L EC. 05: A RRAYS 0

C ONTENT  Features of arrays  Declaring, creating and using arrays  Enhanced for -construct  Variable length parameter-list 1

A RRAYS  An array ( 陣列 ) is a contiguous memory space with fixed size used to store a group of data with compatible data type.  An array is the simplest and basic data structure ( 資料 結構 ) used to store one or more logical-related data.  An array can be used to store all the objects representing the students of a department.  An array can be used to store all the objects representing the employees of a company.  Arrays support random access ( 隨機存取 ). 2

A RRAYS IN J AVA  An array is a collection of variables of the compatible type, referred to by a common identifier.  Each individual variable is called an element ( 元素 ).  Java only supports one-dimensional arrays.  Java supports mechanism, called array-of-arrays, for building multi-dimensional arrays from one-dimensional arrays.  Arrays are used for a variety of purposes because they offer a convenient means of grouping together related variables.  Using an array to hold the records of all the students in a class.  Each array is an object.  The class of an array object is automatically created by Java platform. 3

D ECLARING ARRAYS  Syntax form for declaring an array []; or [] ;  The is the type of array elements, so called the base type.  Example  String strs[], addresses[], str; // str is not an array  int[] vector, score; // both vector and list are arrays  As arrays are objects in Java, the variables declared as arrays are reference variables referred to objects which represent arrays.  When an array is declared, the array is not created. 4

C REATING ARRAYS  An array is created by using the new operator with the following form. new [ ];  Example  new String[10]  new int[10*2]  The must be an int-expression representing the number of elements in the array.  The new operator dynamically allocates memory for an array and returns the reference of the array.  Array declaration and creation can be combined into a single statement.  String strs[] = new String[10];  The length of an array cannot be changed once it is created. 5

L ENGTH OF AN ARRAY  The length of an array is the number of data item can be stored in the array.  Each array has a instance field named length to represent the length of the array.  The length field is final.  A final field in Java is a field that can be set only once.  Setting the length field of an array is illegal.  Example  The output of the following segment is int vector[] = new int[10]; System.out.println(vector.length);

A CCESSING ARRAY ELEMENTS  Each individual element within an array is accessed by using an index( 索引 ).  An index describes the position of an element within an array.  In Java, all arrays have zero as the index of their first element.  This index style is called zero-index.  The type of indices must be compatible to int.  To access an array element, specify the index of the element you want, surrounded by square brackets.  Syntax form [ ]  Example  strs[0] = “Hello”;  vector[3] = vector[0] + vector[2]; 7

A CCESSING ARRAY ELEMENTS  The must be an int -expression.  The value of must be within the range [0, length – 1], where length is the number of elements in an array.  If the is out of bound, an exception (run-time error) occurs.  The best way to process array elements is to use the for - construct.  Example  Let nums be an int array, then the following segment can find the maximum and minimum elements in nums. int min, max; min = max = nums[0]; for(int i = 1; i < nums.length; i++) { if(min > nums[i]) min = nums[i]; if(max < nums[i]) max = nums[i]; } 8

I NITIALIZING ARRAYS  Syntax form 1 = { };  Syntax form 2 = new []{ };  The initializers are separated by using comma.  The length of an array is determined by the number of initializers.  The length cannot be specified if an initializer is given.  Example  String strs[] = {“hello”, “an”, “example”}; //legal  String strs[]; strs = {“hello”, “an”, “example”}; //illegal  int vector[] = new int[]{1, 2, 3, 4, 5, 6}; //legal  int vector[]; vector = new int[]{1, 2, 3, 4, 5, 6}; //legal 9

E XAMPLE OF PROCESSING ARRAYS : BUBBLE SORTING  Bubble sorting is a simple, but may not be efficient for large size arrays, sorting method.  Bubble sorting proceeds iteratively.  In the i th iteration  Comparing the adjacent pair of elements, starting from the right-most two elements, and swapping them if they are out of order  Stopping the iteration when the (i-1) th element is compared 10

F IGURE OF BUBBLE SORTING ( FIRST ITERATION ) 11

C LASS FOR SORTING class Sorting { public static int[] bubbleSort( int[] vector ) { // a class method int sorted[]; // declare an array sorted = new int[vector.length]; // create an array //duplicating vector to sorted System.arraycopy(vector, 0, sorted, 0, vector.length); for(int i = 1; i < sorted.length; i++) { for(int j = sorted.length – 1; j >= i; j--) { // ith iteration if(sorted[j–1] > sorted[j]) { //out of order, swap them int temp = sorted[j]; sorted[j] = sorted[j–1]; sorted[j–1] = temp; } return sorted; } 12

M ORE EFFICIENT VERSION 13 class Sorting { public static int[] bubbleSort( int[] vector ) { // a class method boolean flag; int sorted[]; // declare an array sorted = new int[vector.length]; // create an array //duplicating vector to sorted System.arraycopy(vector, 0, sorted, 0, vector.length); loop: for(int i = 1; i < sorted.length; i++) { // label it flag = true; for(int j = sorted.length – 1; j >= i; j--) { // ith iteration if(sorted[j–1] > sorted[j]) { //out of order, swap them flag = false; int temp = sorted[j]; sorted[j] = sorted[j–1]; sorted[j–1] = temp; } if(flag) break loop; // stop earlier } return sorted; }

M ULTIDIMENSIONAL ARRAYS  In Java, a multidimensional array is implemented by using the concept of array of arrays.  In a 2-dimensional array, i.e. a matrix( 矩陣 ), each row can be treated as a 1-dimensional array.  So, a 2-dimensional array with m rows can be treated as a set of m 1-dimensional arrays.  A 2-dimensional array can be implemented as a 1- dimensional array in which each element is a 1- dimensional array.  The concept for implementing 2-dimensional arrays can be easily extended to any n-dimensional arrays. 14

D ECLARING MULTIDIMENSIONAL ARRAYS  Syntax form [][]…[];  Example  String empSkill[][]; // a 2-dimensional array  int cube[][][]; // a 3-dimensional array 15

C REATING REGULAR MULTIDIMENSIONAL ARRAYS  Syntax form new [ ][ ]…[ ]  Example  empSkill = new String[10][2];  Cube = new int[8][4][10]; 16

C REATING IRREGULAR MULTIDIMENSIONAL ARRAYS  In Java, you can create upper-level arrays and then create lower-level arrays separately.  Before upper-level array is created, no lower-level array can be created.  Example 1 17 int matrix[][]; matrix = new int[3][]; // matrix = new int[][10]; is illegal matrix[0] = new int[4]; matrix[1] = new int[8]; matrix[2] = new int[6];

C REATING IRREGULAR MULTIDIMENSIONAL ARRAYS  Example 2 18 int cube[][][]; cube = new int[2][][]; //create one 1-dimensional array with 2 elements // cube = new int[2][][10]; is illegal cube[0] = new int[2][3]; // totally create 3 1-dimensional arrays cube[1] = new int[3][]; // create one 1-dimensional array with 3 elements cube[1][0] = new int[2]; // create one 1-dimensional array with 2 elements cube[1][1] = new int[3]; // create one 1-dimensional array with 3 elements Cube[1][2] = new int[2]; // create one 1-dimensional array with 2 elements

L ENGTH OF ARRAYS  Based on the examples on pages 17 and 18  matrix.length equals 3.  matrix[1].length equals 8.  matrix[1][0].length is illegal since matrix[1][0] is not an array.  cube.length equals 2.  cube[1].length equals 3.  cube[0][2].length equals 2.  Remember that. can access the instance field of an object pointed by a reference variable.  Since matrix and matrix[1] points to array objects, matrix.length and matrix[1].length read the lengths of array objects.  Since matrix[1][0] is int, matrix[1][0].length is illegal. 19

A CCESSING ELEMENTS OF MULTIDIMENSIONAL ARRAY  For multidimensional arrays, each dimension needs a separate index set.  To access an element of a multidimensional array, you need to specify the index for each dimension.  Syntax form [ ][ ]…[ ]  Example  matrix[0][0] = matrix[1][1] + matrix[2][1];  cube[0][1][2] = 3 * cube[2][1][0]; 20

I NITIALIZING MULTIDIMENSIONAL ARRAYS  A multidimensional array can be initialized by enclosing each dimension’s initializer list within its own set of curly braces.  Example 21 String empSkill[][] = { {“John”, “Mary”, “Ken”}, // for empSkill[0] {“Network”, “Security”, “OS”} // for empSkill[1] } int cube[][][] = { { {1}, // cube[0][0][0] is 1 {1, 2} // cube[0][1][0] is 1, cube[0][1][1] is 2 }, { {22, 21, 34}, // cube[1][0][0] is 22 {30, 1}, // cube[1][1][1] is 1 {85, 34, 22, 56} // cube[1][2][3] is 56 }

E XAMPLE OF USING 2- DIMENSIONAL ARRAYS : MANIPULATING MATRICES class Matrix { int matrix[][]; Matrix(int noRow, int noCol) { //constructor matrix = new int[noRow][noCol]; //create a 2-dim array to represent the matrix for(int i = 0; i < noRow; i++) { // initialize the matrix for(int j = 0; j < noCol; j++) { matrix[i][j] = 0; } Matrix(int[][] arr) { // convert a 2-dimensional array to a Matrix object matrix = new int[arr.length][]; for( int i = 0; i < arr.length; i++) { matrix[i] = new int[arr[i].length]; System.arraycopy(arr[i], 0, matrix[i],0, arr[i].length); // copy the ith row } public void setElement(int row, int col, int val) { matrix[row][col] = val; } public int getElement(int row, int col) { return matrix[row][col]; } 22

E XAMPLE OF USING 2- DIMENSIONAL ARRAYS : MANIPULATING MATRICES public void add(Matrix addend) { // add addend to this object for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[i].length; j++) { matrix[i][j] += addend[i][j]; } public void printMatrix() { // print this object in matrix form for( int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[i].length; j++) { System.out.print(matrix[i][j]+” “); } System.out.println(); } 23

F OR - EACH STYLE for - CONSTRUCT  Beginning with J2SDK v. 1.5, Java supports a new for-each style for -construct, so called enhanced for -construct.  The enhanced for -construct cycles through a collection of objects, such as an array, in strictly sequential fashion, from start to finish.  Syntax form for( : ) statement;  The specifies the type of.  The specifies the name of an iteration variable that will receive the elements from, one at a time, from beginning to end.  The collection being cycled through is specified by.  There are various types of collections, such as primitive and class types, that can be used with the for. 24

F OR - EACH STYLE for - CONSTRUCT  The base type of must be compatible with the type of iteration variable.  With each iteration of the loop, the next element in the collection is retrieved and stored in.  The loop repeats until all elements in the collection have been obtained.  Its iteration variable is “read-only” as it relates to the underlying array.  An assignment to the iteration variable has no effect on the underlying array.  Example  The following segment can add up all the data in a int array name nums. 25 int sum = 0; for(int var : nums) { sum += var; }

F OR - EACH STYLE for - CONSTRUCT  Since the type of can be arrays, the enhanced for -construct can also apply to process multidimensional arrays as the following example.  Example  Let matrix be a 2-dimensional array. 26 int sum = 0; for(int row[]: matrix) { for(int val : row) { sum += val; }