Variables Pepper Variable A variable box holds a

Variables Pepper

Variable • A variable – box – holds a certain type of value – value inside the box can change • Example – A = 2 B+1 – Slope = change in y / change in x – monopoly square – holds different monopoly pieces

Data types • type: A category or set of data values. – Examples: integer, real number, string. • Internally, the computer stores all data as 0 s and 1 s. – examples: 42 "hi" 101010 0110100001101001

Java's primitive types • primitive types: Java's built-in simple data types for numbers, text characters, and logic. – Java has eight primitive types. – Types that are not primitive are called object types. • Four primitive types we will use: – – Name int double char boolean Description Examples integers (whole numbers) 42, -3, 0, 926394 real numbers 3. 14, -0. 25, 9. 4 e 3 single text characters 'a', 'X', '? ', 'n' logical values true, false

Types type kind memory range byte integer 1 byte -128 to 127 • short integer 2 bytes -32768 to 32767 integer 4 bytes -2147483648 to 2147483647 long integer 8 bytes -9223372036854775808 to -9223372036854775807 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 10324 single character 2 bytes all Unicode characters 1 bit char boolean true or false

Create a variable • create a variable – In Java: <type> <variable name> ; example: int my. Int. Var; – declare with correct type: int, double, char, boolean, (not capitalized) – declare Object variables with types such as : String, Scanner, Random, (capitalized) • You try: – Create a program that creates the following variables: • • • int my. Quantity double my. Dollars boolean answer char one. Letter String my. Name

Change a variable • change variable contents: – In Java: <variable name> = <some expression>; example: my. Int. Var = 5 ; . – Equal sign means set equal to – Left side gets put into right side • You try – set those variables = to something • • • my. Quantity = 1 my. Dollars = 3. 3 answer = true one. Letter = ‘a’ my. Name = “pepper” • Now, print those variables with: – System. out. println(my. Quantity + my. Dollars + answer + one. Letter + my. Name) • What happens when you put single or double quotes where there were none, or take away the quotes?

Use variables in a real program • On Paper: Average 2 + 4 + 6 • Pay attention to your steps

Writing the Average program Specification – 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 these values • Divide the sum by the number of values • Print the result Each of these steps will be a different statement.

Flow chart • Done in Gliffy • https: //www. gliffy. com/ • Show

Writing the Average program sum = 2 + 4 + 6; • Add up these values • Divide the sum by the number of values • Print the result 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: + * / Examples: 2 + 5 3 4 * value x / y Addition Subtraction Multiplication Division

Writing the Average program 1 • sum = 2 + 4 + 6; • Divide the sum by the average = sum / 3; number of values • Print the result Names that describe what the values represent

Writing the Average program 2 • sum = 2 + 4 + 6 • average = sum / 3; • Print the result System. out. println(″The average is ″ + average); The output method variable name

Writing the Average program 3 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 the Average program 4 public class Average 3 { 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 the Average program 5 public class Average 3 a { 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.

You try • Add 1 to the average and then print it again. average = average + 1

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. )

Spelling Conventions • • Name constants Variables start lower case Classes uppercase Word boundaries upper case (number. Of. Pods)

Some Illegal Identifiers Illegal Identifier my age 2 times four*five Reason Suggested Identifier Blanks are not allowed Cannot begin with a number * is not allowed my. Age time&ahalf & is not allowed times 2 or two. Times four. Times. Five time. And. AHalf

Assignment int number 1 = 33; double number 2; number 2 = number 1; byte short int long float double char

Dividing • int / int (even if you assign it to a double) • float / int float • int / 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 / 2 * a + 3) variable op = expression count += 1 count += (6 / 2 * a + 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:

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

Summary • A variable holds one value • Variables have certain types • Literals of different types are entered differently (i. e. quotes for String) • The system keeps variable values until you change them. • Constants are a special kind of variable – no changing