Java: Introduction to Arrays

What are 1-D arrays? An array is an object that is used to store a list of values of the same type . It is made out of a contiguous block of memory that is divided into a number of "slots“ or variables. Each slot can be accessed by using its index (or subscript) . If there are N slots in an array, the indexes will be 0 through N-1. The diagram on the right shows a 5-element array. The indexes are from 0 to 4 An array that uses a single subscript is called a one dimensional array. Two dimensional arrays, three dimensional arrays, and higher dimensional arrays also exist:

1-D Array Declaration For any type T, T[ ] is a class , whose instances are arrays of type T. Thus, the following statement declares a reference variable, b, of type T array : T[] b; For any positive integer n, the following expression creates a new T[ ] object of size n and stores its reference in b : b = new T[n] ; As usual, the two expressions can be combined together as: T[] b = new T[n] ; For example, the following declares an int[] , grades, of size 10: int[ ] grades = new int[10];

1-D Array Declaration (cont’d) The declaration of an array of size n creates n variables of the base type T. These variables are indexed starting from 0 to n-1. They are called subscripted variables . A subscripted variable can be used anywhere an ordinary variable of the same type can be used. The slots of an array are initialized to the default value for their type. Each slot of a numeric array is initialized to zero. Each array object has a public instance variable, length, that stores the size of the array. Thus, the following statement prints 10, the size of grades: System.out.println(grades.length); Once an array has been constructed, its length does not change. Other examples of 1D-array declaration are: double[ ] price = new double[500]; // each element is initialized to 0.0 boolean[ ] flag = new boolean[20]; // each element is initialized to false

Accessing elements of a 1-D Array A particular variable is accessed by indexing the array reference with the index (subscript) of the variable in square brackets : grades[4] = 20; The following example, re-initializes each variable with twice its index: int[] grades = new int[10]; for(int i = 0; i < grades.length; i++) grades[i] = 2*i ; The use of grades.length instead of 10 makes the code more general.

Accessing elements of a 1-D Array (cont'd ) The following prints the values of the array re-initialized by the example in the previous slide. for(int i = 0; i < grades . length; i++) System.out.print(grades[i] + “ “); Output: 0 2 4 6 8 10 14 16 18 Note: Trying to access an element with an invalid index causes a run-time error: ArrayIndexOutOfBoundsException: int x = grades[10]; // causes run-time error

Accessing elements of a 1-D Array (cont'd ) The indexes (subscripts) of an array must always be of integer type except long ( int, char, short, byte ). An array index can be any expression that evaluates to an integer within the allowed index range: double[] val = new double[20]; int k = 3; val[0] = 1.5; val[1] = 10.0; val[2] = 15.5; val[k] = val[k-1] + val[k - 2]; // same as val[3] = val[2] + val[1] val[ k*2 + 3 ] = 24 + val[13]; // same as: val[ 9 ] = 24 + 0.0 System.out.println( "val[" + k + "] == " + val[k] ); // output: val[3] = 25.5 Note: the following are invalid because the index is not of type int, char, short, or byte: long k = 5L; val[k] = 33.2; val[8.0] = 75.5;

Using Subscripts with an Array Enhanced for loop Cycle through array Without specifying starting and ending points for loop control variable for(int val : scoreArray) System.out.println(val);

Initializer List Initializer list can be used to instantiate and initialize an array in one step: int[ ] prime = {2 , 3, 5, 7, 11, 13, 17, 19, 23, 29} ; char[ ] letterGrade = { ’ A ’ , ‘ B ’ , ‘C’, ‘ D ’ , ‘ F ’ }; It is actually the compiler that fills the gap. Thus, in the first example, the compiler would add the following: int[] prime = new int[10]; prime[0] = 2; prime[1] = 3; ... prime[9] = 29; Observe that when an initializer list is used: The new operator is not required. The size is not required; it is computed by the compiler.

Copying one array to another Consider the following declarations: int[ ] array1 = {22, 3, 50, 7, 11, 13, 17}; int[ ] array2 = {3, 5, 8, 20, 0, 100, 40}; The assignment statement : array1 = array2; does not copy the contents of array2 to array1 ; it makes the reference array1 to refer to the array object referenced by array2 . The object that was referenced by array1 becomes garbage. To copy the contents of array2 to array1 , code like the following is used: for(int k = 0; k < array2.length; k++) array1[k] = array2[k];

Array used as a Parameter Remember that a parameter is data that is supplied to a method just before it starts running. The following method prints the content of an int array passed to it as parameter: class MyArray { public void print (int[] x) { for(int i = 0; i < x.length; i++) System.out.print(x[i]+ " "); System.out.println(); } } The method is written using the parameter x to refer to the actual data that it will be supplied with. The method is written without referring to any particular data. When the print() method is running it has a reference to an array object in its parameter x .

Example(1) class ArrayDemo { public static void main ( String[] args ) { MyArray operate = new MyArray(); int[] ar1 = { -20, 19, 1, 5, -1, 27 } ; System.out.print ("The array is: " ); operate.print( ar1 ); } } public static void print (int[] x) { for(int i = 0; i < x.length; i++) System.out.print(a[x]+ " "); System.out.println(); } MyArray print(x) operate -20 19 1 5 -1 27 ar1

Parameter Connected to New Data Could the main() method create a second array and use the print() method with it? Example (2): What is the output? class ArrayDemo { public static void main ( String[] args ) { MyArray operate = new MyArray(); int[] ar1 = { -20, 19, 1, 5, -1, 27 } ; int[] ar2 = {1, 2, 6, 3, 9, 5}; System.out.print("The array is: " ); operate.print( ar1 ); System.out.print("The second array is: " ); operate.print( ar2 ); } } The array is: -20 19 1 5 -1 27 The second array is: 1 2 6 3 9 5

Using Two-Dimensional and Multidimensional Arrays One-dimensional or single-dimensional array Picture as column of values Elements accessed using single subscript Two-dimensional arrays Two or more columns of values Rows and columns Use two subscripts Matrix or table int[][] someNumbers = new int[3][4];

View of a Two-Dimensional Array in Memory

Using Two-Dimensional and Multidimensional Arrays (continued) int[][] rents = { {400, 450, 510}, {500, 560, 630}, {625, 676, 740}, {1000, 1250, 1600} }; public static void displayScores(int[][]scoresArray)

Using Two-Dimensional and Multidimensional Arrays (continued) Multidimensional arrays More than two dimensions Create arrays of any size Keep track of order of variables needed as subscripts Don’t exhaust computer’s memory

Java: Introduction to Arrays

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Java: Introduction to Arrays