CS 1713/1711 Introduction to Computer Science Lecture 28

CS 1713 Lecture 28: Searching Arrays of Objects
November 3, 1999

Searching: - finding the position of a specified value in an array.

Example: Return the position of the minimum value in the array a or -1 if a has 0 elements.

     public int findMinimumPosition(double[] a) {
         if (a.length == 0)
            return -1;
         int minpos = 0;
         for (int i = 1; i < a.length i++) {
             if (a[minpos] > a[i]) {
                minpos = i;
             }
         }    
         return minpos;
     }

What is the scope of sizeArray? For an array of with n elements, how many entries do you have to examine (on average) to find the position of the minimum value?

Example: Return the position of val in the array a or -1 if val is not in a. We will do this by scanning the array starting at the beginning until we find val. This technique is called linear search.
```
     public int linearSearch(double[] a, double val) {
         for (int i = 0; i < a.length; i++) {
             if (a[i] == val)
                return i;
         }
         return -1;
     }
```
How many entries do you have to examine (on average) to find the position of val? How many entries do you have to examine (on average) to determine that val is not in a?
Example: How would you feel about using the linear search technique to look up a name in the phone book? What do you actually do? Next week we will discuss sorting and the binary search algorithm.
Arrays of Objects: Arrays can contain objects, as well as primitive types. You create the objects in the array as well as the array.

Example: Create an array to hold the names of the days of the week.

    String [] dayNames = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday",
                         "Friday", "Saturday"};

Here the items in dayNames are references to these literals.

Example: Create an array of 5 Button objects with the names "Cola", "Orange", "Diet Cola", "Root Beer" and "Cherry Cola". (The definition for Button is given at the end of the lecture.)

    VButton [] machine = new VButton[5];
    machine[0] = new VButton("Cola");
    machine[1] = new VButton("Orange");
    machine[2] = new VButton("Diet Cola");
    machine[3] = new VButton("Root Beer");
    machine[4] = new VButton("Cherry Cola");

Example: Clear the buttons in the machine of the previous example.

    for (int i = 0; i < machine.length; i++) 
       machine[i].clearButton();

Example: Write a function called findButtonNumber that returns the position of the button named val in an array of VButton objects, a. Call the function to find the number of the VButton named "Orange" in the array machine of the previous example.

     public int findButtonNumber(VButton[] a, String val) {
         for (int i = 0; i < a.length; i++) {
             if (val.equals(a[i].getName()))
                return i;
         }
         return -1;
     }

The calling code is:

     int position = findButtonNumber(machine, "Orange");

VButton.java referred to in previous examples.

import java.util.*;

public class VButton {

    // fields (data)
    private boolean myIamPushed;
    private Date myCreationDate;
    private String myName;

    // constructor
    public VButton (String name) {
        myIamPushed = false;
        myCreationDate = new Date();
        myName = new String(name);
    }

    // accessor methods
    public boolean amIPushed(){
        return myIamPushed;
    }
 
    public String getName() {
        return myName;
    }
 
    public String getWhenMade() {
        return myCreationDate.toString();
    }
 
    public String toString() {
       String state;
       if (amIPushed())
          state = "pushed";
       else
          state = "cleared";
       return myName + ": created " + myCreationDate + " is " + state;
    }
 
    // modifier methods
    public void clearButton() {
        myIamPushed = false;
    }
 
    public void pushButton() {
        myIamPushed = true;
    }
}

Objective: Learn to manipulate and search an array of objects.

Last revision: November 4, 1999 at 8:30 pm