Introduction to arrays Data in economy size packages

Array Data structure that contains a collection of data items –All items are of the same data type –Access to the entire collection via the array name –Access to individual elements via their indexes (or subscripts) Behaves like a list of variables with a uniform naming mechanism

Example Suppose you needed to store 20 distinct whole-number values; what can you do? –Declare 20 distinct int variables, giving each a unique name –Declare a single int array with the capacity to hold 20 elements Which is preferable? What if the problem involved 200 numbers? 2000? 2,000,000?

Array declaration The syntax for array declaration is: data type [ ] identifier; –“data type” is any built-in, library or user- defined data type (int, double, char, String, etc.) –“identifier” is a valid Java identifier –Java supports a variation of this syntax, which is closer to C/C++ syntax: data type identifier[ ];

Array creation An array is a reference data type, similar to a class newDeclaration doesn’t allocate any memory; this is done, as with objects, with the new operator Examples: double [] averages = new double[10]; char [] alphabet; alphabet = new char[26]

Array creation The number in square brackets when an array is created indicates the number of elements that can be stored in the array Any positive int expression can be used to allocate an array Example: public static final int LETTERS = 26; char [] asciiAlphabet = new char [LETTERS * 2];

Array indexing & length The length of the array is determined when it is created; the value is stored in a public instance variable, length asciiAlphabet.length has the value 52 If an array is created with size n, the subscripts of its elements are numbered (in order) from 0 to n-1 So, for example, the first element in the asciiAlphabet array is element 0, and the last element is element 51

One overloaded operator We have already seen two uses for the indexing operator ([]): –Array declaration; for example, int [] nums; –Array creation; nums = new int [20]; This operator has one more use: access to individual array elements, or indexing

Accessing individual array values An element is accessed via its index or subscript The notation for accessing an element via its subscript is: arrayName[subscript] where –“arrayName” is the array’s identifier –“subscript” is a number between 0 and size-1 The first element has subscript 0, the second has subscript 1, the third has subscript 2, and so forth up to the last element, which has subscript length-1

Examples asciiAlpha[0] = ‘A’; System.out.print(“” + asciiAlpha[8]); averages[7] *= 0.5; Note: although these examples use literal values, a subscript can be any int expression It is up to the programmer to ensure that the subscript value is within the bounds of the array

Filling an array with initial values char [] upperAlpha = new char [26]; for (char c = ‘A’; c <= ‘Z’; c++) upperAlpha[(int)(c – ‘A’)] = c; Trace: cc – ‘A’value stored ‘A’ 0 ‘A’ ‘B’ 1 ‘B’ ‘C’ 2 ‘C’... ‘Z’ 25 ‘Z’ ‘[‘ 26

Initializing at declaration char [] lowerAlpha = {‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’, ‘j’, ‘k’, ‘l’, ‘m’, ‘n’, ‘o’, ‘p’, ‘q’, ‘r’, ‘s’, ‘t’, ‘u’, ‘v’, ‘w’, ‘x’, ‘y’, ‘z’}; List all values in a block of code, terminating with a semicolon: Compiler automatically sizes array to hold the specified data; array will remain this size, even if different data assigned: for (int x = 0; x < 10; x++) lowerAlpha[x]=‘!’

13 What values are assigned? float [] temps = float[ 5 ] ; // allocates memory for array int m ; for (m = 0; m < 5; m++) { temps[ m ] = m  0.2 ; } temps[0] temps[1] temps[2] temps[3] temps[4] ? ? ? ? ?

14 Now what values are printed? float [] temps = float[ 5 ] ; // allocates memory for array int m ;..... for (m = 4; m >= 0; m-- ) { System.out.println(“” + temps[ m ]) ; } temps[0] temps[1] temps[2] temps[3] temps[4]

15 Variable Subscripts float [] temps = new float[ 5 ] ;//allocates memory for array int m = 3 ; What is temps[ m + 1] ? What is temps[ m ] + 1 ? temps[0] temps[1] temps[2] temps[3] temps[4]

Arrays of objects Arrays can be collections of elements of any data type, including classes For example, suppose we wanted to create an array of Fraction objects: –Array declaration: Fraction [] ratios = new Fraction[100]; but not the individual Fraction objects it will contain –This creates the array, but not the individual Fraction objects it will contain; these have to be created individually, as shown on the next slide

Initializing an array of objects for (int x = 0; x < ratios.length; x++) ratios[x] = new Fraction(); // initializes entire array with values 1/1 Random rg = new Random(); for (int y = 0; y < ratios.length; y++) ratios[y] = new Fraction (rg.nextInt(10) +1, rg.nextInt(20) + 1); // initializes array with random values

Methods & array parameters An array element can be passed to any method that takes its base type as an argument For example, in the Fraction class the plus method takes a Fraction argument; elements of the array declared on the previous slide could serve as both calling object and argument: ratios[0] = ratios[1].plus(ratios[2])

Methods & array parameters An entire array can be passed as an argument to a method, if such a parameter is specified – example: public static void fillFractionArray (Fraction [] fracs) { Random rg = new Random(); for (int x = 0 ; x < fracs.length; x++) fracs[x] = new Fraction (rg.nextInt(10)+1,rg.nextInt(20)+1); A call to this method: Fraction [] ratios = new Fraction[25]; Fraction.fillFractionArray(ratios); Note that the square brackets are not used when an entire array is passed as an argument

public static void main (String [] args) { What is args? A. A String B. An array of Strings C. Nothing D. None of the above