Exercises Software Development I. 06 Arrays. Declaration, Initialization, Usage // Multi-dimensional Arrays. November 14, 2012

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1 Exercises Software Development I 06 Arrays Declaration, Initialization, Usage // Multi-dimensional Arrays November 4, 202 Software Development I Winter term 202/203 Institute for Pervasive Computing Johannes Kepler University Linz {farrahi,riener,schmittner}@pervasive.jku.at

2 Arrays: Data Structures Problem: So far only primitive variables (int age, float value, etc.) Feasible structures for associated data needed Example: date (4..202: day, month, year) int day, month, year - this declaration does not allow to infer the relation between these variables In the given notation, a second date would be declared as int day2, month2, year2 - not very intuitive to read or use Better: The entire date should form a coherent unit 'Date' should be usable as a single parameter like a primitive variable Data structures for large number of elements Example: hotel with 500 rooms; reservation system which room is available? boolean availroom, availroom2,..., availroom500; Solution: Arrays An array is a container object that holds a fixed number of values of a single type The length of an array is established when the array is created; i.e., after creation its length is fixed Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 2

3 Arrays: Declaration Each item in an array is called an element, and each element is accessed by its numerical index. Example: Declare date as array of three integer values int[] date = new int[3]; // indices from 0 to 2 date[0] = 4; date[] = ; assigning values date[2] = 202; Example2: Second data array int[] date2 = new int[3]; // a second date Explicit allocation with new operator The array is of type int[] Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 3

4 Arrays: Allocation and Initialization Declaration and initialization can be done separately int[] date;... // data array with 3 elements (e.g., day, month, year) date = new int[3];... // not fixed; can be redeclared to store 6 elements, // e.g., day, month, year, hour, minute, second date = new int[6]; // 'old' date array: garbage collector! Initialization can be also done at declaration time int[] date = { 4,, 202 }; // length implicit, fixed int[] daysofmonth = {3, 28, 3, 30, 3, 30, 3, 3, 30, 3, 30, 3} Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 4

5 Arrays: References Given: 2 arrays int[] array = new int[0]; int[] array2 = new int[0]; array array What happens in case of an assignment? array2 = array; Content of array2 is lost; garbage collector cleans up memory array array Ok, both variables are of the same data type int[] Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 5

6 Arrays: Copying and what happens in this situation? int[] array = { 0,, 2, 3, 4, 5, 6, 7, 8, 9 }; int[] array2 = new int[0]; array array Element-wise assignment for (int i = 0; i < 0; ++i) { array2[i] = array[i]; } array array Fields of type 'int' assigned element by element Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 6

7 Arrays: Two-dimensional Arrays In Java, an element of an array can be itself an array Two-dimensional arrays ("matrices") int[] anarray = new int[0]; //[0]...[9] int[][] amatrix = new int[0][0]; //[0][0]..[9][9] Non-quadratic matrices are also possible (see later) Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 7

8 Arrays: Identity Matrix () Code to generate the identity matrix static int[][] identitymatrix(int dim) { // dim > 0 int[][] m = new int[dim][dim]; for(int row = 0; row < dim; ++row) { for(int col = 0; col < dim; ++col) { if(row == col) { } m[row][col] = ; } else { m[row][col] = 0; } } } return m; Are there more efficient ways of initialization? Use the "conditional operator (?-operator) Elements are automatically initialized with value 0 only the elements in the main diagonal need to be set to "" if(row == col) Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 8

9 Arrays: Multidimensional Arrays Java implements multidimensional arrays (2, 3, n dimensional) as arrays-ofarrays. This means that multidimensional arrays need not be rectangular! Example: short[][] triangle = {{}, {2, 3}, {4, 5, 6}, {7, 8, 9, 0}}; A jagged array has rows with varying number of elements Possible whenever multi-dimensioned arrays actually appear as arrays of arrays C, C++, C#, Java support jagged arrays Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 9

10 Arrays: Array of Objects So far we have looked at arrays of primitive types (int, float, double, etc.) But we often want to store array of objects, such as Students, Books, People, etc. Is this possible (in Java)? Sure, see later OOP Special case: Array of Strings ) Declaration of the String array private String[] strings; 2) Creation (initialization) of the array strings = new String[0]; // allocates memory to hold references to 0 String objects 3) Create String objects and add them to the array: strings[0] = new String("Hello World"); strings[i] =... Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 0

11 Arrays: String Arrays (Memory Organization) Example String array countries holding 0 Strings String[] countries = new String[0]; countries[0] = "Canada"; countries[5] = "Australia"; countries[9] = "Italy"; countries null Canada Australia Italy Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) //

12 Arrays: String Arrays How to use Strings in Java: String str = new String("Hello World"); int len = str.length(); // char ch = str.charat(6); // 'W String s = str.substring(3, 9); // lo Wor String t = str.replace('o', 'z'); // Hellz Wzrld boolean b = str.contains("ello"); // true int i = str.indexof('o'); // 4 int j = str.lastindexof('l'); // 9 String[] array = str.split(" "); // { Hello, World } array.length // 2 String u = array[0]; // Hello array[] = null; // { Hello, null} Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 2

13 Arrays: Summary () An array... Is a collection of homogeneous elements Is an object (complex data types) Is passed by reference and not by value Be careful on Assignments (=) Comparisions (==) Array parameters An array is often used to structure data (remember the date example) Most often used for a huge number of elements of the same data type (especially instead of many variables), e.g. hotel with 000 rooms: use boolean[] booked Often a better alternative is to use classes (see OOP) Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 3

14 Arrays: Summary (2) The length of the array needs to be known at the initialization time (latest at runtime!) length = Input.readInt(); int[] array = new int(length); This length field is a final field and cannot be changed (length is a field, not a method!) It cannot be resetted or changed by assigning a.length = 7; Arrays allows direct access to every single element Be careful with the index; runs from 0 to.length- Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 4