Variables and Assignments CSC 121 Fall 2015 Howard Rosenthal

Transcription

1 Variables and Assignments CSC 121 Fall 2015 Howard Rosenthal

2 Lesson Goals Understand variables Understand how to declare and use variables in Java Programs Learn how to formulate assignment statements Learn how to read in data from a terminal Creating your first object Using the Scanner object for interactive input Compatibility and casting 2

3 What is a Variable A variable (either primitive or an object) is a named memory location capable of storing data of a specified type Variables indicate logical, not physical addresses Those are taking care of in the JVM and the OS Variables in Java always have a specific data type Variables must always be declared before they can be used Remember that a data type is a scheme for using bit patterns to represent a value or a reference. Think of a variable as a little box made of one or more bytes that can hold a value using a particular data type. Cost_of_Home $128,

4 What is a Variable (2) A declaration of a variable is where a program says that it needs a variable The value of a variable is always its last declared or assigned value in a program public class Example1_CH3 { public static void main ( String [] args ) { long payamount; //the declaration of the variable payamount = 123; // variable assignment System.out.println("The variable payamount contains: " + payamount ); payamount = ; // variable reassignment } } 4

5 Declaring Variables Variables must be declared before the are used Basic syntax: Type name1, name2, name3 ; Example: double dinner_bill, credit_balance, money_in_bank; Variables can be explicitly initialized when they are used long rent = 75; double milkprice_half = 1.99; boolean testimony = true; Can t do multiple assignments at once when declaring variables int x=y=z=0; // Incorrect syntax int x,y,z=60; // only z is initialized int x=60, y=60, z=60; //all three are initialized Don t try to assign a value that that is illegal short big_number = ; This gives an error why? 5

6 Naming Variables Use only the characters 'a' through 'z', 'A' through 'Z', '0' through '9', character '_', and character '$'. A variable name cannot contain the space character. Do not start with a digit. An identifier can be any length. Java is case sensitive. Upper and lower case count as different characters. SUM and Sum are different identifiers. A variable name can not be a reserved word. Don t use the same name twice, even as different types 6

7 Assigning Variables Assignment statements look like this: variablename = expression ; //The equal sign = is the assignment operator. variablename is the name of a variable that has been declared previously in the program. Remember: An expression is a combination of literals, operators, variable names, and parentheses used to calculate a value must be syntactically correct Assignment is accomplished in two steps: Evaluate the expression Store the value in the variable which appears to the left of the = sign Examples: total = 3 + 5; // total =8 price = 34.56; tax = price*0.05; //tax = While you can t do multiple initializations at once, you can do multiple assignments: x=y=z=0; // works as an assignment statement provided the variables have been declared previously these work right to left int x=y=z=3; // This will generate a syntax error because you are first declaring the variables here x=y=z=14+5; // First calculate the value of the expression, then do the assignments 7

8 Assigning Variables (2) What does the following program fragment write? value = 5; System.out.println("value is: " + value ); value = value + 10; System.out.println("value is: " + value ); 8

9 Final Variables Final variables are really constants They are assigned a value that doesn t change final double PI = ; final int PERFECT = 100; Final variables are by convention written with all capitals 9

10 An Example of a Simple Program public class Fibonacci { // Print out the Fibonacci sequence for values < 50 public static void main(string[] args) { int lo = 1; int hi = 1; System.out.println(lo); while (hi < 50) { System.out.println(hi); hi = lo + hi; // new hi lo = hi - lo; /* new lo is (sum - old lo) i.e., the old hi */ } } } Note: The Fibonacci sequence is 1,1,2,3,5,8,13,21,34,55. The next number is the sum of the previous two numbers. 10

11 Obtaining Input Data Using Scanner A Scanner is an object. You must include the following statement to use the object: import java.util.scanner or import java.util.* You are importing a class with all it s methods that can then be used to create objects Declare a Scanner object: Scanner ScannerName = new Scanner (System.in) new means you are creating a new object new is a reserved word System.in is a variable that tell the Java program that this input stream is coming the keyboard You can now access seven types of input: byte, short, int, long, double, float and boolean using predefined methods associated with object A Scanner object doesn t read characters The ScannerName is just another identifier 11

12 There are 7 Input Methods Available ScannerName.nextByte() ScannerName.nextShort() ScannerName.nextInt() ScannerName.nextLong() ScannerName.nextDouble() ScannerName.nextFloat() ScannerName.nextBoolean() What you are looking at above is the standard way objects access methods: objectname.method(par1, par2, ) - There are no parameters required for these methods 12

13 An Example Using Scanner import java.util.scanner; // Make the Scanner class available. public class Interest2withScanner { public static void main(string[] args) { Scanner keyboard = new Scanner( System.in ); // Create the Scanner. double principal; // The value of the investment. double rate; // The annual interest rate. double interest; // The interest earned during the year. System.out.print("Enter the initial investment: "); principal = keyboard.nextdouble(); System.out.print("Enter the annual interest rate "); rate = keyboard.nextdouble(); rate = rate/100; interest = principal * rate; // Compute this year's interest. principal = principal + interest; // Add it to principal. System.out.print("The value of the investment after one year is $"); System.out.println(principal); // } } // end of class Interest2withScanner 13

14 CasPng (1) The value of a smaller numerical type may be assigned to a higher numerical data type automatically Do you know the order? Casting down must be done explicitly Example: int natural, bigger; double odds, othernumber; odds = 20.3; natural = 5; othernumber = (int)(odds)*natural; bigger = (int)othernumber; System.out.println(otherNumber); //prints System.out.println(bigger); //prints 100 othernumber = odds*natural; bigger = (int)othernumber; System.out.println(otherNumber); //prints System.out.println(bigger); //prints

15 CasPng (2) Character data may be assigned to any of the types short, int, long, double or float When this happens the ASCII/Unicode value is assigned to the numerical value double x = A ; System.out.print(x); The output is 65.0 Why? 15

16 Shortcuts Operator Shortcut For += x+=10 x=x+10 - = x- =10 x=x- 10 *= x*=10 x=x*10 /= x/=10 x=x/10 %= x%=10 x=x%10 16

17 Prefix and PosRix for IncremenPng and DecremenPng There are two mechanisms for adding or subtracting 1 from a variable ++number; //prefix form for adding 1 to number - - number; //prefix form for subtracting 1 from number number++; //postfix form for adding 1 to number number- - ; //postfix form for subtracting 1 from number What s the difference? In assignment statements prefix action take effect before the assignment In assignment statements postfix action take effect after the assignment 17

18 Prefix and PosRix for IncremenPng and DecremenPng (2) Prefix Incrementing int number =5,result; result = 3*(++number); System.out.println(result); 18 System.out.println(number); 6 Postfix Incrementing int number =5,result; result = 3*(number++); System.out.println(result); 15 System.out.println(number); 6 Use postscript incrementing carefully, it can be confusing However, it is very useful when dealing with loops 18

19 Programming Exercise Class (1) Exercise 11. Running Sums Write a program that accepts ten integers n1, n2, n3,. n10 and prints a running sum that is, your program should display ten sums: n1, n1+n2, n1+n2+n3 and so on. For example, if the input is 3, 28, 5, 8, 9, 10, 12, 2, 1, - 19 then the output is

20 10. Larger or Smaller Programming Exercise Lab(1) Write a program that accepts 5 integers, and for each integer following the first, prints true or false depending on whether or not that integer is greater than the previous one. This program can be written more simply after reading chapter 4. 20