COMP 110: Introduction to Programming Tyler Johnson January 28, 2009 MWF 11:00AM-12:15PM Sitterson...

COMP 110:Introduction to Programming

Tyler JohnsonJanuary 28, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Lab 1 due tomorrow by midnight

TA Office Hours (SN 043)Nick Dragan - Tues/Thurs 2-3pm• [email protected]

Georgi Tsankov – Tues 4-6pm• [email protected]


Questions?


Today in COMP 110

Documentation and Style

Branching


Documentation & Style

The programs you write should be correct

They should also be easy to read and understand


Variable Names

Part of creating good programs is choosing meaningful variable names

If a variable stores a count, call it “count”, if a variable represents a cost, call it “cost”


Variable Names

Variables are typically named using only numbers and digits

int count;int grade1, grade2;

Underscores are generally avoided in variable names


Capitalization in Variable Names

Variable and object names are generally written in lower case

double cost, ratio;

Names consisting of multiple words should use capital letters at word boundaries

int numberOfTries;double costOfLiving;


Capitalization in Class Names

Class names generally have the first letter capitalized

Scanner, String


Naming Examples

String sentence;

Scanner keyboard;

String personalGreeting;

YourClass yourObject;


Comments

The best programs are self-documenting

It should be obvious what the program does just by reading the program itself


Comments

Choosing meaningful variables names is sometime not enough

Comments allow the programmer to leave notes in the code to aid in understanding


Comments in Java

Two Types

Single-line comment• Begins with “//”

int i; //this is a single-line comment, write whatever you want

Multi-line comment• Everything within /* */

public class Program { /*

This is a multi-line comment. The compiler will completely ignore this text*/public static void main(String[] args) {


Comments

Using a program header is a great way to give basic information about a program

AuthorDescription of the programInputsOutputs


Comments

You should use comments to explain any non-obvious details in your programs

double radius; //in inchesdouble area; //in square inches


Indentation

Programs have a nested structureYour indentation should reflect this

import java.util.Scanner;public class FirstProgram{ public static void main(String[] args) { System.out.println("Hello out there."); System.out.println("I will add two numbers for you."); System.out.println("Enter two whole numbers on a line:");

int n1, n2;

Scanner keyboard = new Scanner(System.in); n1 = keyboard.nextInt(); n2 = keyboard.nextInt();

System.out.println("The sum of those two numbers is"); System.out.println(n1 + n2); }}


Placement of Braces

Some prefer

public class FirstProgram{ …}

Others prefer

public class FirstProgram { …}

It doesn’t matter what you choose, just be consistent


Using Named Constants

You should always give a name to constants such as PI

It may not be obvious to the reader where such a number comes from

final double PI = 3.14159;

area = PI * radius * radius; //is much clearer thanarea = 3.14159 * radius * radius;


Naming Constants

The style for naming constants if different than for variables

Typically use all caps with underscores separating words

final int DAYS_PER_WEEK = 7;final double MASS_OF_EARTH = 5.9742e24; //kgfinal float INTEREST_RATE = 5.55;


Branching

Sometimes, it is necessary to make decisions in programs

ExampleThe remainder operator can be used to determine if a number n is even or odd

If n%2 equals 0, n is even

If n%2 equals 1, n is odd


Branching

int result = n % 2;

Evaluate

result is 0?

Execute

Print “n is even”

Execute

Print “n is odd”

true false


If-Else Statement

An if-else statement allows us to make decisions in a program

int result = n % 2;

if(result == 0)System.out.println(“That number is even!”);

elseSystem.out.println(“That number is odd!”);


If-Else Example

int n = 2int result = n % 2;



System.out.println(“Finished!”);

result = 0


If-Else Example

int n = 3int result = n % 2;



System.out.println(“Finished!”);

result = 1


Boolean Expressions

(result == 0) is a boolean expression

Boolean expressions evaluate to either true or false

Examples10 > 5, (true)4 > 6, (false)Integers are whole numbers, (true)


Java Comparison Operators

Math

Java Name

= == Equal to

≠ != Not equal to

> > Greater than

≥ >= Greater than or equal to

< < Less than

≤ <= Less than or equal to


If-Else Statement Syntax

Syntaxif(Boolean_Expression)Statement_1

elseStatement_2

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


Compound Statements

Multiple statements can be included in each branch

Called a compound statementEnclose between {…}

if(Boolean_Expression) {Statements_1

}else { Statements_2}


If without Else

Syntax

if(Boolean_Expression)Statement_1

Example

if(accntBalance < 0) //subtract a fee for overdraftsaccntBalance = accntBalance – FEE;


Common Comparison Mistakes

Don’t confuse the assignment operator (=) with the comparison operator (==)!

if(x == y) //valid

if(x = y) //syntax error



Don’t use a semi-colon here

if(x == y) ; //syntax errorSystem.out.println(“x equals y”);



Don’t use == to compare StringsUse string.equals(A_String) or string.equalsIgnoreCase(A_String)Example

String s1 = keyboard.next(); //read in a stringif(s1.equals(“Hello”))

System.out.println(“The String is Hello.”);else

System.out.println(“The String is not Hello.”);


The && Operator (AND)

We can check for multiple conditions using the && (AND) operator

Meaning is similar to that of English “and”

if (temperature > 50 && temperature < 75) {// walk to school if 50 < temperature < 75

}else {

//otherwise drive to school}


The || Operator (OR)

We can also join boolean expression with || (OR)

Meaning is similar to that of English “or”

boolean raining, runningLate;

…if (raining || runningLate) {

//drive to school}


The ! Operator (NOT)

Boolean negation!false is true!true is false

Example

boolean cloudy;

…

if (!cloudy) {// walk to school if it’s not cloudy

}


Effect of Boolean Operators

A B A && B

A || B !A

true true true true false

true false false true false

false true false true true

false false false false true


Boolean Expression Examples

Using x = 5, y = 10, z = 15

(x < 5 && y > x)• (false && true) -> false

(x <= 5 || y > x)• (true || true) -> true

(x > 3 || z != 15)• (true || false) -> true

(!(x > 3) && x + y == z)• (false && true) -> false


Avoiding the Negation Op

!(A < B) -->

!(A <= B) -->

!(A > B) -->

!(A >= B) -->

!(A == B) -->

!(A != B) -->

(A >= B)

(A > B)

(A <= B)

(A < B)

(A != B)

(A == B)

It’s best to avoid use of the negation operator (!) when possible


Nested If-Statements

It’s possible to have if statements inside other if statements

if(Boolean_Expression_1) {

if(BooleanExpression_2) {…

}}


Nested If Statements

ExampleWe want to perform some checks on the user’s account balance

If the balance is >= 0, we’ll add interest only if the interest rate is >= 0, and print an error message if interest rate is < 0. If the balance is < 0, we’ll subtract a fee.



Evaluate

balance >= 0

Evaluate

INTEREST_RATE >= 0

Execute

balance -= FEE

true false

Execute

balance = balance + balance*INTEREST_RATE

Execute

Print Error Message a

true false



if(balance >= 0) {if(INTEREST_RATE >= 0) balance = balance +

INTEREST_RATE*balance;else

System.out.println(“Negative Interest!”);}else

balance = balance – FEE;



What if we didn’t need to print the error message?

if(balance >= 0 && INTEREST_RATE >= 0) balance = balance +

INTEREST_RATE*balance;else

balance = balance – FEE;


Multi-Branch If Statements

What if we need to decide between many possibilities?

ExampleGiven a numeric score (0..100) determine whether the grade is an A,B,C,D, or F



A if: score >= 90B if: 90 > score >= 80C if: 80 > score >= 70D if: 70 > score >= 60F in all other cases



We could write this as follows

if(score >=90) grade = ‘A’;else if(score >=80) grade = ‘B’; else if(score >=70) grade = ‘C’; else if(score >=60) grade = ‘D’; else grade = ‘F’;



The preferred way to write this is

if(score >=90)grade = ‘A’;

else if(score >=80)grade = ‘B’;

else if(score >=70)grade = ‘C’;

else if(score >=60)grade = ‘D’;

elsegrade = ‘F’;


Multi-Branch If Statement

Syntax

if(Boolean_Expression_1)Action_1

else if(Boolean_Expression_2) Action_2

…else if(Boolean_Expression_n)

Action_nelseDefault_Action


Programming Demo

Write a program to read in three distinct nonnegative integers from the keyboard and display them in increasing order


Programming Demo

Call the three numbers a,b,c

Designing the algorithmDetermine which of a,b,c is the smallest• If it’s not a, is it b? If it’s not a or b, must be c

Determine which of the two remaining integers is smaller than the other


Programming Demo

Evaluate

a is smallest?

true false

a < b < c

a < c < b

Evaluate

b < ctrue false

b < a < c

b < c < a

Evaluate

a < ctrue false

c < a < b

c < b < a

Evaluate

a < btrue false

Evaluate

b is smallest?

true false


Programming Demo

PseudocodeAsk user for three integers, a,b,cDetermine which of a,b,c is the smallestDetermine which of the remaining two is smallerPrint a,b,c in ascending order


Friday

Recitation

BringLaptop (fully charged)TextbookAny questions about Program 1

COMP 110: Introduction to Programming Tyler Johnson January 28, 2009 MWF 11:00AM-12:15PM Sitterson...

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