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Variables

Learn about Java's variables, data types, and how to create and manipulate them. Understand primitive and object types, declaration rules, changing variable contents, and writing programs using variables in real-world applications.

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Variables

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  1. Variables Pepper

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

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

  4. 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 Description Examples • int integers (whole numbers) 42, -3, 0, 926394 • double real numbers 3.14, -0.25, 9.4e3 • char single text characters 'a', 'X', '?', '\n' • boolean logical values true, false

  5. Types

  6. Create a variable • create a variable • In Java: <type> <variable name> ;  example: int myIntVar; • 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 myQuantity • double myDollars • boolean answer • char oneLetter • String myName

  7. Change a variable • change variable contents: • In Java: <variable name> = <some expression>; example:  myIntVar = 5 ;. • Equal sign means set equal to • Left side gets put into right side • You try – set those variables = to something • myQuantity = 1 • myDollars = 3.3 • answer = true • oneLetter = ‘a’ • myName = “pepper” • Now, print those variables with: • System.out.println(myQuantity + myDollars + answer + oneLetter + myName) • What happens when you put single or double quotes where there were none, or take away the quotes?

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

  9. 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.

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

  11. Writing the Average 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;

  12. Assignment Statements • Assignment statements take the form: variable=expression Memory location where the value is stored Combination of constants and variables

  13. Expressions • Expressions combine values using one of several operations. • The operations being used is indicated by the operator: + Addition - Subtraction * Multiplication / Division Examples: • + 5 • 4 * value x / y

  14. Writing the Average program 1 • 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

  15. 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

  16. 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.

  17. Writing the Average program 4 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

  18. Writing the Average program 5 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.

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

  20. 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)

  21. 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.)

  22. Spelling Conventions • Name constants • Variables start lower case • Classes uppercase • Word boundaries upper case (numberOfPods)

  23. IllegalIdentifier Reason Suggested Identifier my age Blanks are not allowed myAge 2times Cannot begin with a number times2 or twoTimes four*five * is not allowed fourTimesFive time&ahalf & is not allowed timeAndAHalf Some Illegal Identifiers

  24. Assignment int number1 = 33; double number2; number2 = number1; byteshortintlongfloatdouble char

  25. 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

  26. Math Operators & PEMDAS • + add • - subtract • * multiply • - division • % remainder Example: base + (rate * hours)

  27. 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

  28. More Fancy Math • Increment ++ • Decrment – • ++n adds 1 before executing • n++ adds 1 after executing Example:

  29. 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

  30. 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

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