COMP 110Introduction to Programming

Mr. Joshua StoughSeptember 19, 2007

Basic Review

• Terms you need to know– variable, type, value, expression,

statement, declaration, assignment, initialization, package, class, method, parameter, return type• What is the return type of

Integer.parseInt(String)• double x = 3.14596*Math.pow(rad,2);

• You need to learn what every line of code you write does.

ReviewFlow of Execution

ReviewRelational Operators

• Less than <• Greater than >• Equal to ==

– not assignment ‘=‘

• Not equal to !=• Less than or equal to <=• Greater than or equal to >=

ReviewBoolean Operators

• NOT! (unary)!(2+2==5)

• AND&& (binary)(2+2==5) && (1+1==2)

• OR || (binary)(2+2==5) || (1+1==2)

true

false

true

Selection

• if statements• if-else statements• Nested if statements• switch statements

• Textbook Reference: Chapter 4 (pgs. 178-234)

Conditional Statements

• Let us choose which statement will be executed next– also called selection statements

• Java's conditional statements:– the if statement– the if-else statement– the switch statement

One-Way Selection

• Syntax: if (expression) statement

The if Statement

if ( condition ) statement;

if is a Javareserved word

The condition must be a boolean expression.It must evaluate to either true or false.

If the condition is true, the statement is executed.If it is false, the statement is skipped.

The if Statement

if (sum > MAX) delta = sum - MAX;System.out.println ("The sum is " + sum);

First, the condition is evaluated. The value of sumis either greater than the value of MAX, or it is not.

If the condition is true, the assignment statement is executed.If it is not, the assignment statement is skipped.

Either way, the call to println is executed next.

Block Statements

Syntax:{

statement1statement2

.

.

.statementn

}

We use curly braces to group a set of individual statements.

This way we can have multiple statements execute based on a decision.

If we don't use curly braces, only a single statement will be executed based on the result of a decision.

The if Statement

if (sum > MAX)delta = sum - MAX;

System.out.println ("The sum is " + sum);

A

if (sum > MAX) {delta = sum - MAX;

}System.out.println ("The sum is " + sum);

B

if (sum > MAX) {delta = sum - MAX;System.out.println ("The sum is " + sum);

}

C

Curly Braces

int num = 87, max = 25;if (num >= max*2) {

System.out.println ("apple");}

System.out.println ("orange");System.out.println ("pear");

int num = 87, max = 25;if (num >= max*2)

System.out.println ("apple");System.out.println ("orange");

System.out.println ("pear");

State.java Example

if Statement Gotcha

if (score >= 90);grade = "A";

No matter the result of the condition,grade will be assigned "A". The semicolonafter the if statement is a semantic error.

In-Class Exercises

What is printed by the following code fragment?

int num = 87, max = 25;if (num >= max*2)

System.out.println ("apple");System.out.println ("orange");

System.out.println ("pear");

Write a code fragment that will print the value of val if val is less than MAX.

appleorangepear

if (val < MAX) {System.out.println (val);

}

Two-Way Selection

The if-else Statement

if ( condition ) statement1;else statement2;

• If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

• One or the other will be executed, but not both

The if-else Statement

if (height <= MAX) {adjustment = 0;

}else {

adjustment = MAX-height;}

In-Class Exercise

• What is the value of adjustment given the following values of height and MAX?

if (height <= MAX) {adjustment = 0;

}else {

adjustment = MAX-height;}

• height = 60, MAX = 80• height = 100, MAX = 75• height = 45, MAX = 45

0

0-25

Nested if Statements

• The statement (or block) executed as a result of an if statement can be another if statement (nested if).

• An else clause is always matched to the closest unmatched if.

• Closing curly brace signals the end of an if statement.

Nested if StatementsThe general syntax of a nested if statement is:

if (condition1) {block1

}else if (condition2) {block2

}else {block3

}

Nested if Statementsif (hasFourLegs) {

if (playsFetch) {System.out.println ("DOG");

} else {

System.out.println ("CAT");}

}else if (hasWings) {

System.out.println ("BIRD");}else {

System.out.println ("FISH");}

has four legs

does not have four legs

plays fetch

doesn't play fetch

has wingsdoesn't have wings

Min.java Example

The switch Statement

The switch Statement

• Provides another means to decide which statement to execute next

• Evaluates an expression, then attempts to match the result to one of several possible cases

• Each case contains a value and a list of statements

• The flow of control transfers to statement associated with the first value that matches

The switch Statement• The general syntax of a switch statement is:

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here

The switch Statement

• Expression evaluated must be integral type– integer or character, not boolean or floating

point

• Case values must be constant (literal), not variable

• Can be implemented with nested if statements, but is much clearer with switch statements

The switch Statement

• default– no case value matches the expression– if no default exists and no case value

matches the expression, no statements in the switch statement are executed

• break– processing jumps to statement

following the switch statement– usually at the end of each case

The switch Statement

System.out.print ("Enter prize code: ");int prize = Integer.parseInt(keyboard.readLine());switch (prize){

case 1:System.out.println (``A Brand New Car!’’);break;

case 2:System.out.println (``A Trip to Hawaii!’’);break;

default:System.out.println (``Sorry, Try Again’’);break;

}

GradeReport.java Example

QuestionsAssume movieRating is an int and movieName is a String

switch (movieRating) {case 1:

System.out.println ("Run away!");if (movieName.equals("Gigli")) {

System.out.println ("Quickly!");}

case 2:System.out.println ("Save your money");break;

case 3:System.out.println ("It's OK");break;

}Run away!Quickly!Save your money

[Nothing]

1. What is printed if movieRating is 1 and movieName is "Gigli"?

2. What is printed if movieRating is 5?

Questionsif ((x > 0) && (y < 0)) {

z = x+y;System.out.println (“z = ” + z);

}System.out.print (“The sum is “);if (x+y < 0) { System.out.println (“negative.”)}else { System.out.println (“positive.”);}

z=1The sum is positive.The sum is negative.The sum is positive.

What is printed with the following values of x and y:1. x = 5, y = -4

2. x = -9, y = 53. x = 5, y = 5

Suggested Practice Problems• Answers are in back of textbook

• Ch 4 : #1, 2, 5, 8, 11

Next Time in COMP 110

• Loops

• Reading Assignment: Chapter 5