavor; //declare a variable of type Flavor … switch(flavor) { case Vanilla: System.out.println(“That’s Vanilla!”); case Chocolate: System.out.println(“That’s Chocolate!”); break; case Strawberry: System.out.println(“That’s Strawberry!”); default: System.out.println(“I don’t recognize that flavor”); }

Programming Demo Write a program that takes two numbers as input from the user The user should then be able to choose from among the following options by entering the appropriate symbol Add the two numbers Subtract the two numbers Multiply the two numbers Divide the two numbers

Programming Demo Pseudocode

Programming Demo Programming

Wednesday Loops

Programming Demo Sample Pseudocode Ask user to input two numbers Read in two numbers Provide the user with a list of arithmetic options Read in their choice Perform the operation selected by the user Output the result