02/09/2005 Introduction to Programming with Java, for Beginners Arrays (of primitives)

02/09/2005CSE 1101 public class Frog{ private boolean formerPrince; private String phrase1; private String phrase2; private String phrase3; private String phrase4; private String phrase5; private String phrase6; private String phrase7; private String phrase8; private String phrase9; private String phrase10;... What if our Frog (from homework 2) could say 10 different things?

02/09/2005CSE 1102 public class CrashCop{ private Taxi taxi1; private Taxi taxi2; private Taxi taxi3; private Taxi taxi4; private Taxi taxi5; private Taxi taxi6; private Taxi taxi7; private Taxi taxi8; private Taxi taxi9; private Taxi taxi10; private Taxi taxi11; private Taxi taxi12; private Taxi taxi13; private Taxi taxi14; private Taxi taxi15; What if our CrashCop (from homework 3) could watch 15 taxis?

02/09/2005CSE 1103 public class Sitter{ private Sitter kid1; private Sitter kid2; private Sitter kid3; private Sitter kid4; private Sitter kid5; private Sitter kid6; private Sitter kid7; private Sitter kid8; private Sitter kid9; pirvate Sitter kid10; private Sitter kid11; private Sitter kid12; private Sitter kid13; private Sitter kid14; private Sitter kid15; private Sitter kid16; private Sitter kid17; private Sitter kid18; private Sitter kid19; pirvate Sitter kid20; What if our Sitter (from homework 4) could watch 20 kids?

02/09/2005CSE 1104 public class IToonList{ private IToon toon1; private IToon toon2; private IToon toon3; private IToon toon4; private IToon toon5; private IToon toon6; private Ttoon toon7; private IToon toon8; private IToon toon9; private Ttoon toon10; private IToon toon11; private IToon toon12; private IToon toon13; private IToon toon14; private IToon toon15; private IToon toon16; private Ttoon toon17; private IToon toon18; private IToon toon19; private Ttoon toon20; private IToon toon21; private IToon toon22; private IToon toon23; private IToon toon24;... What if we want to store 500 Itoons (from homework 2)?

02/09/2005CSE 1105 What if we want to store lots of things… …but we don’t want to declare a separate variable for each one? That’s what arrays are good for.

02/09/2005CSE 1106 int[] data; data = new int[5]; data[0] = 6; data[1]= 10; data[2] = 12; What is an Array? It’s an easy way to declare lots of variables that all have the same type.

02/09/2005CSE 1107 Array Elements and Indices The “data” array has 5 elements: data[0] whose value is 6 data[1] whose value is 10 data[2] whose value is 12 data[3] whose value is 0 data[4] whose value is 0 When accessing an element, the number within square brackets is called an index The valid indices are 0 thru (array length - 1)

02/09/2005CSE 1108 int[] data; data is a reference variable whose type is int[], meaning “array of ints”. At this point its value is null. data = new int[5]; The new operator causes a chunk of memory big enough for 5 ints to be allocated on the heap. Here, data is a assigned a reference to the heap address. data[0] = 6; data[1]= 10; data[2] = 12; Initially, all five ints have the value 0. Here, three of them are assigned other values. int[] info = {6, 10, 12, 0, 0}; // another way An Array is an Object (with unique [] syntax)

02/09/2005CSE 1109 int x = data[0]; data[3] = data[2]; data[4] = sum(1,5); data[4] = sum(1, data[4]); System.out.println(“data[4] is “ + data[4]); x = data[0] * data[1] * data[2]; Person p = new Person; p.setAge(data[3]); Operations on Array Elements An “element” of an array of ints can be used virtually anywhere an expression of type int is valid. Likewise for arrays of other types.

02/09/2005CSE int[] data; data = new int[5]; data[0] = 6; data[1]= 10; data[2] = 12; int result = 0; // the value of data.length is 5 for (int i =0; i < data.length; i++){ result = result + data[i]; } An Array’s “length” Instance Variable

02/09/2005CSE double[] temps; temps = new double[24]; temp[0] = 78.5; temp[1] = 84.2; boolean[] answers = new boolean[6]; answers[0] = true; if (answers[0) foo(); char[] buffer = new char[500]; open a file for reading while (more chars in file & buffer not full) buffer[i++] = char from file Arrays of Other Primitive Types