Arrays and Strings

Arrays and Strings • Introducing Arrays • Declaring Arrays • Creating Arrays • Initializing Arrays • Array of Objects • Copying Arrays • Multidimensional Arrays • Command-Line Parameters

Introducing Arrays In computer science, an array is a data structure that represents a collection of the same types of data. Java treats these arrays as objects. An Array of 10 Elementsof type double double[] myList = new double[10]

Declaring Arrays • datatype[] arrayname; Example: int[] myList; • datatype arrayname[]; Example: int myList[];

Creating Arrays arrayName = new datatype[arraySize]; Example: myList = new double[10]; An array is considered to be an object. Thus: myList is really a reference to 10 doubles and a field called length that contains the array’s size. For example, myList.length contains 10.

Declaring and Creatingin One Step datatype[] arrayname = new datatype[arraySize]; double[] myList = new double[10]; or datatype arrayname[] = new datatype[arraySize]; double myList[] = new double[10];

Initializing Arrays • Using a loop: for (int i = 0; i < myList.length; i++) myList[i] = (double)i; • Declaring, creating, initializing in one step: double[] myList = {1.9, 2.9, 3.4, 3.5};

Enhanced for statement • Used to iterate through the elements of an array or collection without using a counter. • Form: for (parameter: arrayName) statement; • Parameter has two parts – a type and an identifier • ArrayName is the array through which to iterate.

Enhanced for Example: total = 0; for (int count=0; count < array.length; count ++) total += array [count]; Is the same as: total = 0; for(int number: array) total += number; number is actually taking on array[0], array[1], etc.

Using Arrays in a Gradebook • Objective: Use an array for a grade book used by a professor to store and analyze a set of student grades. • Figure 7.14 in Java book (page 317…) and Figure 7.15 on page 321.

Array of Objects • Declaring and creating: Circle[] circleArray = new Circle[10]; • Initializing: for (int i=0; i<circleArray.length; i++) { circleArray[i] = new Circle(); }

Copying Arrays Using a loop: int[] sourceArray = {2, 3, 1, 5, 10}; int[] targetArray = new int[sourceArray.length]; for (int i = 0; i < sourceArrays.length; i++) targetArray[i] = sourceArray[i];

The arraycopy Utility arraycopy(sourceArray, src_pos, targetArray, tar_pos, length); Example: System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length);

Multidimensional Arrays Multidimensional arrays with two dimensions are often used to represent tables of values consisting of information arranged in rows and columns called a two-dimensional array. Example: int[][] matrix = new int[10][10]; or int matrix[][] = new int[10][10]; for (int i=0; i<matrix.length; i++) for (int j=0; j<matrix[i].length; j++) { matrix[i][j] = (int)(Math.random()*1000); }

Nested array initializers A 2D array can be initialized when it is declared. Example: int b[][] = { {1, 2}, {3, 4}, {5, 6}}; creates the following array containing the values shown

How does Java treat a 2D array? • A 2D array is thought of as an array of arrays. • For example if b is an array with 3 rows and 2 columns, • it is considered to be a one dimensional array with 3 objects. • Each object is an array with 2 elements. b[0]  {1, 2} b[1]  {3, 4} b[2]  {5, 6}

2D arrays with rows of varying lengths • The way Java represents 2D arrays makes them very flexible. Lengths of rows are not required to be the same: int b[][] = {{1, 2}, {3, 4, 5, 6}}; makes b look like: b[0]  {1, 2} b[1]  {3, 4, 5, 6}

2D Array creation examples int a[][] = new int[4][2]; int a[][]; a = new int [4][2]; int c[][]; c = new int[3][]; //create 3 rows c[0] = new int[3];//create 3 columns for row 0 C[1] = new int[2];//create 2 columns for row 1 C[2] = new int[4];//create 4 columns for row 2

length field for 2D arrays • For a 2D array declared as: int B[][]=new int[2][3]; there are several “length” fields B.length contains 2, the number of rows B[0].length contains 3, the number of columns in row 0. B[1].length contains 3, the number of columns in row 1, etc.

Working with 2D arrays int total = 0; for (int row = 0; row < a.length; row++) { for (int column = 0; column < a[row].length; column++) total += a[row][length]; { Or for (int rows[] : a) //loop thru rows of a { //loop thru columns of the current row for (int oneValue : rows) total += oneValue; }

Example: • Revisit grade book using 2D arrays • Figures 7.18 and 7.19

Command-Line Parameters class TestMain { public static void main(String[] args) { ... } } java TestMain arg0, arg1, arg2, ..., argn

ProcessingCommand-Line Parameters In the main method, get the arguments from args[0], args[1], ..., args[n], which corresponds to arg0, arg1, ..., argn in the command line.

Example: Using Command-Line Parameters • Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers. Java TestCommandParameters + 2 3 Java TestCommandParameters - 2 3 Java TestCommandParameters / 2 3

Arrays and Strings