comp 110: spring 20091 announcements lab 1 was due at noon lab 2 on friday (bring laptops)...

COMP 110: Spring 20091

Announcements

Lab 1 was due at noonLab 2 on Friday

(Bring Laptops)

Assignment 1 is onlineTA Office hours online30-min quiz at end of class

select all that applymultiple choice


Questions?

Last time we coveredTypecastingStringsBinary/Unary operatorsI/OStyle


Today in COMP 110

Branchingif – else control structurebooleansrelational operators


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

We know how to calculate n % 2

int result = n % 2;

But what next to do next?


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

==

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-withdrawal < 0) //overdraft feeaccntBalance = accntBalance – FEE;


Common Comparison Mistakes

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

if(x == y) //valid

if(x = y) //syntax error


Common Comparison Mistakes

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”

if (raining || runningLate) {//drive to school

}


The ! Operator (NOT)

Boolean negation!false is True!true is false

Example

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 statementsExample

We want to perform some checks on the user’s account balanceIf the balance is >= 0, we’ll add interest 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.


Nested If Statements

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;

//whats wrong here?


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

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)Any questions about Program 1

comp 110: spring 20091 announcements lab 1 was due at noon lab 2 on friday (bring laptops)...

Documents