Java Variables and Expressions

1
Java Variables and Expressions

• CSC160
• Professor Pepper
• (presentation adapted from Dr. Siegfried)

2
Average

• On Paper Average 2 4 6
• Pay attention to your steps

3
A very simple average program

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

4
Noun types
Name Type Formal Name
Program ? Average1
Value 1 Int Dont need
Value 2 Int Dont need
Value 3 Int Dont need
Sum Int sum
Average Double average
5
Writing Our Second Program

• Add up these values
• Divide the sum by the number of values
• Print the result

sum 2 4 6
an assignment statement
sum 2 4 6
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Assignment Statements

• Assignment statements take the form
• variable expression

Memory location where the value is stored
Combination of constants and variables
7
Expressions

• Expressions combine values using one of several
  operations.
• The operations being used is indicated by the
  operator
• Addition
• - Subtraction
• Multiplication
• / Division

8
Expressions Some Examples

• 2 5
• 4 value
• x / y

9
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 what the values represent
10
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
11
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.
12
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
13
Writing Our Second Program

• public 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.
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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)

15
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.)

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Some Illegal Identifiers
17
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 to3.40282347 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  

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What type to use?

• Repeat a value often ? worry about the size ?
• Float and Double imprecise ? not for big money!

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Assignment

• int number1 33
• double number2
• number2 number1
• byte?short?int?long?float?double
• char

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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

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Math Operators PEMDAS

• add
• - subtract
• multiply
• - division
• remainder
• Example base (rate hours)

22
Fancy Math

• variable variable op (expression)
• count count 1
• count count (6 / 2a 3)
• variable op expression
• count 1
• count (6 / 2a 3)
• Example
• int count 1
• count 2
• The value of count is now
• 3

23
More Fancy Math

• Increment
• Decrment
• n adds 1 before executing
• n adds 1 after executing
• Example

24
Characters

• Lets talk in words, not numbers!
• char single character
• Note it with single quotes (ex a, 1)
• Cant 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

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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.

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STRINGS

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

G O O D B Y E W O R L D
0 1 2 3 4 5 6 7 8 9 0 1 2

• 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

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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()

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Updated Avg Pgm

• public 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)

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Escape Characters 1

• BUT, I really want a quote inside my string!
• \ is -? abc\def -? abcdef
• \ is -? abc\def -? abcdef
• \\ is \ -? abc\def -? abc\def

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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)

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Constants

• Constant doesnt 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 -gt just do it.

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Spelling Conventions

• Name constants
• Variables start lower case
• Classes uppercase
• Word boundaries upper case (numberOfPods)

33
Comments

• // -gt comment line ex // this is a comment
• / xxx / ? comment between marks ex
• / these are a bunch of comments
• xy
• that line above is meaningless /
• Space liberally

34
Another Version of Average

• Lets rewrite the average program so it can find
  the average any 3 numbers we try
• First, make up examples
• We now need to
• Find our three values
• Add the values
• Divide the sum by 3
• Print the result

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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)

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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

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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
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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

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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 value
• 1.2.1 System.out.println
• ("Enter the second value ?")
• 1.2.2 Read in the second value
• 1.3.1 System.out.println
• ("Enter 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

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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)

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Reading from the keyboard

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

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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 values
• 3. Divide the sum by 3
• 4. Print the result

43
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
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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
45
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)

46
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()
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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)
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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

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Examples
Pay Rate Hours Gross
6.75 10 67.50
6.75 0 0
100 40 4000
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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
51
Developing The Payroll Program
1.1.1 Prompt the user for the rate 1.1.2 Read the
rate 1.2.1 Prompt the user for the
hours 1.2.2 Read the hours
We can substitute

• 1.1 Get the rate
• 1.2 Get the hours
• 2. Calculate the gross pay
• 3. Print the gross pay

52
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.

53
Developing The Payroll Program (continued)

• 1.1.1 Prompt the user for the rate
• 1.1.2 Read the rate
• 1.2.1 Prompt the user for the hours
• 1.2.2 Read the hours
• 2. Calculate the gross pay
• 3. Print the gross pay

System.out.println("What is your hourly pay
rate?") double rate keyb.nextDouble()
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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 hours
• 1.2.2 Read the hours
• 2. Calculate the gross pay
• 3. Print the gross pay

System.out.println("How many hours did you
work?") double hours keyb.nextDouble()
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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 pay
• 3. Print the gross pay

double gross rate hours
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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)
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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)
58

• 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()

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• // 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)

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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.

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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

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Examples for pounds to kilograms

• Weight in pounds (int) Weight in kilograms
• 0 0
• -22 -10
• 220 100
• 225 102.27

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Pounds to Kilograms Program (continued)

• Our program must
• Get the weight in pounds
• Calculate the weight in kilograms
• Print the weight in kilograms

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Pounds to Kilograms Program (continued)

• Our program must
• Get the weight in pounds
• Calculate the weight in kilograms
• Print the weight in kilograms

1.1 Prompt the user for the weight in
pounds 1.2 Read the pounds
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Pounds to Kilograms Program (continued)

• Our program must
• 1.1 Prompt the user for the weight in pounds
• 1.2 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()
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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
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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")
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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")
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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 0
• 100 20 5
• 300 100 3

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Our Programs Steps

1. Find the length and width
2. Calculate the area
3. Print the area

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Our Programs Steps (continued)

1. Find the length and width
2. Calculate the area
3. Print the area

1.1 Find the length 1.2 Find the width
72
Our Programs Steps (continued)

• 1.1 Find the length
• 1.2 Find the width
• 2. Calculate the area
• 3. 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
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Our Programs 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 width
• 2. Calculate the area
• 3. Print the area

System.out.println("Enter the length?") double
length keyb.nextDouble()
System.out.println("Enter the width?") double
width keyb.nextDouble()
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Our Programs 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 area
• 3. Print the area

double area length width
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Our Programs 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)
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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." )
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// Get the inputs System.out.println(
"Enter the length?") double length
keyb.nextDouble() System.out.println("En
ter the width?") double width
keyb.nextDouble() // Calculate and
display the area double area length
width System.out.println("The area is "
area)
78
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

79
Try Scanner

• Tell the user to Type an integer and then a
  word, and press Enter
• Print it back to them with You typed ltthe first
  number they typedgt and ltthe word they typedgt.
• 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.)

80
Scanner Play solution

• import 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)