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CS 106Introduction to Computer Science I
10 / 29 / 2007
Instructor: Michael Eckmann
Michael Eckmann - Skidmore College - CS 106 - Fall 2007
Today’s Topics• Comments and/or Questions?• midterm exam grading explanation• binary search examples• Object Oriented programming introduction
– data– methods/behaviours
Michael Eckmann - Skidmore College - CS 106 - Fall 2007
0-4.0 scale grade coversion• Your exam was graded on a curve.• The total points: 68
• The mean: 55.47
• The standard deviation: 7.61
• Low: 34
• High: 67.5
• Your RawScore is the number of points you earned out of 68 (the numerator on the front of your test.)
(RawScore - 55.47) ECMin(4.33, 3.17 + --------------------- + ----------- ) 7.61 68
- can't get higher than 4.33.
Michael Eckmann - Skidmore College - CS 106 - Fall 2007
0-4.0 scale grade coversion
These are the minimum numbers for each letter grade.
e.g.
a 2.9 is a B
a 3.53 is an A-
0.51 D -
0.84 D
1.17 D +
1.51 C-
1.84 C
2.17 C +
2.51 B-
2.84 B
3.17 B+
3.51 A-
3.84 A
4.17 A+
Binary Search // method to perform binary search of an array
public static int binarySearch( int array2[], int key )
{
int low = 0; // low element subscript
int high = array2.length - 1; // high element subscript
int middle; // middle element subscript
// loop until low subscript is greater than high subscript
while ( low <= high )
{
// determine middle element subscript
middle = ( low + high ) / 2;
// if key matches middle element, return middle location
if ( key == array2[ middle ] )
return middle;
// if key less than middle element, set new high element
else if ( key < array2[ middle ] )
high = middle - 1;
// key greater than middle element, set new low element
else
low = middle + 1;
} // end while loop
return -1; // key not found
} // end method binarySearch
Searching arrays (Binary search)
• Let’s analyze the binary search.• To simplify the discussion, we can count the 2
comparisons in the if/else/if/else together to be 1 comparison.
• How long (that is, how many comparisons) does it take to find the value?– What’s the minimum number of comparisons it would take?– What’s the maximum number of comparisons it would take?
Michael Eckmann - Skidmore College - CS 106 - Fall 2006
Program 1• Comments about the extra credit method. This method is supposed
to ignore case and ignore some punctuation and whitespace to determine if a string is a palindrome.
A man, a plan, a canal, Panama.
– This is a palindrome if we ignore case of letters, spaces, commas and periods.
• The only additional method needed, besides what we learned, may be to call the toUpperCase() method in the Character class. No other methods in the String class besides .charAt are needed.
Michael Eckmann - Skidmore College - CS 106 - Fall 2006
Program 1• toUpperCase() method of the Character class takes in a character as an argument
and returns a characer. The character returned is:
– the uppercase version of that character if the argument was a lowercase letter
– the original character if the argument was not a lowercase letter• Examples:
– Character.toUpperCase('c') returns 'C'
– Character.toUpperCase('C') returns 'C'
– Character.toUpperCase('#') returns '#'
– Character.toUpperCase('\n') returns '\n'
– Character.toUpperCase('z') returns 'Z'
Object orientation
• Object orientation is a programming paradigm that views the pieces of large programs as objects with “attributes” and “behaviors”.
• Java is an object-oriented language. C++ is another widely used object-oriented language. Other languages with which you may be familiar, like C, Pascal, and Fortran, are not object-oriented. These are what are called procedural languages.
• So far in this course, we have used Java in a very procedural way and did not use it to its potential as an object-oriented language.
Object orientation
• An example of an object.• If we wanted to write a program that worked with
rectangles:– The attributes of a rectangle include its length and
width.
–Other attributes might be its color, ...
–Behaviors of the rectangle could include: changing its size, computing its area, etc...
Object orientation
• So, in our example, a rectangle would have as its data:– length
–width
– color
• As its methods we would have things like:– set_length
– set_width
– set_color
– calculate_area
Object orientation• This translates to creating a class called Rectangle
which has length, width and color as instance variables. This class will represent a generic rectangle.
• The methods in the class Rectangle would be:– set_length
– set_width
– set_color
– calculate_area
– among other possible methods
Partial class definition of Rectangle// note that this is an incomplete class definition to allow it to fit on one slide, // for instructional purposes
public class Rectangle{ private double length, width; // instance variables for the rectangle private String color; // another instance variable
public void set_length( double new_length) {
length = new_length; } public double calculate_area( ) {
return length * width; }}
Class Rectangle• The Rectangle class would be defined in its own file called
Rectangle.java• The Rectangle class represents what it means to be a rectangle
-- it means having a length width and color and it means that we can change it's length width and color. We can also calculate the area of a rectangle. If we wanted to be able to do more things to a Rectangle then we could add more methods such as calculating the perimeter of the rectangle.
Object of type Rectangle• An object of type Rectangle is ONE rectangle with a specific
length, width, and color. An object of type Rectangle must be created to use the data and methods in the Rectangle class.
• To do this, in some other class, we could instantiate an object of type Rectangle, by declaring a variable of type Rectangle (and calling it's constructor – we'll get to that shortly).
• Then, through this object reference, we would have access to the methods within the Rectangle class to set its length, set its width, compute its area, etc... for that particular instance of the class.
• We might want multiple variables of type rectangle, each with their own width, length and color. These would each be objects of type Rectangle.
Constructor methods
• A constructor method is a special method of a class that is called when an object of the class is instantiated (created.)
• Constructor methods usually initialize the instance variables.
• Constructor methods have the same name as the class and are defined WITHOUT a return type.
• A constructor for our class Rectangle should contain code that sets the values of length, width and color.
Constructor methods
• for example, we might have a constructor that looks like the following:
public Rectangle ( double len, double wid, String col ){
length = len;width = wid;color = col;
}• Recall that the instance variables were named length, width
and color that is why they are on the left side of the assignment statements --- they're the variables being set.
Constructor methods
• We might also want a default constructor (one that takes no arguments) so that we can create an object of type Rectangle without specifying its length, width and color.
public Rectangle ( ){
length = 1.0;width = 1.0;color = “Green”;
}• This says the default rectangle is 1 by 1 and Green.
Constructor methods
• Assuming that the two constructors just described are defined within the Rectangle class, to instantiate an object of type Rectangle we would do one of the following:
// this would create a rectangle with the specified values. Rectangle some_rect = new Rectangle ( 3.2, 5.1, “Red”);
// this would create a “Green” rectangle with length = 1 // and width = 1 Rectangle another_rect = new Rectangle ( );
example• Let’s create a program containing two classes
– Rectangle• let's add the code from the previous slides into
Rectangle and also provide a method that prints the length, width and color of the rectangle.
– RectangleTester• will contain the main method and instantiate a
couple of objects of type Rectangle with which we'll call the methods of Rectangle.
class / object / constructor terminology
• Programmer defined classes can be used as types, just like classes in the Java API can be used as types (e.g. String.)
• Objects have attributes (data) and behaviors (methods.)• The data are the instance variables in a class.• An object of a class is instantiated by declaring a variable of
that type and initializing the object by calling a constructor of that class. – e.g. Time1 time_object = new Time1( );
• here, we are creating an object (time_object), of a programmer defined class (Time1).
