comp 110: introduction to programming

COMP 110:Introduction to Programming

Tyler JohnsonFeb 2, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Program 1 due Wed by midnightCouple things


Questions?


Today in COMP 110

The type boolean

switch statements

Enumerations


Review

If-StatementsUsed to make decisions or check conditions in a program• E.g. check the value of a variable

Syntaxif(Boolean_Expression)

Statement_1else

Statement_2

If Boolean_Expression is true, Statement_1 is executed; otherwise Statement_2 is executed


If Statement Exercise

What is the output?

if(x > 5) { System.out.print(‘A’); if(x < 10) System.out.print(‘B’);}else System.out.print(‘C’);

x Output

4 C

5 C

6 AB

9 AB

10 A

11 A


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))Why is this probably not what the programmer intended?It’s the same as (x < 100)


The Type boolean

boolean is a primitive type in JavaStores the value true or false

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 readif(temp <= 100 && thrust >= 12000 && cabinPressure

> 30)System.out.println(“Launch”);

elseSystem.out.println(“Abort”);


Booleans

Booleans can be used inside boolean expressions

boolean systemsGo = temp <= 100 && thrust >= 12000 && cabinPressure > 30;

//much easier to readif(systemsGo)

System.out.println(“Launch”);else

System.out.println(“Abort”);


True and False

The words true and false are also reserved words in javaWe 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)System.out.println(“Launch”);


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

Examplescore >= 80 && score < 90The expressions to the left and right of && are evaluated first


Operator Precedence

Highest 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 > 94 < 1 – 10 || 4 * -2 > 94 < 1 – 10 || -8 > 94 < -9 || -8 > 9false || -8 > 9false || falsefalse


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



Java uses what’s called short-circuit evaluation when evaluating boolean expressions

If at any point in the evaluation of a boolean expression the outcome is determined, any remaining subparts are not evaluated



ExampleWe’re computing an average homework scorePrint “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 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_1break;…

case Case_Label_n:Statements_nbreak;


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”);

break;case 3: System.out.println(“You are a junior”);

break;case 4: System.out.println(“You are a senior”); break;default: System.out.println(“This is a default case”); break;

}



int year = 1;…switch(year) {





}



int year = 3;…switch(year) {





}


Switch Statement Syntax

switch(Controlling_Expression) {case Case_Label: Statements break;case Case_Label : Statements break;…default: Statements break;

}


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 onetwo



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”);

break;default: System.out.println(“Invalid input”); break;

}


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”);

elseSystem.out.println(“Invalid input”);


If/Switch

How to know whether to use a switch or an if-statement?

Switch statementCan only be used with type char or intWhen you want to choose between many, specific values such as 5,6, 'y' etc.

If statementCan only be used with boolean expressionsWhen the number of choices is relatively smallWhen 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; //vanillaflavor = 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!”); break;case Chocolate: System.out.println(“That’s Chocolate!”); break;case Strawberry: System.out.println(“That’s Strawberry!”); break;default: System.out.println(“I don’t recognize that flavor”); break;

}


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

Add the two numbersSubtract the two numbersMultiply the two numbersDivide the two numbers


Programming Demo

PseudocodeAsk user to input two numbersProvide the user with a list of the optionsPerform the operation selected by the userOutput the result


Programming Demo

Programming


Wednesday

Loops

Keep up with the readingWe’ll start Ch 4 on Wed

comp 110: introduction to programming

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