1. Chapter 8: Collections: Arrays
Searching and Sorting
Arrays as Arguments

2. Aggregate Data Types – re-visited
Some objects are made up of other objects – e.g. a computer system. Aggregation is sometimes described as a “has-a” relationship. For instance, a car has 4 wheels.
In software, we define an aggregate data type consisting of one or more scalar data type objects

3. Run-Time Dimensioning
The size of an array can also be entered interactively at run time
An entered value can be used to allocate space for an array using the new operator

4. public static void main(String[] args) { int i, numgrades; int grade[]; // declare the array String s1; s1 = JOptionPane.showInputDialog("Enter the number of grades” +
“ to be processed: "); numgrades = Integer.parseInt(s1); grade = new int[numgrades]; // allocate the array

5. The Arrays Class: Searching and Sorting
Java-provided class
Distinct from the Array class we have been implementing
Called helper class because it provides a number of extremely useful methods that can be a great help in processing arrays
Contains methods for:
Sorting an array
Searching a sorted array for a particular item

6. The sort() and binarySearch() Methods
sort() method
Arranges elements of array into ascending (increasing) order
Uses modified quicksort algorithm – we’ll look at this in more detail in 8.9
Only works for:
Primitive data types
Strings

7. The sort() and binarySearch() Methods (continued)
Searches array for a specified value
Requires a sorted array
Returns:
Item index if found
Negative number if not found

8. Search & Sort
Generally the sort( ) method is called immediately before the binarySearch( ) method.
Use either
import java.util.*; or
import java.util.Arrays;
Example:

9. String s1; int i, numels; int variousNums[]; int item, location; // set up the basic input stream BufferedReader br = new BufferedReader( new InputStreamReader(System.in)); System.out.print("Enter the number of array values: "); s1 = br.readLine(); numels = Integer.parseInt(s1); // allocate the array variousNums = new int[numels]; // read the array values for (i = 0; i < numels; i++) { System.out.print("Enter element " + (i+1) + ": "); s1 = br.readLine();_ variousNums[i] = Integer.parseInt(s1); }

10. // sort the array Arrays
// sort the array Arrays.sort(variousNums); // display the sorted array System.out.print("The values in sorted order are:"); for (i = 0; i < numels; i++) System.out.print(" " + variousNums[i]); // now locate a desired element System.out.print("\n\nEnter the item you are searching for: "); s1 = br.readLine(); item = Integer.parseInt(s1); location = Arrays.binarySearch(variousNums, item); if (location > 0) System.out.println("This item is at location " (location + 1) + " in the sorted array."); else System.out.println("This item is not in the list."); } }

11. Arrays as Arguments
Individual array elements are passed to a called method in the same manner as individual scalar variables
Passed by value – i.e. method receives a copy of the stored value, can manipulate it, but cannot directly change the value in the original array.
e.g. findMax (number[5], number[7])

12. Passing an entire array
A complete array can be passed by reference
The called method may change items in the original array
double salary [ ] = new double [5]; ….
findMax(salary);
Must alert the method that an array is coming in header line:
double findMax (double vals [ ])

13. public class ArrayArgument { public static void main(String[] args) { int nums[ ] = {2, 18, 1, 27, 16}; // the call is made here System.out.println("The maximum value is " + findMaximum(nums)); } // this is the called method used to find the maximum value // stored in an array of integers public static int findMaximum(int vals[]) { int i, max = vals[0]; for (i = 1; i < vals.length; i++) if (max < vals[i]) max = vals[i]; return max; } }

14. figure 8.14 on page 418 illustrates that the method references the original array.

15. Class Exercise
p. 416 # 1, 2
p. 420 # 1, 3, 5, 7