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Java Variables and Expressions

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.

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Java Variables and Expressions

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  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. 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

  4. Noun types

  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;

  6. 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.

  14. 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.)

  16. 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

  17. Types

  18. What type to use? • Repeat a value often  worry about the size  • Float and Double imprecise  not for big money!

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

  20. 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

  21. 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 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

  25. 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.

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

  27. 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()

  28. 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); } }

  29. 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

  30. 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)

  31. 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.

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

  33. 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

  34. 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: • Find our three values • Add the values • Divide the sum by 3 • Print the result

  35. 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)

  36. Writing Average3b This first step becomes: 1.1Find 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

  37. Noun types

  38. 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 1.1.2Read 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

  39. Writing Avg3 (continued) We can prompt the user with: 1.1.1 System.out.println ("Enter the first value ?"); 1.1.2Read 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.Addthe values 3. Divide the sum by 3 4. Print the result

  40. 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);

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

  42. 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

  44. 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();

  47. 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); } }

  48. 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

  49. Examples

  50. Our Design for payroll • Get the inputs • Calculate the gross pay • Print the gross pay We can substitute: 1.1 Get the rate 1.2 Get the hours

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