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COMP 110: Introduction to Programming Tyler Johnson January 28, 2009 MWF 11:00AM-12:15PM Sitterson 014

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COMP 110: Spring Announcements Lab 1 due tomorrow by midnight TA Office Hours (SN 043) Nick Dragan - Tues/Thurs 2-3pm Georgi Tsankov – Tues 4-6pm

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COMP 110: Spring Questions?

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COMP 110: Spring Today in COMP 110 Documentation and Style Branching

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COMP 110: Spring Documentation & Style The programs you write should be correct They should also be easy to read and understand

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COMP 110: Spring 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

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COMP 110: Spring Variable Names Variables are typically named using only numbers and digits int count; int grade1, grade2; Underscores are generally avoided in variable names

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COMP 110: Spring 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;

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COMP 110: Spring Capitalization in Class Names Class names generally have the first letter capitalized Scanner, String

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COMP 110: Spring Naming Examples String sentence; Scanner keyboard; String personalGreeting; YourClass yourObject;

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COMP 110: Spring Comments The best programs are self- documenting It should be obvious what the program does just by reading the program itself

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COMP 110: Spring Comments Choosing meaningful variables names is sometime not enough Comments allow the programmer to leave notes in the code to aid in understanding

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COMP 110: Spring 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) {

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COMP 110: Spring Comments Using a program header is a great way to give basic information about a program Author Description of the program Inputs Outputs

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COMP 110: Spring Comments You should use comments to explain any non-obvious details in your programs double radius; //in inches double area; //in square inches

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COMP 110: Spring Indentation Programs have a nested structure Your 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); } }

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COMP 110: Spring Placement of Braces Some prefer public class FirstProgram { … } Others prefer public class FirstProgram { … } It doesnt matter what you choose, just be consistent

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COMP 110: Spring 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 = ; area = PI * radius * radius; //is much clearer than area = * radius * radius;

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COMP 110: Spring 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 = e24; //kg final float INTEREST_RATE = 5.55;

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COMP 110: Spring Branching Sometimes, it is necessary to make decisions in programs Example The 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

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COMP 110: Spring Branching int result = n % 2; Evaluate result is 0? Execute Print n is even Execute Print n is odd true false

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COMP 110: Spring 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!); else System.out.println(That number is odd!);

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COMP 110: Spring If-Else Example int n = 2 int result = n % 2; if(result == 0) System.out.println(That number is even!); else System.out.println(That number is odd!); System.out.println(Finished!); result = 0

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COMP 110: Spring If-Else Example int n = 3 int result = n % 2; if(result == 0) System.out.println(That number is even!); else System.out.println(That number is odd!); System.out.println(Finished!); result = 1

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COMP 110: Spring Boolean Expressions (result == 0) is a boolean expression Boolean expressions evaluate to either true or false Examples 10 > 5, (true) 4 > 6, (false) Integers are whole numbers, (true)

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COMP 110: Spring Java Comparison Operators Math Java Name ===Equal to !=Not equal to >>Greater than >=Greater than or equal to <

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COMP 110: Spring If-Else Statement Syntax Syntax if(Boolean_Expression) Statement_1 else Statement_2 If Boolean_Expression is true, Statement_1 is executed; otherwise Statement_2 is executed

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COMP 110: Spring Compound Statements Multiple statements can be included in each branch Called a compound statement Enclose between {…} if(Boolean_Expression) { Statements_1 } else { Statements_2 }

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COMP 110: Spring If without Else Syntax if(Boolean_Expression) Statement_1 Example if(accntBalance < 0) //subtract a fee for overdrafts accntBalance = accntBalance – FEE;

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COMP 110: Spring Common Comparison Mistakes Dont confuse the assignment operator (=) with the comparison operator (==)! if(x == y) //valid if(x = y) //syntax error

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COMP 110: Spring Common Comparison Mistakes Dont use a semi-colon here if(x == y) ; //syntax error System.out.println(x equals y);

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COMP 110: Spring Common Comparison Mistakes Dont use == to compare Strings Use string.equals(A_String) or string.equalsIgnoreCase(A_String) Example String s1 = keyboard.next(); //read in a string if(s1.equals(Hello)) System.out.println(The String is Hello.); else System.out.println(The String is not Hello.);

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COMP 110: Spring 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 }

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COMP 110: Spring 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 }

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COMP 110: Spring The ! Operator (NOT) Boolean negation !false is true !true is false Example boolean cloudy; … if (!cloudy) { // walk to school if its not cloudy }

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COMP 110: Spring Effect of Boolean Operators ABA && BA || B!A true false truefalse truefalse truefalsetrue false true

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COMP 110: Spring Boolean Expression Examples Using x = 5, y = 10, z = 15 (x x) (false && true) -> false (x x) (true || true) -> true (x > 3 || z != 15) (true || false) -> true (!(x > 3) && x + y == z) (false && true) -> false

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COMP 110: Spring Avoiding the Negation Op !(A !(A > B) --> !(A >= B) --> !(A == B) --> !(A != B) --> (A >= B) (A > B) (A <= B) (A < B) (A != B) (A == B) Its best to avoid use of the negation operator (!) when possible

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COMP 110: Spring Nested If-Statements Its possible to have if statements inside other if statements if(Boolean_Expression_1) { if(BooleanExpression_2) { … }

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COMP 110: Spring Nested If Statements Example We want to perform some checks on the users account balance If the balance is >= 0, well add interest only if the interest rate is >= 0, and print an error message if interest rate is < 0. If the balance is < 0, well subtract a fee.

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COMP 110: Spring Nested If Statements Evaluate balance >= 0 Evaluate INTEREST_RATE >= 0 Execute balance -= FEE truefalse Execute balance = balance + balance*INTEREST_RATE Execute Print Error Message a truefalse

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COMP 110: Spring Nested If Statements if(balance >= 0) { if(INTEREST_RATE >= 0) balance = balance + INTEREST_RATE*balance; else System.out.println(Negative Interest!); } else balance = balance – FEE;

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COMP 110: Spring Nested If Statements What if we didnt need to print the error message? if(balance >= 0 && INTEREST_RATE >= 0) balance = balance + INTEREST_RATE*balance; else balance = balance – FEE;

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COMP 110: Spring Multi-Branch If Statements What if we need to decide between many possibilities? Example Given a numeric score (0..100) determine whether the grade is an A,B,C,D, or F

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COMP 110: Spring Multi-Branch If Statements A if: score >= 90 B if: 90 > score >= 80 C if: 80 > score >= 70 D if: 70 > score >= 60 F in all other cases

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COMP 110: Spring Multi-Branch If Statements 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;

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COMP 110: Spring Multi-Branch If Statements 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; else grade = F;

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COMP 110: Spring Multi-Branch If Statement Syntax if(Boolean_Expression_1) Action_1 else if(Boolean_Expression_2) Action_2 … else if(Boolean_Expression_n) Action_n else Default_Action

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COMP 110: Spring Programming Demo Write a program to read in three distinct nonnegative integers from the keyboard and display them in increasing order

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COMP 110: Spring Programming Demo Call the three numbers a,b,c Designing the algorithm Determine which of a,b,c is the smallest If its not a, is it b? If its not a or b, must be c Determine which of the two remaining integers is smaller than the other

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COMP 110: Spring Programming Demo Evaluate a is smallest? true false a < b < ca < c < b Evaluate b < c true false b < a < cb < c < a Evaluate a < c true false c < a < bc < b < a Evaluate a < b true false Evaluate b is smallest? truefalse

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COMP 110: Spring Programming Demo Pseudocode Ask user for three integers, a,b,c Determine which of a,b,c is the smallest Determine which of the remaining two is smaller Print a,b,c in ascending order

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COMP 110: Spring Friday Recitation Bring Laptop (fully charged) Textbook Any questions about Program 1

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