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Chapter 7

Chapter 7. Arrays…. The entire array has a single name. Each value has a numeric index. 79 87 94 82 67 98 87 81 74 91. scores. Arrays. An array is an ordered list of values. 0 1 2 3 4 5 6 7 8 9.

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Chapter 7

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  1. Chapter 7 Arrays…

  2. The entire array has a single name Each value has a numeric index 79 87 94 82 67 98 87 81 74 91 scores Arrays • An array is an ordered list of values 0 1 2 3 4 5 6 7 8 9 An array of size N is indexed from zero to N-1 This array holds 10 values that are indexed from 0 to 9

  3. Arrays • A particular value in an array is referenced using the array name followed by the index in brackets • For example, the expression scores[2] refers to the value 94 (the 3rd value in the array) • That expression represents a place to store a single integer and can be used wherever an integer variable can be used

  4. Arrays • For example, an array element can be assigned a value, printed, or used in a calculation: scores[2] = 89; scores[first] = scores[first] + 2; mean = (scores[0] + scores[1])/2; System.out.println ("Top = " + scores[5]);

  5. Arrays • The values held in an array are called array elements • An array stores multiple values of the same type – the element type • The element type can be a primitive type or an object reference • Therefore, we can create an array of integers, an array of characters, an array of String objects, an array of Coin objects, etc. • In Java, the array itself is an object that must be instantiated

  6. 79 scores 87 94 82 67 98 87 81 74 91 Arrays • Another way to depict the scores array:

  7. Declaring Arrays • The scores array could be declared as follows: int[] scores = new int[10]; • The type of the variable scores is int[] (an array of integers) • Note that the array type does not specify its size, but each object of that type has a specific size • The reference variable scores is set to a new array object that can hold 10 integers • An array is an object, therefore all the values are initialized to default ones (here 0)

  8. Declaring Arrays • Some other examples of array declarations: float[] prices = new float[500]; boolean[] flags; flags = new boolean[20]; char[] codes = new char[1750];

  9. Using Arrays • The iterator version of the for loop can be used when processing array elements for (int score : scores) System.out.println (score); • This is only appropriate when processing all array elements from top (lowest index) to bottom (highest index) • See BasicArray.java(page 372)

  10. final int LIMIT = 15, MULTIPLE = 10; int[] list = new int[LIMIT]; // Initialize the array values for (int index = 0; index < LIMIT; index++) list[index] = index * MULTIPLE; list[5] = 999; // change one array value // Print the array values for (int value : list) System.out.print (value + " ");

  11. Bounds Checking • Once an array is created, it has a fixed size • An index used in an array reference must specify a valid element • That is, the index value must be in range 0 to N-1 • The Java interpreter throws an ArrayIndexOutOfBoundsException if an array index is out of bounds • This is called automatic bounds checking

  12. problem Bounds Checking • For example, if the array codes can hold 100 values, it can be indexed using only the numbers 0 to 99 • If the value of count is 100, then the following reference will cause an exception to be thrown: System.out.println (codes[count]); • It’s common to introduce off-by-one errors when using arrays for (int index=0; index <= 100; index++) codes[index] = index*50 + epsilon;

  13. Bounds Checking • Each array object has a public constant called length that stores the size of the array • It is referenced using the array name: scores.length • Note that length holds the number of elements, not the largest index • See ReverseOrder.java(page 375) • See LetterCount.java(page 376)

  14. ReverseOrder Scanner scan = new Scanner (System.in); double[] numbers = new double[10]; System.out.println ("The size of the array: " + numbers.length); for (int index = 0; index < numbers.length; index++) { System.out.print ("Enter number " + (index+1) + ": "); numbers[index] = scan.nextDouble(); } System.out.println ("The numbers in reverse order:"); for (int index = numbers.length-1; index >= 0; index--) System.out.print (numbers[index] + " ");

  15. Char type String st = "abcd"; for(int i =0; i < st.length (); i++ ) { char c = st.charAt (i); System.out.print(c); System.out.print(" "); System.out.print((int) c); System.out.print(" "); System.out.println(c - 'a'); } a 97 0 b 98 1 c 99 2

  16. Char Type for(int i =40; i < 130; i++) { System.out.print(i + ":\'" + (char) i + "\'\t"); if ((i+1) % 10 == 0) System.out.println(); } 40:'(' 41:')' 42:'*' 43:'+' 44:',' 45:'-' 46:'.' 47:'/' 48:'0' 49:'1' 50:'2' 51:'3' 52:'4' 53:'5' 54:'6' 55:'7' 56:'8' 57:'9' 58:':' 59:';' 60:'<' 61:'=' 62:'>' 63:'?' 64:'@' 65:'A' 66:'B' 67:'C' 68:'D' 69:'E' 70:'F' 71:'G' 72:'H' 73:'I' 74:'J' 75:'K' 76:'L' 77:'M' 78:'N' 79:'O' 80:'P' 81:'Q' 82:'R' 83:'S' 84:'T' 85:'U' 86:'V' 87:'W' 88:'X' 89:'Y' 90:'Z' 91:'[' 92:'\' 93:']' 94:'^' 95:'_' 96:'`' 97:'a' 98:'b' 99:'c' 100:'d' 101:'e' 102:'f' 103:'g' 104:'h' 105:'i' 106:'j' 107:'k' 108:'l' 109:'m' 110:'n' 111:'o' 112:'p' 113:'q' 114:'r' 115:'s' 116:'t' 117:'u' 118:'v' 119:'w' 120:'x' 121:'y' 122:'z' 123:'{' 124:'|' 125:'}' 126:'~' 127:'' 128:'?' 129:'?'

  17. Letter Count final int NUMCHARS = 26; Scanner scan = new Scanner (System.in); int[] upper = new int[NUMCHARS]; int[] lower = new int[NUMCHARS]; char current; // the current character being processed int other = 0; // counter for non-alphabetics System.out.println ("Enter a sentence:"); String line = scan.nextLine();

  18. Letter Count // Count the number of each letter occurence for (int ch = 0; ch < line.length(); ch++) { current = line.charAt(ch); if (current >= 'A' && current <= 'Z') upper[current-'A']++; else if (current >= 'a' && current <= 'z') lower[current-'a']++; else other++; }

  19. Letter Count // Print the results System.out.println (); for (int letter=0; letter < upper.length; letter++) { System.out.print ( (char) (letter + 'A') ); System.out.print (": " + upper[letter]); System.out.print ("\t\t" + (char) (letter + 'a') ); System.out.println (": " + lower[letter]); } System.out.println (); System.out.println ("Non-alphabetic characters: " + other);

  20. Alternate Array Syntax • The brackets of the array type can be associated with the element type or with the name of the array • Therefore the following two declarations are equivalent: float[] prices; float prices[]; • The first format generally is more readable and should be used

  21. Initializer Lists • An initializer list can be used to instantiate and fill an array in one step • The values are delimited by braces and separated by commas • Examples: int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};

  22. Initializer Lists • Note that when an initializer list is used: • the new operator is not used • no size value is specified • The size of the array is determined by the number of items in the initializer list • An initializer list can be used only in the array declaration • SeePrimes.java(page 381)

  23. Primes int[] primeNums = {2, 3, 5, 7, 11, 13, 17, 19}; System.out.println ("Array length: " + primeNums.length); System.out.println ("The first few prime numbers are:"); for (int prime : primeNums) System.out.print (prime + " ");

  24. Arrays as Parameters • An entire array can be passed as a parameter to a method • Like any other object, the reference to the array is passed, making the formal and actual parameters aliases of each other • Therefore, changing an array element within the method changes the original • An individual array element can be passed to a method as well, in which case the type of the formal parameter is the same as the element type

  25. Arrays as Parameters public static void doubleValues(int[] x) { for (int i = 0; i < x.length; i++) { x[i] *= 2; } } int[] x = {1, 3, 5}; doubleValues(x); for(int val:x) System.out.print(val + " ");

  26. Arrays of Objects • Consider: class String3 { String value0, value1, value2; } String3 sts3 = new String3(); System.out.println(sts3.value0); // outputs null sts3.value0 = new String("abc");

  27. Arrays of Objects • The elements of an array can be object references • The following declaration reserves space to store 3 references to String objects String[] words = new String[3]; • It does NOT create the String objects themselves • Initially an array of objects holds null references • Each object stored in an array must be instantiated separately

  28. Previous Code using string array: String[] sts3 = new String[3]; System.out.println(sts3[0]); sts3[0] = new String("abc");

  29. Arrays of Objects • The words array when initially declared: - words - - • At this point, the following reference would throw a NullPointerException: System.out.println (words[0].length());

  30. “friendship” “loyalty” Arrays of Objects • After some String objects are created and stored in the array: words “honor”

  31. Arrays of Objects • Keep in mind that String objects can be created using literals • The following declaration creates an array object called verbs and fills it with four String objects created using string literals String[] verbs = {"play", "work", "eat", "sleep"};

  32. Array of Objects • The following example creates an array of Grade objects, each with a string representation and a numeric lower bound • See GradeRange.java (page 384) • See Grade.java (page 385)

  33. Grade public class Grade { private String name; private int lowerBound; //----------------------------------------------------------------- // Constructor: Sets up this Grade object with the specified // grade name and numeric lower bound. //----------------------------------------------------------------- public Grade (String grade, int cutoff) { name = grade; lowerBound = cutoff; } //----------------------------------------------------------------- // Returns a string representation of this grade. //----------------------------------------------------------------- public String toString() { return name + "\t" + lowerBound; } }

  34. Grade[] grades = { new Grade("A", 95), new Grade("A-", 90), new Grade("B+", 87), new Grade("B", 85), new Grade("B-", 80), new Grade("C+", 77), new Grade("C", 75), new Grade("C-", 70), new Grade("D+", 67), new Grade("D", 65), new Grade("D-", 60), new Grade("F", 0) }; for (Grade letterGrade : grades) System.out.println (letterGrade);

  35. CD Collection • Let’s write methods that will help us maintain information about music CD’s. • Specifically, we want to keep track of the title, the artist name, and price of each CD. • Let’s keep these information in three different arrays, titles, artists, and prices. • The idea is the same location i is used for storing the information of CD number i in three different arrays. • Write methods for adding a new CD, deleting one given its title, and displaying information given its title.

  36. Example Use String[] ts = new String[100]; String[] as = new String[100]; double[] prices = new double[100]; int count = 0; CDCollection.addCD("title1", "artist1", 1,count, ts, as, prices); count++; CDCollection.addCD("title2", "artist2", 2,count, ts, as, prices); count++; CDCollection.addCD("title3", "artist3", 3,count, ts, as, prices); count++; CDCollection.printCD("title3", count, ts, as, prices); CDCollection.deleteCD("title2", count, ts, as, prices); count--; CDCollection.printCD("title2", count, ts, as, prices);

  37. public class CDCollection { public static void addCD(String title, String artist, double price, int count, String[] titles, String[] artists, double[] prices) { titles[count] = title; artists[count] = artist; prices[count] = price; } public static boolean deleteCD(String title,int count, String[] titles, String[] artists, double[] prices) { int loc = findCD(title, count, titles); if (loc == -1) return false; for(int i = loc + 1; i < count; i++) { titles[i-1] = titles[i]; artists[i-1] = artists[i]; prices[i-1] = prices[i]; } return true; }

  38. public static void printCD(String title,int count, String[] titles, String[] artists, double[] prices) { int loc = findCD(title, count, titles); if (loc == -1) System.out.println("CD not found"); else System.out.println("\n" + titles[loc] + " " + artists[loc] + " " + prices[loc]); } private static int findCD(String title,int count,String[] titles) { for(int i = 0; i < count; i++) if (titles[i].equals(title)) return i; return -1; }

  39. Notes • We can change the contents of the array, but not the primitive type int for count. • It is not convenient for the user to maintain the count variable himself. • One solution is to put the count into an int array • This way, the user can do : String[] ts = new String[100]; String[] as = new String[100]; double[] prices = new double[100]; int[] count = new int[1]; CDCollection.addCD("title1", "artist1", 1,count, ts, as, prices);

  40. public static void addCD(String title, String artist, double price, int[] count, String[] titles, String[] artists, double[] prices) { titles[count[0]] = title; artists[count[0]] = artist; prices[count[0]] = price; count[0]++; } public static boolean deleteCD(String title,int[] count, String[] titles, String[] artists, double[] prices) { int loc = findCD(title, count[0], titles); if (loc == -1) return false; for(int i = loc + 1; i < count[0]; i++) { titles[i-1] = titles[i]; artists[i-1] = artists[i]; prices[i-1] = prices[i]; } count[0]--; return true; }

  41. Notes • It is still inconvenient for the user to maintain the arrays and the count itself. • We can let the class contain the arrays itself

  42. Using static variables class CDCollection4 { private static int count = 0; private static String[] titles = new String[100]; private static String[] artists = new String[100]; private static double[] prices = new double[100]; public static void addCD(String title, String artist, double price) { titles[count] = title; artists[count] = artist; prices[count] = price; count++; }

  43. Using static variables CDCollection.addCD("title1", "artist1", 1); CDCollection.addCD("title2", "artist2", 2); CDCollection.addCD("title3", "artist3", 3); CDCollection.printCD("title3"); CDCollection.deleteCD("title2"); CDCollection.printCD("title2"); • Since the variables are static, and belong to the class (therefore there is only once copy), a program can only have one collection at a time.

  44. Using Objects • A better way is to make the variables and methods non-static, so that objects, not the class owns the information. So different objects will have their own arrays, letting one to use several collections at the same time • We also improve the code so that it grows the storage area as the number of CDs exceed the capacity.

  45. Using… CDCollection cc1 = new CDCollection(); CDCollection cc2 = new CDCollection(); cc1.addCD("title1", "artist1", 1); cc2.addCD("title2", "artist2", 2); cc1.addCD("title3", "artist3", 3); cc1.printCD("title3"); cc2.deleteCD("title2"); …

  46. Tunes CDCollection - collection : CD[] - count : int - totalCost : double + main (args : String[]) : void + addCD (title : String, artist : String, cost : double, tracks : int) : void + toString() : String - increaseSize() : void CD - title : String - artist : String - cost : double - tracks : int 1 * + toString() : String Arrays of Objects • A UML diagram for the Tunes program:

  47. CDCollection public class CDCollection { private CD[] collection; private int count; private double totalCost; //----------------------------------------------------------------- // Constructor: Creates an initially empty collection. //----------------------------------------------------------------- public CDCollection () { collection = new CD[100]; count = 0; totalCost = 0.0; }

  48. //----------------------------------------------------------------- //----------------------------------------------------------------- // Adds a CD to the collection, increasing the size of the // collection if necessary. //----------------------------------------------------------------- public void addCD (String title, String artist, double cost, int tracks) { if (count == collection.length) increaseSize(); collection[count] = new CD (title, artist, cost, tracks); totalCost += cost; count++; }

  49. public String toString() { NumberFormat fmt = NumberFormat.getCurrencyInstance(); String report = "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n"; report += "My CD Collection\n\n"; report += "Number of CDs: " + count + "\n"; report += "Total cost: " + fmt.format(totalCost) + "\n"; report += "Average cost: " + fmt.format(totalCost/count); report += "\n\nCD List:\n\n"; for (int cd = 0; cd < count; cd++) report += collection[cd].toString() + "\n"; return report; }

  50. //----------------------------------------------------------------- //----------------------------------------------------------------- // Increases the capacity of the collection by creating a // larger array and copying the existing collection into it. //----------------------------------------------------------------- private void increaseSize () { CD[] temp = new CD[collection.length * 2]; for (int cd = 0; cd < collection.length; cd++) temp[cd] = collection[cd]; collection = temp; } }

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