BUILDING JAVA PROGRAMS CHAPTER 7: ARRAYS 1

2 OUT-OF-BOUNDS INDEXES The indexes that are legal to access in an array are those in the range of 0 to the array's length - 1.  Reading or writing any index outside this range will throw an ArrayIndexOutOfBoundsException. Example: int[] data = new int[10]; System.out.println(data[0]); // okay System.out.println(data[-1]); // exception System.out.println(data[9]); // okay System.out.println(data[10]); // exception 2 index value

3 THE ARRAYS CLASS The Arrays class in package java.util has several useful static methods for manipulating arrays: 3 Method nameDescription binarySearch( array, value ) returns the index of the given value in this array (< 0 if not found) equals( array1, array2 ) returns true if the two given arrays contain exactly the same elements in the same order fill( array, value ) sets every element in the array to have the given value sort( array ) arranges the elements in the array into ascending order toString( array ) returns a string representing the array, such as "[10, 30, 17]"

4 ARRAYS.TOSTRING The Arrays.toString method is useful when you want to print an array's elements.  Arrays.toString accepts an array as a parameter and returns the String representation, which you can then print.  Example: int[] a = {2, 5, 1, 6, 14, 7, 9}; for (int i = 1; i < a.length; i++) { a[i] += a[i - 1]; } System.out.println("a is " + Arrays.toString(a)); Output: a is [2, 7, 8, 14, 28, 35, 44] 4

5 ARRAY TRAVERSAL traversal: An examination of each element of an array.  Traversal algorithms often take the following form: for (int i = 0; i.length; i++) { do something with [i]; } Traversals are useful in many situations:  printing the elements of an array  searching an array for a specific value  rearranging the elements of an array  computing the sum, product, etc. of an array's elements ... 5

6 PRINTING ARRAY ELEMENTS Example (print each element of an array on a line): int[] myarray = {4, 1, 9, 7}; for (int i = 0; i < myarray.length; i++) { System.out.println(i + ": " + myarray[i]); } Output: 0: 4 1: 1 2: 9 3: 7 How could we change the code to print the following? 4, 1, 9, 7 (Do not use Arrays.toString.) 6

7 EXAMINING ARRAY ELEMENTS Example (find the largest even integer in an array): int[] list = {4, 1, 2, 7, 6, 3, 2, 4, 0, 9}; int largestEven = 0; for (int i = 0; i < list.length; i++) { if (list[i] % 2 == 0 && list[i] > largestEven) { largestEven = list[i]; } } System.out.println("Largest even: " + largestEven); Output: Largest even: 6 7

8 STRING TRAVERSAL String s are like arrays of char s; they also use 0-based indexes.  We can write algorithms to traverse strings to compute information: // string stores voters' votes // (R)epublican, (D)emocrat, (I)ndependent String votes = "RDRDRRIDRRRDDDDIRRRDRRRDIDIDDRDDRRDRDIDD"; int[] counts = new int[3]; // R -> 0, D -> 1, I -> 2 for (int i = 0; i < votes.length(); i++) { char c = votes.charAt(i); if (c == 'R') { counts[0]++; } else if (c == 'D') { counts[1]++; } else { // c == 'I') counts[2]++; } System.out.println(Arrays.toString(counts)); Output: [17, 18, 5] 8

9 OUTPUT PARAMETERS output parameter: An array or object passed as a parameter that has its contents altered by the method.  We can pass in an array to a method and the method can change its contents in useful ways for us.  Examples: The methods Arrays.fill and Arrays.sort. int[] nums = {4, -1, 2, 7, 3, 6}; System.out.println(Arrays.toString(nums)); Arrays.sort(nums); // modifies contents of nums System.out.println(Arrays.toString(nums)); Arrays.fill(nums, 42); System.out.println(Arrays.toString(nums)); Output: [4, -1, 2, 7, 3, 6] [-1, 2, 3, 4, 6, 7] [42, 42, 42, 42, 42, 42] 9

10 ARRAYS AS RETURN VALUES An array can also be returned from a method.  Syntax (declaration): public static [] ( ) {  Example: public static int[] readAllNumbers(Scanner input) {  Syntax (call): [] = ( );  Example: Scanner fileScan = new Scanner(new File("temp.dat")); int[] numbers = readAllNumbers(fileScan); 10

11 ARRAY RETURN EXAMPLE Example (digit-counting problem from an earlier slide): public static int[] countDigits(int n) { int[] counts = new int[10]; while (n > 0) { int digit = n % 10; n = n / 10; counts[digit]++; } return counts; } public static void main(String[] args) { int number = ; int[] tally = countDigits(number); System.out.println(Arrays.toString(tally)); } Output: [2, 1, 3, 1, 0, 0, 0, 1, 0, 1] 11

12 STRING METHODS WITH ARRAYS These String methods return arrays: String s = "long book"; 12 Method nameDescriptionExample toCharArray() separates this string into an array of its characters s.toCharArray() returns {'l', 'o', 'n', 'g', ' ', 'b', 'o', 'o', 'k'} split( delimiter ) separates this string into substrings by the given delimiting string s.split(" ") returns {"long", "book"} s.split("o") returns {"l", "ng b", "", "k"}

13 STRING/ARRAY PROBLEMS Write a method named areAnagrams that accepts two String s as parameters and returns whether those strings contain the same letters (in any order).  areAnagrams("bear", "bare") returns true  areAnagrams("sale", "sail") returns false  Use the methods from the previous slide, as well as any relevant methods from the Arrays class. Write a method named wordCount that accepts a String as its parameter and returns the number of words in that string. Words are separated by spaces.  wordCount("the quick brown fox") returns 4  Use the methods from the previous slide. 13

SHIFTING ELEMENTS IN AN ARRAY reading:

15 CONCEPT OF AN ARRAY ROTATION Imagine we want to 'rotate' the elements of an array; that is, to shift them left by one index.  The element that used to be at index 0 will move to the last slot in the array.  For example, {3, 8, 9, 7, 5} becomes {8, 9, 7, 5, 3}. Before: After:  Shifting elements is useful when inserting and removing values from arrays after they have already been filled with data. 15 index01234 value38975 index01234 value89753

16 SHIFTING ELEMENTS LEFT A left shift of the elements of an array: Let's write the code to do the left shift.  Can we generalize it so that it will work on an array of any size?  Can we write a right-shift as well? 16 index01234 value38975 index01234 value89753

17 SHIFTING PRACTICE PROBLEM Write a method insertInOrder that accepts a sorted array a of integers and an integer value n as parameters, and inserts n into a while maintaining sorted order. In other words, assume that the element values in a occur in sorted ascending order, and insert the new value n into the array at the appropriate index, shifting to make room if necessary. The last element in the array will be lost after the insertion.  Example: calling insertInOrder on array {1, 3, 7, 10, 12, 15, 22, 47, 74} and value 11 produces {1, 3, 7, 10, 11, 12, 15, 22, 47}. 17