Java Variables and Expressions

CSC160

Professor Pepper

(presentation adapted from Dr. Siegfried)

Average

• On Paper: Average 2 + 4 + 6

• Pay attention to your steps

A very simple average program

Problem – write a program which can find the average of three numbers.

Let’s list the steps that our program must perform to do this:• Add up the three values• Divide the sum by the number of values• Print the resulting average

Each of these steps will be a different statement.List the nouns to find your objects – program, value

1, value 2, value 3, sum, average

Noun types

Name Type Formal Name

Program ? Average1

Value 1 Int Don’t need

Value 2 Int Don’t need

Value 3 Int Don’t need

Sum Int sum

Average Double average

Writing Our Second Program

• Add up these values

• Divide the sum by the number of values

• Print the result

sum = 2 + 4 + 6;

sum = 2 + 4 + 6; an assignment statement

Assignment Statements

• Assignment statements take the form:variable = expression

Memory location where the value is stored Combination of constants

and variables

Expressions

• Expressions combine values using one of several operations.

• The operations being used is indicated by the operator:+ Addition- Subtraction* Multiplication/ Division

Expressions – Some Examples

2 + 5

4 * value

x / y

Writing Our Second Program

• sum = 2 + 4 + 6;• Divide the sum by the

number of values

• Print the result

average = sum / 3;

Names that describe whatthe values represent

Writing Our Second Program

• sum = 2 + 4 + 6• average = sum / 3;• Print the result

System.out.println(″The average is ″ + average); The output method variable

name

Writing Our Second Program

public static void main(String[] args) {

-------------------- sum = 2 + 4 + 6; average = sum / 3; System.out.println("The average is "

+ average); }

We still need to add a declare our variables. This tells the computer what they are.

Writing Our Second Program

public class Average3 { public static void main(String[] args) { int sum, average; sum = 2 + 4 + 6; average = sum / 3; System.out.println("The average is " +

average); }}

Tells the computer that sum and average are integers

Writing Our Second Programpublic class Average3a {

public static void main(String[] args) {

int sum;

int average;

sum = 2 + 4 + 6;

average = sum / 3;

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

}

}

We could also write this as two separate declarations.

Variables and Identifiers

• Variables have names – we call these names identifiers.

• Identifiers identify various elements of a program (so far the only such element are the variables.

• Some identifiers are standard (such as System)

Identifier Rules

• An identifier must begin with a letter or an underscore _

• Java is case sensitive upper case (capital) or lower case letters are considered different characters. Average, average and AVERAGE are three different identifiers.

• Numbers can also appear after the first character.• Identifiers can be as long as you want but names

that are too long usually are too cumbersome.• Identifiers cannot be reserved words (special

words like int, main, etc.)

Some Illegal Identifiers

timeAndAHalf& is not allowed

time&ahalf

fourTimesFive* is not allowedfour*five

times2 or twoTimes

Cannot begin with a number

2times

myAgeBlanks are not allowed

my age

Suggested IdentifierReasonIllegal Identifier

Types

•

type kind memory range

byte integer 1 byte -128 to 127

short integer 2 bytes -32768 to 32767

int integer 4 bytes -2147483648 to 2147483647

long integer 8 bytes -9223372036854775808 to-9223372036854775807    

float floating point 4 bytes ±3.40282347 x 1038 to±3.40282347 x 10-45  

double floating point 8 bytes ±1.76769313486231570 x 10308 to ±4.94065645841246544 x 10-324     

char single character

2 bytes all Unicode characters

boolean true or false 1 bit  

What type to use?

• Repeat a value often worry about the size

• Float and Double imprecise not for big money!

Assignment

int number1 = 33;

double number2;

number2 = number1;

byteshortintlongfloatdoublechar

Dividing

• int / int int (even if you assign it to a double)

• float / int float

• int / float float

Solution: Cast it

ans = n / (double) m

Math Operators & PEMDAS

• + add

• - subtract

• * multiply

• - division

• % remainder

Example: base + (rate * hours)

Fancy Math

variable = variable op (expression)count = count + 1 count = count + (6 / 2a + 3)variable op = expressioncount += 1 count += (6 / 2a + 3)

Example:int count = 1;count += 2;The value of count is now 3

More Fancy Math

• Increment ++

• Decrment –

• ++n adds 1 before executing

• n++ adds 1 after executing

Example:

Characters

Let’s talk in words, not numbers!

• char = single character

• Note it with single quotes (ex: ‘a’, ‘1’)

• Can’t move to byte or short

• We can store single characters by writing:char x, y;– x and y can hold one and only one character

Character Strings

• We are usually interested in manipulating more than one character at a time.

• We can store more than one character by writing:String s;

• If we want s can hold to have some initial value, we can write:String s = “Initial value";

• For now, we use character data for input and output only.

STRINGS • Type : String Holds text• Enter with double quotes “abc”• Really a class, so capitalize String• Just a list of chars. Example byeString:

GOODB YE W O R L D

• Start at 0• byeString.charAt(3) is D• byeString.length() is 13• byeString.equals(somethingOtherString) is either true or false• byeString.toUpperCase() is GOODBYE WORLD• byeString.toLowerCase() is goodbye world

0 1 2 3 4 5 6 78 9 0 1 2

Average Pgm with String

• Change the AVG program to put “the average is” into a string first, and convert the string to Uppercase using toUpperCase()

Updated Avg Pgmpublic class Average3 { public static void main(String[] args) { int sum, average;

String averageLabel = “The average is “; averageLabel = averageLabel.toLowerCase();

sum = 2 + 4 + 6; average = sum / 3; System.out.println( averageLabel + average); }}

Escape Characters 1

• BUT, I really want a quote inside my string!

\” is “ - “abc\”def” - abc”def

\’ is ‘ - “abc\’def” - abc’def

\\ is \ - “abc\def” - abc\def

Escape Characters 2

How do I get new lines and tabs?

\n= new line (go to beginning of next line)

\r =carriage return (go to beginning of this line)

\t = tab (go to next tab stop)

Constants

Constant doesn’t change

Why use a variable if

massive changes later

show meaning

avoid Hard coding

public static final int MAX_PEOPLE = 20;

Capitalize by convention only -> just do it.

Spelling Conventions

• Name constants

• Variables start lower case

• Classes uppercase

• Word boundaries upper case (numberOfPods)

Comments

• // -> comment line ex: // this is a comment

• /* xxx */ comment between marks ex:

/* these are a bunch of comments

x=y;

that line above is meaningless */

• Space liberally

Another Version of Average• Let’s rewrite the average program so it

can find the average any 3 numbers we try:

• First, make up examples• We now need to:

1. Find our three values2. Add the values3. Divide the sum by 34. Print the result

Examples for Average

• 0 + 0 + 0 = 0/3 = 0 (Try zeroes)

• 100 -50 -29 = 21/3 = 7 (Try + and -)

• 2 + 4 + 6 = 12/3 = 4 (Try normal)

Writing Average3b

This first step becomes:

1.1 Find the first value

1.2 Find the second value

1.3 Find the third value

2. Add the values

3. Divide the sum by 3

4. Print the result

Noun types

Name Type Formal Name

Program ? Average1

Value 1 int value1

Value 2 int value2

Value 3 int value3

Sum int sum

Average double average

Writing Avg3 (continued)

Since we want the computer to print out some kind of prompt, the first step becomes:

1.1.1 Prompt the user for the first value

1.1.2 Read in the first value

1.2.1 Prompt the user for the second value

1.2.2 Read in the second value

1.3.1 Prompt the user for the third value

1.3.2 Read in the third value

2. Add the values

3. Divide the sum by 3

4. Print the result

Writing Avg3 (continued)We can prompt the user with:

1.1.1 System.out.println ("Enter the first value ?");

1.1.2 Read in the first value1.2.1 System.out.println ("Enter the second value ?");

1.2.2 Read in the second value1.3.1 System.out.println ("Enter the third value ?");

1.3.2 Read in the third value2.Add the values3. Divide the sum by 34. Print the result

The Scanner Class

• Most programs will need some form of input.

• At the beginning, all of our input will come from the keyboard.

• To read in a value, we need to use an object belonging to a class called Scanner:

Scanner keyb = new Scanner(System.in);

Reading from the keyboard

• Once we declare keyb as Scanner, we can read integer values by writing:variable = keyb.nextInt();

Writing the input statements in Average3b

We can read in a value by writing:System.out.println ("What is the first value\t?");int value1 = keyb.nextInt();System.out.println ("What is the second value\t?");int value2 = keyb.nextInt();System.out.println ("What is the third value\t?");int value3 = keyb.nextInt();

2. Add the values3. Divide the sum by 34. Print the result

Writing the assignments statements in Average3b

System.out.println ("What is the first value\t?");

int value1 = keyb.nextInt();

System.out.println

("What is the second value\t?");

int value2 = keyb.nextInt();

System.out.println

("What is the third value\t?");

int value3 = keyb.nextInt();

sum = value1 + value2

+ value3;3. Divide the sum by 3

4. Print the result Adding up the three values

Writing the assignments statements in Average3b

System.out.println ("What is the first value\t?");

int value1 = keyb.nextInt();

System.out.println

("What is the second value\t?");

int value2 = keyb.nextInt();

System.out.println

("What is the third value\t?");

int value3 = keyb.nextInt();

sum = value1 + value2 + value3;

average = sum / 3;4. Print the result

Calculating the average

Writing the output statement in Average3b

System.out.println

("What is the first value\t?");

int value1 = keyb.nextInt();

System.out.println

("What is the second value\t?");

int value2 = keyb.nextInt();

System.out.println

("What is the third value\t?");

int value3 = keyb.nextInt();

sum = value1 + value2 + value3;

average = sum / 3;

System.out.println("The average is "

+ average);

import java.util.Scanner;

public class Average3b { public static void main(String[] args) { int sum, average; Scanner keyb = new Scanner(System.in); System.out.println ("What is the first value\t?"); int value1 = keyb.nextInt(); System.out.println ("What is the second value\t?"); int value2 = keyb.nextInt();

System.out.println ("What is the third value\t?"); int value3 = keyb.nextInt(); sum = value1 + value2 + value3; average = sum / 3; System.out.println("The average is " + average); }}

Another example – calculating a payroll

• We are going to write a program which calculates the gross pay for someone earning an hourly wage.

• We need two pieces of information:– the hourly rate of pay

– the number of hours worked.

• We are expected to produce one output: the gross pay, which we can find by calculating:– Gross pay = Rate of pay * Hours Worked

Examples

Pay Rate Hours Gross

$6.75 10 $67.50

$6.75 0 0

$100 40 $4000

Our Design for payroll

1. Get the inputs

2. Calculate the gross pay

3. Print the gross pay

1.1 Get the rate

1.2 Get the hours

We can substitute:

Developing The Payroll Program

1.1 Get the rate

1.2 Get the hours

2. Calculate the gross pay

3. Print the gross pay

1.1.1 Prompt the user for the rate1.1.2 Read the rate1.2.1 Prompt the user for the hours1.2.2 Read the hours

We can substitute

Coding the payroll program

• Before we code the payroll program, we recognize that the values (rate, hours and gross) may not necessarily be integers.

• We will declare these to be double, which means that they can have (but do not have to have) fractional parts.

• In Java, we usually declare our variables where they first appear in the program.

Developing The Payroll Program (continued)

1.1.1 Prompt the user for the rate1.1.2 Read the rate1.2.1 Prompt the user for the hours1.2.2 Read the hours2. Calculate the gross pay3. Print the gross pay

System.out.println("What is your hourly pay rate?");double rate = keyb.nextDouble();

Developing The Payroll Program (continued)

System.out.println ("What is your hourly pay rate?");double rate = keyb.nextDouble();1.2.1 Prompt the user for the hours1.2.2 Read the hours2. Calculate the gross pay3. Print the gross pay

System.out.println("How many hours did you work?");double hours = keyb.nextDouble();

Developing The Payroll Program (continued)

System.out.println ("What is your hourly pay rate?");double rate = keyb.nextDouble();System.out.println ("How many hours did you work?");double hours = keyb.nextDouble();

2. Calculate the gross pay3. Print the gross pay

double gross = rate * hours;

Developing The Payroll Program (continued)

System.out.println ("What is your hourly pay rate?");double rate = keyb.nextDouble();System.out.println ("How many hours did you work?");double hours = keyb.nextDouble();

double gross = rate * hours;3. Print the gross pay

System.out.println("Your gross pay is $" + gross);

import java.util.Scanner;

public class Payroll { public static void main(String[] args) { Scanner keyb = new Scanner(System.in); System.out.println ("What is your hourly pay rate?"); double rate = keyb.nextDouble(); System.out.println ("How many hours did you work?"); double hours = keyb.nextDouble(); double gross = rate * hours; System.out.println("Your gross pay is $“ + gross); }}

import java.util.Scanner;

public class Payroll {

/** This program calculates the gross pay for an * hourly worker * Inputs - hourly rate and hours worked * Output - Gross pay */ public static void main(String[] args) { Scanner keyb = new Scanner(System.in); // Get the hourly rate System.out.println ("What is your hourly pay rate?"); double rate = keyb.nextDouble();

// Get the hours worked System.out.println ("How many hours did you work?"); double hours = keyb.nextDouble(); // Calculate and display the gross pay double gross = rate * hours; System.out.println("Your gross pay is $" + gross); }}

Using Stepwise Refinement to Design a Program

• You should noticed that when we write a program, we start by describing the steps that our program must perform and we subsequently refine this into a long series of more detailed steps until we are writing individual steps. This is called stepwise refinement.

• Stepwise refinement is one of the most basic methods for developing a program.

Example – A program to convert pounds to kilograms

• Our program will convert a weight expressed in pounds into kilograms.– Our input is the weight in pounds.– Our output is the weight in kilograms– We also know that

Kilograms = Pounds / 2.2

Examples for pounds to kilograms

Weight in pounds (int) Weight in kilograms

0 0

-22 -10

220 100

225 102.27

Pounds to Kilograms Program (continued)

• Our program must:1. Get the weight in pounds

2. Calculate the weight in kilograms

3. Print the weight in kilograms

Pounds to Kilograms Program (continued)

• Our program must:1. Get the weight in pounds

2. Calculate the weight in kilograms

3. Print the weight in kilograms

1.1 Prompt the user for the weight in pounds1.2 Read the pounds

Pounds to Kilograms Program (continued)

• Our program must:1.1 Prompt the user for the weight in pounds1.2 Read the pounds 2. Calculate the weight in kilograms 3. Print the weight in kilograms

System.out.println ("What is the weight in pounds?");double lbs = keyb.nextInt();

Pounds to Kilograms Program (continued)

System.out.println

("What is the weight in pounds?");

double lbs = keyb.nextInt(); 2. Calculate the weight in kilograms 3. Print the weight in kilograms

double kg = lbs / 2.2;

Pounds to Kilograms Program (continued)

System.out.println

("What is the weight in pounds?");

double lbs = keyb.nextInt();double kg = lbs / 2.2; 3. Print the weight in kilograms

System.out.println("The weight is " + kg + " kilograms");

import java.util.Scanner;

public class ConvertPounds {

// Convert pounds to kilograms // Input - weight in pounds // Output - weight in kilograms public static void main(String[] args) { Scanner keyb = new Scanner(System.in); // Get the weight in pounds System.out.println ("What is the weight in pounds?"); double lbs = keyb.nextInt(); // Calculate and display the weight in // kilograms double kg = lbs / 2.2; System.out.println("The weight is " + kg + " kilograms"); }}

Another Example – The Area of A Rectangle

• Our program will calculate the area of a rectangle.– Our input is the length and width.– Our output is the area.– We also know that

Area = Length * Width

0 = 0 * 0100 = 20 * 5300 = 100 * 3

Our Program’s Steps

1. Find the length and width

2. Calculate the area

3. Print the area

Our Program’s Steps (continued)

1. Find the length and width2. Calculate the area3. Print the area

1.1 Find the length1.2 Find the width

Our Program’s Steps (continued)

1.1 Find the length1.2 Find the width 2. Calculate the area3. Print the area

1.1.1 Prompt the user for the length

1.1.2 Read the length

1.2.1 Prompt the user for the width

1.1.2 Read the width

Our Program’s Steps (continued)

1.1.1 Prompt the user for the length

1.1.2 Read the length

1.2.1 Prompt the user for the width

1.1.2 Read the width2. Calculate the area3. Print the area

System.out.println("Enter the length?");double length = keyb.nextDouble(); System.out.println("Enter the width?");double width = keyb.nextDouble();

Our Program’s Steps (continued)

System.out.println("Enter the length?");

double length = keyb.nextDouble();

System.out.println("Enter the width?");

double width = keyb.nextDouble();

2. Calculate the area3. Print the area

double area = length * width;

Our Program’s Steps (continued)

System.out.println("Enter the length?");

double length = keyb.nextDouble();

System.out.println("Enter the width?");

double width = keyb.nextDouble();double area = length * width;

3. Print the area

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

import java.util.Scanner;

public class CalculateArea { // Calculates the area of a rectangle // Inputs - The length and width of the rectangle // Output - The area of the rectangle public static void main(String[] args) { Scanner keyb = new Scanner(System.in); // Print an explanatory message for the user System.out.println ("Given the width and length of a rectangle"); System.out.println ("this program calculates its area." );

// Get the inputs System.out.println("Enter the length?"); double length = keyb.nextDouble(); System.out.println("Enter the width?"); double width = keyb.nextDouble(); // Calculate and display the area double area = length * width; System.out.println("The area is " + area); }}

More on Scanner

• You can read:– nextInt()– nextLong()– nextByte()– nextDouble()– next() – up to next whitespace (delimiter) – nextLine() – up to “\n”– useDelimiter()

• Throw in nextLine() to get down a line

Try Scanner• Tell the user to “Type an integer and then a

word, and press Enter”• Print it back to them with “You typed <the

first number they typed> and <the word they typed>.”

• Then, ask for a whole line and print it back.• See that you need to be careful with the Enter

keystroke. (Capture it with keyb.nextLine.)

Scanner Play solutionimport java.util.Scanner;public class ScannerPlay{ public static void main(String[] args) { Scanner keyb = new Scanner(System.in); System.out.println ("Type an integer and then a word, and press Enter"); int number1 = keyb.nextInt(); String word1 = keyb.next(); System.out.println("You typed " + number1 + " and " + word1 + "."); System.out.println("Type something else and Enter"); keyb.nextLine(); // skip a line String line1 = keyb.nextLine(); System.out.println("You typed " + line1); }}