itm352 conditionals lecture #5. 1/28/03itm352 fall 2003 class 5 – control flow 2 announcements r...

ITM352ConditionalsLecture #5

1/28/03 ITM352 Fall 2003 Class 5 – Control Flow 2

Announcements Assignment 1

Create a Zip file of your JBuilder project directory (the folder that contains the .jpr or .jpx file, your source code, etc.) and email it as an attachment to [email protected]

If you don’t know how to create a Zip file or email an attachment ask a friend or contact me

Assignment 1 is due by 11:59pm TONIGHT ** beware ** I use a very sophisticated program that checks to see if your

code is the same as another's. It cannot be easily fooled by just changing the names of variables or moving lines around.

Short, in class quizzes will start TODAY Mostly on terminology Will be easy if you have been paying attention You may not get a quiz every class

Lab class dates have changed!!! Check the “Weekly Schedule” for details Go to E 102, NOT this classroom!!!


Announcements (cont.) The “last of the last minute man” homework curse

continues…. DO NOT PUT OFF PROGRAMMING ASSIGNMENTS

TO THE DAY THEY ARE DUE!!!! It’s far too risky, too many things can go wrong.

I VERY strongly recommend you always test and review anything that is given out to you (homework, examples, etc.) and anything you submit Get in the habit of compiling and testing your code as you write it Be generous with your code comments (you are never penalized

for too many comments)


** Important Notice **

Section 2 On 2/4, 2/6, 2/11 please go to the section 1

lecture from 12:00-1:15pm, there will be no 1:30-2:45pm lecture on those dates

If you absolutely cannot make these lectures, come see me RIGHT AWAY


Today

Review Basic Java Program Variables, types, casting SavitchIn I/O

Documentation and Coding StyleControl Flow

Conditionals (decisions) Loops <if time>


Review


Basic Java Program Structure

public class MyClass {

public static void main (String args[]) {

<your code here>

} // end of main method

} // end of MyClass def


Types: Which Ones to Know for Now

int just whole numbers may be positive or negative no decimal point

char just a single character uses single quotes for examplechar letterGrade = `A`;

double real numbers, both positive and

negative has a decimal point (fractional

part) two formats

number with decimal point, e.g. 514.061

e (or scientific, or floating-point) notation, e.g. 5.14061 e2, which means 5.14061 x 102

boolean two values “true” or “false”

Display in text is for reference; for now stick to these simple primitive types:


Specialized Assignment Operators

A shorthand notation for performing an operation on and assigning a new value to a variable

General form: var <op>= expression; equivalent to: var = var <op> (expression); <op> is +, -, *, /, or %

Examples:

amount += 5;

//amount = amount + 5;

amount *= 1 + interestRate;

//amount = amount * (1 + interestRate); Note that the right side is treated as a unit (put parentheses around the entire

expression)


Returned Value Expressions return values: the number produced by an

expression is “returned”, i.e. it is the “return value.”int numberOfBaskets, eggsPerBasket, totalEggs;

numberOfBaskets = 5;

eggsPerBasket = 8;

totalEggs = numberOfBaskets * eggsPerBasket; in the last line numberOfBaskets returns the value 5 and eggsPerBasket returns the value 8

numberOfBaskets * eggsPerBasket is an expression that returns the integer value 40

Similarly, methods return valuesSavitchIn.readLine(); is a method that returns a string read

from the keyboard


Explicit casting is required to assign a higher type to a lower

ILLEGAL: Implicit casting to a lower data type

int n;

double x = 2.1;

n = x;//illegal in java

It is illegal since x is double, n is an int, and double is a higher data type than integer

LEGAL: Explicit casting to a lower data type int n;

double x = 2.1;

n = (int)x;//legal in java You can always use an explicit cast where an implicit one will be done

automatically, but it is not necessary


Truncation When Casting a doubleto an Integer

Converting (casting) a double to integer does not round; it truncates the fractional part is lost (discarded, ignored, thrown away)

For example:

int n;

double x = 2.99999;

n = (int)x;//cast is required, x is truncated the value of n is now 2

This behavior is useful for some calculations, as demonstrated in Case Study: Vending Machine Change


Examples of Expressions

Ordinary MathExpression

Java Expression(preferred form)

Java FullyParenthesizedExpression

rate2 + delta rate*rate + delta (rate*rate) + delta

2(salary + bonus) 2 * (salary + bonus) 2 * (salary + bonus)

3mass time

1

1/(time + 3 * mass) 1/(time + (3 * mass))

9v t

7 - a

(a - 7) / (t + 9 * v) (a - 7) / (t +( 9 * v))


Increment and Decrement Operators

Shorthand notation for common arithmetic operations on variables used for counting

Some counters count up, some count down, but they are integer variables The counter can be incremented (or decremented) before or after using its

current value

int count;

…

++count preincrement count: count = count + 1 before using it

count++ postincrement count: count = count + 1 after using it

--count predecrement count: count = count -1 before using it

count-- postdecrement count: count = count -1 after using it


Indexing Characters within a String The index of a character is the position of a character within a string Strings are 0 based meaning the first index is at position 0. The charAt(Position)method returns the char at the specified

position substring(Start, End)method returns the string from position

Start to position End For example:

String greeting = "Hi, there!";greeting.charAt(0)returns Hgreeting.charAt(2)returns ,greeting.substring(4,6)returns “th”

H i , t h e r e !

0 1 2 3 4 5 6 7 8 9


More String Methods

length() equals(Other_String) equalsIgnoreCase(Other_String) toLowerCase() toUpperCase() charAt(position) substring(Start) substring(Start, End) indexOf(Other_String) indexOf(Other_String, Start)

See pp. 82-83 for more details!


Concatenating (Appending) StringsThe + operator is used for string concatenation (merging two strings):

String name = "Mondo";String greeting = "Hi, there!";System.out.println(greeting + name + "Welcome");causes the following to display on the screen:>Hi, there!MondoWelcome> Note that you have to remember to include spaces if you want it to look

right:System.out.println(greeting + " " + name + " Welcome"); causes the following to display on the screen:>Hi, there! Mondo Welcome>


I/O Classes

We have been using an output method from a class that automatically comes with Java:

System.out.println() But Java does not automatically have an input class, so one must be added

SavitchIn is a class specially written to do keyboard input SavitchIn.java is provided with the text - see Appendix 2 Examples of SavitchIn methods for keyboard input:

readLineInt()readLineDouble()readLineNonwhiteChar()

Gotcha: remember Java is case sensitive, for example readLineNonWhiteChar()will not work


End of Review


Documentation and Style

Use meaningful names for variables, classes, etc. Use indentation and line spacing as shown in the examples

in the text Always include a “prologue” (an brief explanation of the

program at the beginning of the file) Use all lower case for variables, except capitalize internal

words (eggsPerBasket) Use all upper case for variables that have a constant value, PI for the value of pi (3.14159…) (see text for more examples)


Comments

Comment—text in a program that the compiler ignores Does not change what the program does, only explains the program Write meaningful and useful comments Comment the non-obvious Assume a reasonably knowledgeable reader // for single-line comments /* … */ for multi-line comments Two kinds of comments: inside or outside of a block Comments outside of a block explain what the block of code does


Indentation

Codes inside a block should be indented.Indent 2 to 4 spacesBe consistent.


Named Constants

Named constant—using a name instead of a value Example: use TAX_RATE instead of 5.25 Advantages of using named constants

Easier to understand program because reader can tell how the value is being used

Easier to modify program because value can be changed in one place (the definition) instead of being changed everywhere in the program.

Avoids mistake of changing same value used for a different purpose


Defining Named Constants

public—has public visibility. There are no restrictions on where this name can be used

static—must be included, but explanation has to waitfinal—the program is not allowed to change the value The remainder of the definition is similar to a variable

declaration and gives the type, name, and initial value. A declaration like this is usually at the beginning of the

file and is not inside the main method definition.

public static final double PI = 3.14159;


Chapter 3

BranchingLoopsexit(n) methodBoolean data type and

expressions

Flow of Control


What is “Flow of Control”? Flow of Control is the execution order of instructions in a

program All programs can be written with three control flow

elements:1. Sequence - just go to the next instruction2. Selection - a choice of at least two

either go to the next instruction or jump to some other instruction

3. Repetition - a loop (repeat a block of code) at the end of the loop

either go back and repeat the block of code or continue with the next instruction after the block

Selection and Repetition are called Branching since these are branch points in the flow of control


Java Flow Control Statements

Sequence the defaultJava automatically

executes the next instruction unless you use a branching statement

Branching: Selection if if-else if-else if-else if- … - else

switch

Branching: Repetition while do-while for


Definition of Boolean Values Branching: there is more than one choice for the next

instruction Which branch is taken depends on a test condition which

evaluates to either true or false In general:

if test is true then do this, otherwise it is false, do something else

Variables (or expressions) that are either true or false are called boolean variables (or expressions)

So the value of a boolean variable (or expression) is eithertrue or false

boolean is a primitive data type in Java


Boolean Expressions Boolean expressions can be thought of as test

conditions (questions) that are either true or false Often two values are compared For example:

Is A greater than B?Is A equal to B?Is A less than or equal to B?etc.

A and B can be any data type (or class), but they should be the same data type (or class)


Java Comparison Operators

Math Notation

Name

Java Notation

Java Examples

= equal to == balance == 0 answer == 'y'

not equal to != income != tax answer != 'y'

> greater than

> income > outgo

greater than or equal to

>= points >= 60

< less than

< pressure < max

less than or equal to <= income <= outgo


Comparing objects Great care must be taken when comparing objects:

Statements like myRobot == theRobot compare references to where the objects are stored, not the objects themselves!

In general the equals() method is used The objects themselves define what it means for objects to be

equal

myRobot theRobot

myRobot == theRobotis false

aRobot

theRobot == aRobotis true


Example: Comparison Methods forString Class

“==“ does not do what you may think for String objects When “==“ is used to test objects (such as String objects) it tests to

see if the storage addresses of the two objects are the same are they stored in the same location? more will be said about this later

Use “.equals” method to test if the strings, themselves, are equalString s1 = “Mondo”;String s2;s2 = SavitchIn.readLine();//s1.equals(s2) returns true if the user enters Mondo, false

otherwise .equals()is case sensitive Use .equalsIgnoreCase()to ignore case


Compound Boolean Expressions Use && to AND two or more conditions Use || to OR two or more conditions See text for definitions of AND and OR For example, write a test to see if B is either 0 or between

the values of B and C :(B == 0) || (A <= B && B < C)

In this example the parentheses are not required but are added for clarity See text (and later slides) for Precedence rules Note the short-circuit, or lazy, evaluation rules in text (and later in

slides) Use a single & for AND and a single | for OR to avoid short-

circuit evaluation and force complete evaluation of a boolean expression


Java if statement

Simple decisions Do the next statement if test is true or skip it if

false Syntax:

if (Boolean_Expression)

Action if true;//execute if true

next action;//always executed

Note the indentation for readability (not compile or execution correctness)


if Example

The body of the if statement is conditionally executed

Statements after the body of the if statement always execute

if(eggsPerBasket < 12)

//begin body of the if statement

System.out.println(“Less than a dozen eggs per basket”);

//end body of the if statement

totalEggs = numberOfEggs * eggsPerBasket;

System.out.println(“You have a total of

+ totalEggs + “ eggs.”);


Java Statement Blocks:Compound Statements

Action if true can be either a single Java statement or a set of statements enclosed in curly brackets (a compound statement, or block)

For example:

if(eggsPerBasket < 12){ //begin body of the if statement System.out.println(“Less than a dozen ...”); costPerBasket = 1.1 * costPerBasket} //end body of the if statement

totalEggs = numberOfEggs * eggsPerBasket;System.out.println(“You have a total of “ + totalEggs + “ eggs.”);

All statements between braces are controlled by if


Two-way Selection: if-else Select either one of two options Either do Action1 or Action2, depending on test value Syntax:

if (Boolean_Expression)

{

Action1 //execute only if true

}

else

{

Action2//execute only if false

}

Action3//always executed


if-else Examples

Example with single-statement blocks:if(time < limit) System.out.println(“You made it.”);else

System.out.println(“You missed the deadline.”);

Example with compound statements:

if(time < limit){ System.out.println(“You made it.”); bonus = 100;}else{ System.out.println(“You missed the deadline.”); bonus = 0;}


Multibranch selection:if-else if-else if-…-else

One way to handle situations with more than two possibilities

Syntax:if(Boolean_Expression_1) Action_1else if(Boolean_Expression_2) Action_2 . . .else if(Boolean_Expression_n) Action_nelse Default_Action


if-else if-else if-…-else Example

if(score >= 90 && score <= 100)

grade = ‘A’);

else if (score >= 80)

grade = ‘B’;


grade = ‘C’;


grade = ‘D’;

else

grade = ‘E’;

Note how the sequence is important here and must use else if rather than just if (why?)


if-else if-else if-…-else Non-Example??

if (profRel.equals(“colleague”) ) greeting = “Daniel”;else if (profRel.equals(“friend”) ) greeting = “Dan”;else if (profRel.equals(“grad student”) ) greeting = “DP”;else if (profRel.equals(“undergrad student”) ) greeting = “Professor”;else

greeting = “Mr. Port”;


Quiz #1 – 5 points (5 mins)

Name: _____________ID #: ______________

1) Write a Java statement that declares a variable of type float named birthYear and initialize it with the year of your birth (explicitly cast your birth year value appropriately)

2) Write out a complete Java program (with class definition, main method, comments, proper coding style, etc.) that prints “I was born in the year” and concatenates it with the variable birthYear

3) Write a compound conditional expression with a type boolean return value of true when a string lastName is your last name and birthYear is greater or equal to 1982.

itm352 conditionals lecture #5. 1/28/03itm352 fall 2003 class 5 – control flow 2 announcements r...

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