CMPT 126: Lecture 6 Arrays

Transcription

1 CMPT 126: Lecture 6 Arrays Tamara Smyth, tamaras@cs.sfu.ca School of Computing Science, Simon Fraser University September 25,

2 Array Elements An array is a construct used to group and organize data. By using an array, we don t have to declare separate variables for large amounts of data. An array is a list of values of the same type, each one stored at a specific numbered position called an index indeces 2 42 array elements Figure 1: An array. An array of size N is indexed from 0 to N 1. CMPT 126: Arrays, Lecture 6 2

3 Declaring and Using Arrays In Java, an array is an object that must be instantiated. To create an array, the reference to the array must be declared: int[] height = new int[11]; where the variable is height, which is declared to be an array of eleven integers. The instantiation of height, using the new operator, reserves the memory space to store 11 integers. The size of an array cannot be changed. To access a value in an array, we use the name of the array followed by the index in square brackets: height[2] = 45; CMPT 126: Arrays, Lecture 6 3

4 Initializer list The array may also be initialized at declaration using an initializer list, in which case the new operator is not used: int[] height = {2, 34, 45, 22, 235; An initializer list can only be used when an array is first declared. The type of each value in the initializer list must match the array element type. CMPT 126: Arrays, Lecture 6 4

5 PrintArray.java It is often convenient to use for loops when handling arrays because the number of positions in the array is constant. public class PrintArray { public static void main(string[] args) { final int LIMIT = 15, MULTIPLE = 10; int[] list = new int[limit]; // initialize the array values for (int i = 0; i< LIMIT; i++) list[i] = i * MULTIPLE; list[5] = 999; //change one value // Print the array values for (int value : list) System.out.print(value + " "); Every Java array is an iterator so the version of for that extracts each value in the specified iterator may be used. CMPT 126: Arrays, Lecture 6 5

6 Bounds Checking Bounds checking ensures that an index used to refer to an array element is in range. The index operator performs automatic bounds checking, though we will still want to perform our own bounds checking. A reference to an array must be greater than or equal to zero, and less than the size of the array. The length of the array may be determined using its length constant. public class PrintArray { public static void main(string[] args) { int[] list = {12, 2, 8, 234, 11; // initialize the array values for (int i = 0; i<list.length; i++) System.out.print(list[i] + " "); CMPT 126: Arrays, Lecture 6 6

7 Alternate Array Syntax There are two ways to declare an array reference in Java. 1. associate the brackets with the type of values stored: int[] grades; 2. associate the brackets with the name of the array. int grades[]; The first is consistent with other types of declarations. CMPT 126: Arrays, Lecture 6 7

8 Arrays as Parameters An entire array can be passed as a parameter to a method. What is actually passed is a copy of the reference to the original array. This implies that a method that receives an array as a parameter can permanently change an element of the array. CMPT 126: Arrays, Lecture 6 8

9 Arrays of Objects Arrays can also be used to store objects as elements, often facilitating the management of more complex structures. an array could contain objects that consist of several variables (which may themselves be other arrays) and methods. Instantiating an array of objects allocates memory to store references only. The objects that are stored in each element must be instantiated separately. CMPT 126: Arrays, Lecture 6 9

10 Instantiating an Array of Objects The new operator creates an array, and reserves the memory, in this example, for 5 String references. It does not create any String objects. String[] authors = new String[5] authors Figure 2: authors: an array of string references. CMPT 126: Arrays, Lecture 6 10

11 Instantiating Array Objects (Strings) Recall that String objects may be represented as string literals, allowing the following declarations: String[] authors = {"Kurt Vonnegut", "Douglas Coupland", "Paul Auster", "Salmon Rushdie", "Julian Barnes"; authors "Kurt Vonnegut" "Douglas Coupland" "Paul Auster" "Salmon Rushdie" "Julian Barnes" Figure 3: authors: an array of string references to instantiated string objects. CMPT 126: Arrays, Lecture 6 11

12 Command-line Arguments Recall from lab 1 that command-line arguments are stored in an array of String objects and are passed to the main method. public class NameTag { public static void main(string[] args) { System.out.println(); System.out.println(" " + args[0]); System.out.println("My name is " + args[1]); Any information on the command line beyond the program name is stored in args when the interpreter is invoked, and may be used to provide input into the program. >> java NameTag Greetings! Tamara Greetings! My name is Tamara If two strings are not provided on the command line, an ArrayIndexOutOfBoundsException will be thrown. CMPT 126: Arrays, Lecture 6 12

13 Variable Length Parameters Lists Java provides a way to define methods that accept variable length parameter lists. public class MyAverage { static double average(int... list) { double result = 0.0; if (list.length!=0) { int sum = 0; for (int num : list) sum += num; result = (double)sum/list.length; return result; public static void main(string[] args) { double avg1 = average(1, 23, 43); double avg2 = average(23, 34, 44, 56, 2, 34, 9, 3); System.out.println(avg1); System.out.println(avg2); Output: CMPT 126: Arrays, Lecture 6 13

14 The parameters are automatically put into an array for easy processing in the method. A method cannot accept two sets of varying parameters, but can accept other parameters. CMPT 126: Arrays, Lecture 6 14

15 Two-Dimensional Arrays Two dimensional arrays have values in two dimensions, which you may think of as rows and columns. Brackets are used to represent each dimension in the array: a 2-D array storing integers is int[][]. one dimension two dimension Figure 4: A one and two dimensional array. In reality, a 2-D array is an array of arrays, and is really a 1-D array of references to arrays. CMPT 126: Arrays, Lecture 6 15

16 Multidimensional Array Though it s a little harder to visualize, arrays can have one, two, three, or more dimensions. This is easier to comprehend when you understand how multidimensional arrays are implemented, and consider each array as having a reference that is part of another array. Alternatively (and equivalently), think of one dimension as being a subdivision of the previous one. Java does not directly support multidimensional arrays. Rather they must be implemented by the programmer as array of references to array object. In some cases you may find it difficult to mange data in this was, and opt to use objects instead. CMPT 126: Arrays, Lecture 6 16

17 The ArrayList Class An ArrayList is similar to an array, but it changes its size dynamically, as needed. CODE: ArrayList band = new ArrayList(); band.add( Paul ); band.add( Pete ); band.add( John ); band.add( George ); System.out.println(band); OUTPUT: [Paul, Pete, John, George] To access an element at a particular index, use the object s get method: System.out.println( At index 1: + band.get(1)); OUTPUT: Pete CMPT 126: Arrays, Lecture 6 17

18 ArrayList is Dynamic Since ArrayList is dynamic, elements may be inserted and removed from any location. CODE: int index = band.indexof( Pete ); band.remove(index); System.out.println(band); OUTPUT: [Paul, John, George] When an element is deleted, the ArrayList collapses so that indeces are still incremental for the remaining elements. To add at a particular index, use the object s add method but with two parameters: the index and the object to be added. CODE: band.add(2, Ringo ); System.out.println(band); OUTPUT: [Paul, John, Ringo, George] CMPT 126: Arrays, Lecture 6 18

19 ArrayList Element Type An ArrayList typically stores a list of references to the Object class, rather than a more specific data type, allowing storage of various data types in one array. CODE: ArrayList myarray = new ArrayList(); myarray.add(2); myarray.add( string ); System.out.println(myarray); OUTPUT: [2, string] Primitive data types are promoted to their corresponding wrapper class. CMPT 126: Arrays, Lecture 6 19

20 Caveat with Unspecified Types To remove an object (which is of type Object), it must be cast to its original class. For example, the following will not compile as it sees the reference to the Object class (obtained using the get method) as being incompatible with the integer 2: System.out.println(myarray.get(0) + 2) Rather, the reference to the Object class must be cast to a reference of type Integer (its original type) or some other compatible type: System.out.println((Integer)myarray.get(0) + 2) CMPT 126: Arrays, Lecture 6 20

21 Specifying an Element Type for ArrayList An ArrayList is a generic type, meaning we can specify a specific element type: ArrayList<Family> reunion = new ArrayList<Family>(); Declaring the element type is usually a good idea because it adds a level of type checking, and eliminates the need to cast when extracting objects. ArrayList<Integer> myarray = new ArrayList<Integer>(); myarray.add(2); System.out.println(myarray.get(0) + 2); CMPT 126: Arrays, Lecture 6 21

22 ArrayList Efficiency/Implementation When an element is inserted into an ArrayList, all of the elements at higher indexes are copied into their new locations to make room for the next element Figure 5: Inserting an element into an ArrayList. A Similar process occurs when an element is removed. If several elements are inserted or deleted, this copying is repeated many times, which can make processing inefficient. Collections classes will be discussed later in the course. CMPT 126: Arrays, Lecture 6 22

23 Polygons and Polylines Arrays are helpful when drawing complex shapes. A polygon is a multisided shape defined in Java using a series of (x,y) points that indicate the vertices of the polygon. Arrays are often used to store the list of coordinates. The methods used to draw a polygon are called drawpolygon and fillpolygon. These are overloaded. One version using arrays, these methods take three parameters. The first two parameters are arrays of integers representing the x and y coordinates, and the third is an integer that indicates how many points are used from each of the two arrays. A polyline is similar to a polygon, but is not a closed shape. CMPT 126: Arrays, Lecture 6 23

24 Rocket.java public class RocketPanel extends JPanel { private int[] xrocket = {100, 120, 120, 130, 130, 70, 70, 80, 80; private int[] yrocket = {15, 40, 115, 125, 150, 150, 125, 115, 40; public RocketPanel() { setbackground(color.black); setpreferredsize(new Dimension(200, 200)); public void paintcomponent(graphics page) { super.paintcomponent(page); page.setcolor(color.cyan); page.fillpolygon(xrocket, yrocket, xrocket.length); Figure 6: Java Rocket output. CMPT 126: Arrays, Lecture 6 24

25 CMPT 126: Arrays, Lecture 6 25