COMP-202: Foundations of Programming. Lecture 5: Arrays, Reference Type, and Methods Sandeep Manjanna, Summer 2015

Transcription

1 COMP-202: Foundations of Programming Lecture 5: Arrays, Reference Type, and Methods Sandeep Manjanna, Summer 2015

2 Announcements Assignment 2 posted and due on 30 th of May (23:30). Extra class tomorrow - 13:05 to 15:25 in TR0100. Midterm Exams : June 4 th (12:35 14:35) Rooms : 1B45 and 306 (Burnside) Final Exams : June 26 th (14:00 17:00) Room : MAAS 112

3 Review Loops 1. In a group of nested loops, which loop is executed the most number of times? A. The outermost loop B. The innermost loop C. All loops are executed same number of times Answer B 2. What value is stored in x at the end of this loop? for (x = 1; x <= 5; x++) A. 1 B. 4 C. 6 D. 5 Answer C

4 Review While Loops int x = 0; Step 1 Initialize Condition Fail while (x < 4) { } System.out.println("Write this again"); x++; Step 2 Condition Check Step 3 Loop body Execution Once the condition fails

5 For Loops for (int x = 0; x < 4; x++) { Step 1 Initialize Step 2 Condition Check Review Step 4 Update Condition Fail System.out.println("Write this again"); Step 3 Loop body Execution } Once the condition fails

6 Review Array of Scores of 8 students: int [ ] scores = {100, 95, 25, 99, 39, 100, 100, 90} scores Indices [0] [1] [2] [3] [4] [5] [6] [7] Here is an array with 8 values in it. Each spot of the array has a value and an index. The value is any legitimate value of the type of the array, in this case int. The index is an integer. Interestingly the counting starts from 0 instead of 1. To get or set values of an array, we will use a similar syntax to a normal variable, except we have to specify which value of the array we wish to get or set. We do this by using the index.

7 Creating Array (Method 1) int[] myarray = {1, 5, 6, 3}; Review 1) Allocate this memory. Find a free spot in memory that has enough space to hold four integers. 2) Store the address of the beginning of the memory that was just allocated into the variable myarray. myarray 3) Set values to this newly allocated memory. myarray

8 Review Creating Array (Method 2) This method is useful when we don t know the values that needs to be saved in the array: int[] myarray; Variable myarray is created to point to an array. myarray = new int[4]; myarray myarray[0] = 1; myarray Indices 1 [0] [1] [2] [3]

9 This Lecture Reference Types Reference vs Primitive Types Multidimensional Arrays Methods One step at a time.

10 Adding to our vocabulary - break break is used to stop the execution of a loop before all the iterations are completed. Example: int i; for(i=0; i<10;i++) { if(i==5) break; } System.out.println(i); break statement is useful in situations like searching in an array for a value and exiting the loop once we find it.

11 Objects Arrays, Scanners, and Strings are Objects. You can tell because you need new to create them. (Well, except Strings. Strings are special.) In fact, except for the primitive data types (the ones that start with lowercase, like int, double, float, byte, boolean, etc.), everything in Java is an Object. Objects are data bundled with methods to work with the data, and properties.

12 Methods and Properties of Objects Some String methods:.equals(),.length(),.tolowercase() Some Scanner methods:.nextline(),.nextint(),.nextdouble() In general: object.method_name() Arrays have no methods. Instead, they have a property called length.length Because this is not a method, there is no () after

13 Reference Types

14 Reference Types vs Primitive Types When we write int x = 10; 10 x what we are technically doing is the following: 1) Creating a space in memory that will store an int 2) Specifying that whenever we use the identifier x later in our program, we want to access that memory location. 3) Storing the value of 10 into that memory location.

15 Objects Using Reference Types The variables of objects, including arrays, don't directly store the values of the objects. Instead, they store a reference to the location in memory containing the value. Think of this as storing an address, which points to where the data is actually located in memory.

16 When we write Reference Types vs Primitive Types int x[] = {10, 3, 5}; what we are technically doing is the following: 1) Creating a space in memory that will store the address of an int array. 2) Specifying that whenever we use the identifier x later in our program, we want to access that space in memory (which stores an address). 3) Creating an array somewhere else in memory. That array will store the values 10, 3, and 5 and have length of 3. That array must be located somewhere in memory. Call that address a. 4) Setting the value of x to be a (In other words, the value of a is stored into the space of memory reserved for the variable x).

17 Why references can get Confusing? There are some interesting consequences of storing addresses into variables instead of the values int[] x = {1, 2, 3}; int x = 5; int[] y = x; int y = x; y[0] = 2; y = 10; System.out.println(x[0]); System.out.println(x); What will the above print? 1. 2! Because changing y[0] also changes x[0]. x and y are reference types Because x and y are primitive types in here.

18 Addresses One Step at a time Memory Values int[] x = {1,2,3} The {1, 2, 3} creates an array in memory. The address of the start of this spot in memory is 1000.

19 Addresses One Step at a time Memory Values int[] x = {1,2,3} int[] x creates a variable of type int array. This means we create space in memory to store the address of an int array. In this case, we use the storage space in memory of 2004, to store the address x

20 Addresses One Step at a time Memory Values int[] x = {1,2,3} int[] y = x; x y This means: 1) Create space in memory to store an int[] variable 2) Store into that int[] variable the value of the expression x

21 Addresses One Step at a time Memory Values int[] x = {1,2,3} int[] y = x; y[0] = 2; x y This means: 1) Assign to the first value of the array referred to by y, the value of the expression 2

22 Can also be seen as x y int[] x = {1,2,3} int[] y = x; y[0] = 2;

23 Consequences: Equality int[] arr1 = {1, 2, 3}; int[] arr2 = {1, 2, 3}; System.out.println(arr1 == arr2); Here the addresses stored in arr1 and arr2 variables are getting compared. Not the values. This is also why we have to use.equals() to compare the values of Strings.

24 Why so Confusing!!? Why would one EVER make things so complicated? It turns out there are some very good reasons that will be crucial to the idea of object oriented programming. One useful feature is the ability to pass addresses of data means it's easier to share (and modify!) data between methods! We will see an example about how this helps once we learn about methods.

25 Try it out!! Write some code that actually copies an array of ints (i.e., not just make it point to the same part of memory). e.g., int[] arr1 = {1, 4, 3, 6, 7}; int[] arr2; // make arr2 have a copy of the data in arr1

26 Common Array Problems 1 Referring to an index that doesn't exist. ArrayIndexOutOfBounds exception int[] x = {1, 2, 3}; System.out.println(x[-1]); System.out.println(x[3]);

27 Common Array Problems 2 Trying to do operations on a null pointer. NullPointerException String[] names = new String[35]; System.out.println(names[0].length());

28 Multidimensional Arrays So far, the elements of all the arrays we have seen have been simple values: primitive types or Strings. Such arrays are one-dimensional However, we can create arrays whose elements are themselves one-dimensional arrays Such an array is really an array of arrays These arrays are two-dimensional arrays (or 2D arrays)

29 Two Dimensional Arrays Elements in a two dimensional array are accessed using two indices. The first index specifies the one-dimensional array containing the element we want to access. The second index specifies the element we want to access within the one-dimensional array specified by the first index.

30 Creating 2D Arrays To create an array using an initializer list, you could write: type[] variablename = {exp1, exp2, exp3,...}, where exp1, exp2, exp3, etc all evaluated to type Well, in this case type is just an array. But we need array of arrays, int[][] board = { {1,2,3}, {1,4}, {6,2} } This is creating an array of int[]. The first array in the array is {1,2,3} the second is {1,4} and the third is {6,2}

31 Creating a 2D Array int[][] board = { {1,2,3}, {1,4}, {6,2} } board board[0] board[1] board[2] We can use new operator to create multidimensional arrays. Example: type[][] variablename= new type[size_1][size_2];

32 Accessing a 2D Array int[][] x = new int[3][4]; //creates an array which contains 3 arrays of size 4. x[0][1] = 3; //assigns to the 2nd spot of the 1st array, the value 3; x[2][3] = 10; //assigns to the 4th spot of the 3rd array, the value of 10; x[10][2] = 5; //causes an array out of bounds exception int[] y = x[2]; //stores into y the address of the 3rd array

33 Two Dimensional Arrays What would be printed??? int [][] x = {{1,2,3},{4,5,2,7},{9,3,5,2,6}}; System.out.println(x.length); System.out.println(x[2].length); System.out.println(x[0]); System.out.println(x[0][1]);

34 String (More about Strings)

35 String vs char[] A String is just a fancy version of char[] both store an array of char values. A String has in addition: Useful methods like.equals(),.tolowercase() A special way to be created, using "" syntax (So we don't have to go {'H', 'e', 'l', 'l', 'o', ',', ' ', 'W', 'o', 'r', 'l', 'd'} as for a char[].)

36 More String Methods.charAt(int i) Gets the ith char of a String (starting from 0).substring(int a, int b) Gets the substring starting at position a ending just before position b..compareto(string other).indexof(char c).tolowercase().touppercase() Tells you which String comes first, orthographically. Gets the index of the first occurrence of c Gets a lower-case version of the String. Gets an upper-case version of the String

37 A Note on Describing Methods When you see something like:.substring(int a, int b) This means this is a method that: 1. takes two inputs (or arguments, parameters). 2. the first is of type int 3. the second is of type int as well You call the method by: "some string".substring(1, 4) The "int"s are just there as reminders. Don't type them in!

38 Try it out!! Write some code that turns a String into a format that is like a name i.e., the first letter is capitalized, and all subsequent ones are not. "bob" "Bob" "BOB" "Bob" "BoB" "Bob" "bob" "Bob" Hint: use the methods Character.isLowerCase(char ch) and/or Character.isUpperCase(char ch) to check the case of a char.

39 Caesar Cipher Used in the ancient Roman army to obscure important messages ATTACKATMIDNIGHT* DWWDFNDWPLGQLJKW Encrypt by shifting all letters by three positions A (1 st letter) D (4 th letter) T (20 th letter) W (23 rd letter) *Ancient Romans didn't use lowercase letters, or spaces

40 Try it out!! Implement the encryption algorithm for a Caesar cipher Steps: Read each character of plaintext (String.charAt(int i)) Convert it to its numerical value (by casting) Add 3 Tricky: implement wrap-around (e.g., Z C) Convert it back to a char, and save it or print it out

41 String[] args So what exactly is args there for? public static void main(string[] args) Another way to pass input data to a program Command line: java NameOfClass arg1 arg2 argn DrJava: run NameOfClass arg1 arg2 argn Examples of uses: Name of file to open when you run the program

42 Command-line Calculator This program takes three arguments. The first represents an int, the second is a single character, in {'+', '-', '*', or '/'}, and the third is another int. It prints the result of computing the calculation. e.g., run Calc run Calc 88 / 5 17 run Calc 2 *

43 Methods

44 Calling Methods You have been calling a lot of methods already Some examples? String.equals() String.length()

45 Using Java Libraries The JDK comes with many libraries, which contain classes and methods for you to use. String library Math library Swing library (used for graphics) Libraries for networking

46 Math Library Contains useful and common methods you can use off-the-shelf (i.e., don't have to write yourself) Also, constants like E or PI How do you access them? import java.lang.math;

47 Documentation Read the Application Programming Interface (API) for the specification of a library. e.g., Math library: html What information do you get from an API?

48 Sample Entry static double abs(double a) Returns the absolute value of a double value. Name of the method

49 Sample Entry static double abs(double a) Returns the absolute value of a double value. The number and type of input arguments The name of the method together with the number and type of input arguments is called the signature of the method.

50 Sample Entry static double abs(double a) Returns the absolute value of a double value. Description of what the method does.

51 Sample Entry static double abs(double a) Returns the absolute value of a double value. The return type of the method

52 Sample Entry static double abs(double a) Returns the absolute value of a double value. This keyword means this method does not have to be called on a particular object. Don't worry too much about it for now.

53 Sample Entry static double abs(double a) Returns the absolute value of a double value. Based on this API entry, we know that we call use this method with something like: Math.abs(-4.0) and that this expression evaluates into the value 4.0 with type double.

54 Sample Entry static double abs(double a) Returns the absolute value of a double value. We also know that Math.abs("5.0") would result in an error.

55 Try it out!! Give the input arguments (with types), and return types of the following Math library methods, and describe what they do. Math.pow Math.random Math.max

56 Summary Reference Types Multidimensional Arrays Introduction to Methods