Arrays. Theoretical Part. Contents. Keywords. Programming with Java module 3

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1 Programming with Java module 3 Arrays Theoretical Part Contents 1 Module Overview One-dimensional arrays Declaring arrays Generating an array Initializing arrays Accessing elements of an array Passing through arrays with loops Determining the length of an array Two- and multidimensional arrays Initialization and generation of a two-dimensional array Reading values in a two-dimensional array Multidimensional arrays Strings as arrays 8 Keywords Data Structure Array Array Index Array Element Array Dimension Array Length Array Iteration Two-dimensional Array

2 Authors: Lukas Fässler, Barbara Scheuner, David Sichau Date: 18 July 2018 Version: 1.1 Hash: d2eb005 The authors try the best to provide an error free work, however there still might be some errors. The Authors are thankful for your feedback and suggestions. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit 2

3 1 Module Overview By using the data types you have already learnt, almost any number or character can be represented. However, related data are often used in programming (e.g. lottery numbers, temperatures, departure times, etc.). One possibility for storing a group of related elements are arrays (Row, Fields). In this way, we do not need to declare an individual variable for each element but we can store them in a single identifier. The data structure array occurs in almost every modern programming language. 1.1 One-dimensional arrays One-dimensional arrays are the simplest form of arrays. They consist of an ordered set of n elements. The elements can be accessed by a so-called index. It indicates the position of an element in the array. In many programming languages (including Java), the first element has the index 0, the second one the index 1 and the last one the index n 1 (see example in table 1). Index Value Table 1: Example for a one-dimensional array with six elements. The following operations are performed typically: Declaring, generating, reading values 1.2 Declaring arrays Arrays need to be declared first. This means that we define the name and the data type. To indicate that not only one element can be stored in the variable, squared brackets [] are used. At the time of declaration, the number of elements is not fixed yet. Data type[] name; Example: The following command declares an arry with the name number of data type Integer: 3

4 int[] numbers; 1.3 Generating an array To generate an array, the operator new is used. The array length (i.e. the number of elements) is written within square brackets [] after the data type. If the length specified once, it cannot be changed anymore. The length of the array has to be of the data type Integer. The length can be stated by a number, a constant or an expression. name = new Data type[number]; Example: The following command generates six storage spaces in the array with the name number in which six elements of type int can be stored: numbers = new int[6]; // or numbers = new int[4+2]; The elements are initialized with default values during the generation of the array. For the data type Integer, this is the value 0. The declaration and the generation of arrays can alternatively also be done with one command Data type[] name = new data typ[number]; Example: The following command generates an array with the name numberof the data type Integer with six elements: int[] numbers = new int[6]; 4

5 1.4 Initializing arrays A value can be assigned to an array element by using its index. In Java, the first element has index 0. An array with six elements has therefore the indices 0, 1, 2, 3, 4 and 5. name[index] = Value; Example: The followings commands assign the value 12 to the first element and the value 13 to the second element of the array: numbers[0] = 12; numbers[1] = 13; In Java, the elements can be initialized directly during the generation of the array by entering the value separated by commas (,) and enclosed by curly brackets {. The size of the array is given by the number of values. The operator new can be dropped in this case. Type[] name = {value1, value2,..., valuen; Example: Declaration, generation and initialization of the array numbers with six elements 12, 13, 15, 17, 22 and 32: int[] numbers = {12, 13, 15, 17, 22, 32; 1.5 Accessing elements of an array The individual elements of an array can be accessed by their index (e.g. i). x[i] therefore provides the element of the array x at the position i. It is important to recognise that indices begin at 0 and end at one fewer than the number of elements of the array. It is possible to invoke individual elements or ranges of the array and to calculate using array elements. 5

6 Example: //Array with 3 elements. int[] c = new int[3]; c[0]=1; c[1]=2; c[2]=3; //Accessing an element (Result: 1). System.out.println(c[0]); //Sum of two elements (Result: 5). System.out.println(c[1]+c[2]); 1.6 Passing through arrays with loops When working with arrays, we usually use for-loops. Thereby, the value of the counter variable corresponds to the index value of the array. Thus, the effort is reduced to just a few commands independent on the number of elements the array contains. Example: By using the following command, six elements of the array numbers can be output below each other on the screen: for (int i=0; i<6; i++) { System.out.println(numbers[i]); The for-loop counts from 0 to 5. In each loop iteration, the variable i is used as an index to print the array element of the corresponding position. 1.7 Determining the length of an array Every array in Java has the property length with which the length of the array can be queried. Example: for (int i=0; i<zahlen.length; i++) { System.out.println(numbers[i]); 6

7 The function length is often used to compute the upper bound of a loop when running through an array. This has the advantage that the loops do not need to be adapted if the array length changes. 2 Two- and multidimensional arrays If an element of an array itself consists of an array, a two-dimensional array is formed. It can be imagined as a table with m n elements which can be accessed via two indices (see example in table 2). Index Table 2: Example for a two-dimensional array with three by six elements. 2.1 Initialization and generation of a two-dimensional array Type[][] name = new Type[numberRows][numberColumns]; Example: The following command generates a two-dimensional array with the name numbers of the data type Integer with 3 by 5 elements: int[][]numbers = new int[3][5]; 2.2 Reading values in a two-dimensional array To access individual elements of a two-dimensional array, two indices (indicating the row and the column number) are used: name[rownumber][columnnumber] = value; 7

8 Example: numbers[0][0] = 22; To run through an array iteratively, nested loops with two index variables are necessary. Example: The following commands output all elements of a two-dimensional array on the screen: for (int i=0; i<3; i++) { for (int j=0; j<6; i++) { System.out.println(numbers[i][j]); 2.3 Multidimensional arrays An array can have more than two dimensions. For each further dimension, a further index is required for an element access. 3 Strings as arrays As already mentioned in module 1, a variable of the type string is a combination of several variables of the type Character (char). In Java, a string is stored in an array of data type Character. Thus, a string can be generated by Character-arrays and the individual letters of a string can be accessed via the index. Example: char[] mytext = { t, h, e,, t, e, x, t ; String mystring = new String(myText); char firstcharacter= mystring.charat(0); In this example, a char-array of length 8 is generated and initialized with 8 characters. Next, a string that contains the text the text is generated based on this array mytext. In the 3rd line, the first character is read out from this string. If you want to read out all characters of a text, you can use a loop. for (int i=0; i<mystring.length(); i++){ System.out.print(myString.charAt(i)); 8