Basics of Programming with Arrays

1. Programming with Arrays 1 This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials. CSS161: Fundamentals of Computing

2. Partially Filled Arrays • The exact size needed for an array may be unknown till run time. • Solution: • Declare a larger array: someArray[MAX] • Keep track of # valid elements in count someArray[0] ~ someArray[count – 1] Where count <= someArray.length CSS161: Fundamentals of Computing

3. An Array Implementation of a List • List is a data structure to maintain a dynamic size of items in a sequential order. 10.0 20.0 30.0 40.0 50.0 60.0 70.0 List.add( ) X 10.0 20.0 30.0 40.0 50.0 60.0 70.0 List.deleteLast( ) X 10.0 20.0 30.0 40.0 50.0 60.0 List.delete( 3 ) 10.0 20.0 30.0 50.0 60.0 List.getElement( 3 ) returns 50.0 CSS161: Fundamentals of Computing

4. List Implementation (1 of 5) /** List.java: demonstrates a use of a partially filled array of doubles. The class enforces the following invariant: All elements are at the beginning of the array in locations 0, 1, 2, and so forth up to a highest index with no gaps. */ public class List { private int maxNumberElements; //Same as a.length private double[] a; private int numberUsed; //Number of indices currently in use /** Sets the maximum number of allowable elements to 10. */ List( ) { maxNumberElements = 10; a = new double[maxNumberElements]; numberUsed = 0; } CSS161: Fundamentals of Computing

5. List Implementation (2 of 5) /** Precondition arraySize > 0. */ List(int arraySize) { if (arraySize <= 0) { System.out.println("Error Array size zero or negative."); System.exit(0); } maxNumberElements = arraySize; a = new double[maxNumberElements]; numberUsed = 0; } List(List original) { if (original == null) { System.out.println("Fatal Error: aborting program."); System.exit(0); } maxNumberElements = original.maxNumberElements; numberUsed = original.numberUsed; a = new double[maxNumberElements]; for (int i = 0; i < numberUsed; i++) a[i] = original.a[i]; } CSS161: Fundamentals of Computing

6. List Implementation (3 of 5) /** Adds newElement to the first unused array position. */ public void add(double newElement) { if (numberUsed >= a.length) { System.out.println("Error: Adding to a full array."); System.exit(0); } else { a[numberUsed] = newElement; numberUsed++; } } public double getElement(int index) { if (index < 0 || index >= numberUsed) { System.out.println("Error:Illegal or unused index."); System.exit(0); } return a[index]; } CSS161: Fundamentals of Computing

7. List Implementation (4 of 5) /** index must be an index in use or the first unused index. */ public void resetElement(int index, double newValue) { if (index < 0 || index >= maxNumberElements) { System.out.println("Error:Illegal index."); System.exit(0); } else if (index > numberUsed) { System.out.println( "Error: Changing an index that is too large."); System.exit(0); } else a[index] = newValue; } public void deleteLast( ) { if (empty( )) { System.out.println("Error:Deleting from an empty array."); System.exit(0); } else numberUsed--; } CSS161: Fundamentals of Computing

8. List Implementation (5 of 5) /** Deletes the element in position index. Moves down all elements with indices higher than the deleted element. */ public void delete(int index) { if (index < 0 || index >= numberUsed) { System.out.println("Error:Illegal or unused index."); System.exit(0); } for (int i = index; i < numberUsed; i++) a[i] = a[i + 1]; // what if numberUsed reached a.length? numberUsed--; } public boolean empty( ) { return (numberUsed == 0); } public boolean full( ) { return (numberUsed == maxNumberElements); } public int getMaxCapacity( ) { return maxNumberElements; } public int getNumberOfElements( ) { return numberUsed; } } CSS161: Fundamentals of Computing

9. List Driver public class ListDriver { public static void main( String[] args ) { List list = new List( 20 ); for ( int i = 0; i < 20; i++ ) list.add( i ); list.deleteLast( ); // what if this statement is omitted? list.delete( 10 ); for ( int i = 0; i < list.getNumberOfElements( ); i++ ) System.out.println( "list(" + i + ") = " + list.getElement( i ) ); } } [mfukuda@perseus css161]$ java ListDriver list(0) = 0.0 list(1) = 1.0 list(2) = 2.0 list(3) = 3.0 list(4) = 4.0 list(5) = 5.0 list(6) = 6.0 list(7) = 7.0 list(8) = 8.0 list(9) = 9.0 list(10) = 11.0 list(11) = 12.0 list(12) = 13.0 list(13) = 14.0 list(14) = 15.0 list(15) = 16.0 list(16) = 17.0 list(17) = 18.0 [mfukuda@perseus css161]$ CSS161: Fundamentals of Computing

10. Question • What if we delete the following statement from ListDriver.java program? list.deleteLast( ); • Keep track of the program execution. CSS161: Fundamentals of Computing

11. ordinary for loop for-each loop “for-each” Loop double[] a = new double[10]; for ( int i = 0; i < a.length; i++ ) a[i] = 0.0; for ( int i = 0; i < a.length; i++ ) System.out.println( a[i] ); for ( int i = 0; i < a.length; i++ ) a[i] = 2 * i; double[] a = new double[10]; for ( double element : a ) element = 0.0; for ( double element : a ) System.out.println( element ); for ( double element : a ) element = 2 * i; // ??? Was i defined ??? CSS161: Fundamentals of Computing

12. Vararg Specification Method with a Variable Number of Parameters • Syntax Type… Array_Name • Examples int… arg double… a String… unwanted public static int max( int… arg ) { // arg can be used as an array of ints } max( score1, score2 ); max( score1, score2, score3 ); score1 score2 score1 score2 score3 CSS161: Fundamentals of Computing

13. Display 6.7 (1 of 2) CSS161: Fundamentals of Computing

14. Display 6.7 (2 of 2) import java.util.Scanner; public class VariableParameterDemo { public static void main(String[] args) { System.out.println("Enter scores for Tom, Dick, and Harriet:"); Scanner keyboard = new Scanner(System.in); int tomsScore = keyboard.nextInt( ); int dicksScore = keyboard.nextInt( ); int harrietsScore = keyboard.nextInt( ); int highestScore = UtilityClass.max(tomsScore, dicksScore, harrietsScore); System.out.println("Highest score = " + highestScore); } } Enter scores for Tom, Dick, and Harriet: 55 100 99 Highest score = 100 CSS161: Fundamentals of Computing

15. Self-Test Exercises • Work on textbook p371’s exercises 16 - 17. CSS161: Fundamentals of Computing

16. Privacy Leaks with Array Instance Variables • Just as when an accessor returns a reference to any private object public double[] getArray() { return anArray;//BAD! } • The example above will result in a privacy leak CSS161: Fundamentals of Computing

17. Privacy-Leak Preventive Example 1 • The previous accessor method would simply return a reference to the array anArray itself • Instead, an accessor method should return a reference to a deep copy of the private array object • Below, both a and count are instance variables of the class containing the getArray method public double[] getArray() { double[] temp = new double[count]; for (int i = 0; i < count; i++) temp[i] = a[i]; return temp } CSS161: Fundamentals of Computing

18. Privacy-Leak Preventive Example 2 • If a private instance variable is an array that has a class as its base type, then copies must be made of each class object in the array when the array is copied: public ClassType[] getArray() { ClassType[] temp = new ClassType[count]; for (int i = 0; i < count; i++) temp[i] = new ClassType(someArray[i]); return temp; } CSS161: Fundamentals of Computing

19. Self-Test Exercises • Work on textbook p375’s exercises 18 ~ 19. CSS161: Fundamentals of Computing