1. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 1 Chapter 10 Arrays Sections 1-4

2. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 2 Array Basics Suppose we have a sequence of data values which all need to be processed in the same way. –We would like to use a loop but how do we process a different variable each time through the loop? An array is an indexed collection of data values. If your program needs to deal with 100 integers, 500 Account objects, 365 real numbers, etc., you can use an array. An array is a collection of data values of the same type.

3. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 3 Arrays of Primitive Data Types Array variables are reference types (like objects) Array Declaration [ ] //variation 1 [ ]//variation 2 Array Instantiation (Creation) = new [ ] Example double[ ] rainfall; rainfall = new double[12]; Variation 1 double rainfall [ ]; rainfall = new double[12]; Variation 2 An array is like an object!

4. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 4 Accessing Individual Elements Each array has a variable named length associated with it. Individual elements in an array accessed by index. double[] rainfall = new double[12]; The index of the first position in an array is 0. rainfall 01234567891011 rainfall[2] This indexed expression refers to the element at position #2 rainfall.length is 12

5. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 5 Reading Data into an Array double[] rainfall = new double[12]; double annualAverage, sum = 0.0; Scanner kbd = new Scanner( System.in); for (int i = 0; i < rainfall.length; i++) { System.out.println("Rainfall for month " + (i+1)); rainfall[i] = kbd.nextDouble(); sum += rainfall[i]; } annualAverage = sum / rainfall.length; The public constant length returns the capacity of an array.

6. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 6 Using an Array of Strings double[] rainfall = new double[12]; String[] monthName = new String[12]; monthName[0] = "January"; monthName[1] = "February"; … double annualAverage, sum = 0.0; Scanner kbd = new Scanner( System.in); for (int i = 0; i < rainfall.length; i++) { System.out.println("Rainfall for month " + monthName[i]); rainfall[i] = kbd.nextDouble(); sum += rainfall[i]; } annualAverage = sum / rainfall.length; The same pattern for the remaining ten months. The actual month name instead of a number.

7. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 7 Use Loops to Process Array Data Compute the average rainfall for each quarter. //assume rainfall is declared and initialized properly double[] quarterAverage = new double[4]; for (int i = 0; i < 4; i++) { sum = 0; for (int j = 0; j < 3; j++) { //compute the sum of sum += rainfall[3*i + j];//one quarter } quarterAverage[i] = sum / 3.0; //Quarter (i+1) average }

8. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 8 Array Initialization Like other data types, it is possible to declare and initialize an array at the same time. int[] number = { 2, 4, 6, 8 }; double[] samplingData = { 2.443, 8.99, 12.3, 45.009, 18.2, 9.00, 3.123, 22.084, 18.08 }; String[] monthName = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" }; number.length samplingData.length monthName.length 4 9 12

9. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 9 Array Size Specification The size of an array can be specified as a constant or an expression The following code prompts the user for the size of an array and declares an array of designated size: int size; Scanner kbd = new Scanner( System.in); int[] number; System.out.println("Size of an array:"); size= kbd.nextInt(); number = new int[size];

10. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 10 Arrays of Objects In addition to arrays of primitive data types, we can declare arrays of objects An array of primitive data is a powerful tool, but an array of objects is even more powerful. The use of an array of objects allows us to model the application more cleanly and logically.

11. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 11 The Person Class We will use Person objects to illustrate the use of an array of objects. Person latte; latte = new Person( ); latte.setName("Ms. Latte"); latte.setAge(20); latte.setGender('F'); System.out.println( "Name: " + latte.getName() ); System.out.println( "Age : " + latte.getAge() ); System.out.println( "Sex : " + latte.getGender() ); The Person class supports the set methods and get methods.c

12. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 12 Creating an Object Array - 1 Code State of Memory Person[ ] person; person = new Person[20]; person[0] = new Person( ); A A Only the name person is declared, no array is allocated yet.

13. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 13 person Creating an Object Array - 2 Code State of Memory Person[ ] person; person = new Person[20]; person[0] = new Person( ); B B Now the array for storing 20 Person objects is created, but the Person objects themselves are not yet created. After is executed B 0123416171819 person

14. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 14 Creating an Object Array - 3 Code State of Memory Person[ ] person; person = new Person[20]; person[0] = new Person( ); C C One Person object is created and the reference to this object is placed in position 0. 0123416171819 person 0123416171819 person After is executed C Person

15. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 15 Person Array Processing – Sample 1 Create Person objects and set up the person array. String name, inpStr; intage; chargender; Scanner kbd = new Scanner( System.in); for (int i = 0; i < person.length; i++) { //read in data values System.out.println("Enter name:" ); name = kbd.next(); System.out.println("Enter age:" ); age= kbd.nextInt(); System.out.println("Enter gender:" ); inpStr= kbd.next(); gender = inpStr.charAt(0); //create a new Person and assign values person[i] = new Person( name, age, gender); }

16. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 16 Person Array Processing – Sample 2 Find the youngest and oldest persons. intminIdx = 0;//index to the youngest person intmaxIdx = 0; //index to the oldest person for (int i = 1; i < person.length; i++) { if ( person[i].getAge() < person[minIdx].getAge() ) { minIdx = i; //found a younger person } else if (person[i].getAge() > person[maxIdx].getAge() ) { maxIdx = i; //found an older person } //person[minIdx] is the youngest and person[maxIdx] is the oldest

17. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 17 Partially Filled Arrays We don't always know how big an array we need –What happens if we aren't using all the elements? Fill the array in order and use a counter variable to keep track of how many elements have been used –Don't leave holes For object arrays, can use the first null element as a sentinal –What happens if we run out of room? Allocate a new block of memory (using new) Copy all the elements from the old array to the new one Set the reference to the new array

18. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 18 Object Deletion – Approach 1 int delIdx = 1; person[delIdx] = null; Delete Person B by setting the reference in position 1 to null. 0123 person A B C D A A 0123 AC D Before is executed A After is executed A

19. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 19 Object Deletion – Approach 2 int delIdx = 1, last = 3; person[delIndex] = person[last]; person[last] = null; Delete Person B by setting the reference in position 1 to the last person. 0123 person A B C D A A 0123 AC D Before is executed A After is executed A

20. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 20 Person Array Processing – Sample 3 Searching for a particular person. int i = 0; while ( person[i] != null && !person[i].getName().equals("Latte") ) { i++; } if ( person[i] == null ) { //not found - unsuccessful search System.out.println("Ms. Latte was not in the array"); } else { //found - successful search System.out.println("Found Ms. Latte at position " + i); }

21. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 21 Passing Arrays to Methods - 1 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } A A At before searchMinimum A arrayOne A. A. Local variable number does not exist before the method execution

22. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 22 Passing Arrays to Methods - 2 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } arrayOne B B The address is copied at B number B. B. The value of the argument, which is an address, is copied to the parameter.

23. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 23 arrayOne number While at inside the method C C Passing Arrays to Methods - 3 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } C C C. C. The array is accessed via number inside the method.

24. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 24 arrayOne number Passing Arrays to Methods - 4 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } D arrayOne At after searchMinimum D D. D. The parameter is erased. The argument still points to the same object.

25. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 25 Arrays are objects What do you think will happen if you do the following? System.out.println( array); –To print the individual elements, you need to use a loop. What do you expect the comparison a==b to do? –To compare the individual elements of a and b you need to write a loop.

26. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 26 Arrays Class The Arrays class has a number of useful static methods –equals does an element by element comparison of two arrays uses equals to compare objects –toString returns a string representation of the array –binarySearch and sort methods will be discussed in Chapter 11 Arrays is in the java.util package

27. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 27 Array Practice Consider the array declared below double [] number = new double[25] a)What is number.length ? b)Show how to set the last element to 6.25. c)Write code to compute the sum of all the negative numbers stored in the array.

28. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 28 for Loops with Arrays We can use a for loop to process all the elements of an array. for (int i=0; i<array.length; i++) { /* process array[i] /* } Java 1.5 added a new loop syntax to be used with arrays and collections for (element : array) { /* process element /* } This loop iterates through all the elements in the array but you don't need to use an index

29. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 29 Two-Dimensional Arrays Two-dimensional arrays are useful in representing tabular information.

30. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 30 Declaring and Creating a 2-D Array Declaration [][] //variation 1 [][] //variation 2 Creation = new [ ][ ] Example double[][] payScaleTable; payScaleTable = new double[4][5]; 3 2 1 0 43210 payScaleTable

31. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 31 Accessing an Element An element in a two-dimensional array is accessed by its row and column index.

32. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 32 Sample 2-D Array Processing Find the average of each row. double[ ] average = { 0.0, 0.0, 0.0, 0.0 }; for (int i = 0; i < payScaleTable.length; i++) { for (int j = 0; j < payScaleTable[i].length; j++) { average[i] += payScaleTable[i][j]; } average[i] = average[i] / payScaleTable[i].length; }

33. Java Implementation of 2-D Arrays The sample array creation payScaleTable = new double[4][5]; is really a shorthand for payScaleTable = new double [4][ ]; payScaleTable[0] = new double [5]; payScaleTable[1] = new double [5]; payScaleTable[2] = new double [5]; payScaleTable[3] = new double [5];

34. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 34 Ragged Arrays Subarrays may be different lengths. Executing triangularArray = new double[4][ ]; for (int i = 0; i < 4; i++) triangularArray[i] = new double [i + 1]; results in an array that looks like:

35. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter 10 - 35 Array Practice Trace the following code fragments for the array declared int [] array = {1, 2, 3, 5, 8, 13} 1) for (int i=0; i<array.length; i++) if (i %2==0) System.out.println( array[i]); 2) for (int i=0; i<array.length; i++) if (array[I] %2==0) System.out.println( array[i]);