CS 206 Introduction to Computer Science II

Transcription

1 CS 206 Introduction to Computer Science II 01 / 21 / 2015 Instructor: Michael Eckmann

2 Today s Topics Introduction of myself Review the syllabus List of Topics we'll cover Start Java review

3 Who is your instructor? I'm Mike Eckmann and this is my eleventh year at Skidmore. Before that I was at Lehigh University in PA. I studied Mathematics and Computer Engineering and Computer Science all at Lehigh University. I was employed as a programmer (systems analyst) for eight years.

4 Syllabus Office hours Text book Classrooms Assignments Hw's & programs Collaboration policy Grading schema Attendance Readings The most up-to-date syllabus will be found on the course web page.

5 Syllabus course topics The course is mainly a course in data structures and algorithms that work efficiently on the data structures. Topics- Java review Quick intro to Algorithm Analysis Big O Linked Lists Trees General trees Binary trees Traversals

6 Syllabus course topics Topics continued Binary Search trees Recursion Dynamic Programming Divide and Conquer Merge Sort Stacks Queues Priority Queues Heaps

7 Syllabus course topics Topics continued Graphs Breadth First Search Depth First Search Dijkstra's Algorithm Hash functions and tables Balanced binary search trees AVL B-trees

8 Suggestions on how to succeed in this (or any? CS) course Keep up on the material before the class period that we cover it. Participate in class -- ask questions in class (best), via or in my office. Start the programs as soon as they are assigned. You will run into snags. Debugging can take a long, long time. These are based on my observations of successful students in past classes.

9 Java review What is a variable? How to declare a variable --- what does declare mean? How to read a value of a variable? How to change the value of a variable? What is a type? What effect does the type have on a variable? What are all the primitive types? Other types? Can we create our own types in Java?

10 Java review What is required in every Java program? Where does a program start its execution? Where does it end?

11 Java review A program's statements will execute sequentially, starting in the main method. One statement after another will execute (read top to bottom) unless the program flow is somehow altered. Name some ways the program flow can change from sequentially proceeding from top to bottom within the main method?

12 Java review If / else statements Loops (for, while, do while) Method calls

13 Java review Let's write a program that asks the user how many numbers he/she will enter and then prompts the user to enter that many numbers and then: prints out the list of those numbers prints out the list of those numbers sorted from low to high prints the average of the numbers

14 Java review - References A reference variable (aka reference) is a variable that stores the memory address at which an object lives. References are variables of class types and arrays. Differentiate references from variable of the eight primitive types. null is the value a reference has when it does not refer to an object.

15 Java review - References Assignment operator = e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs c1 = c3; // assign reference c3 to c1 When a reference is assigned to another reference what happens?

16 Java review - References Assignment operator = e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs c1 = c3; // assign reference c3 to c1 When a reference is assigned to another reference what happens? Both references refer to the SAME object. That is, the memory address is copied. c1 and c3 are aliases for the same object.

17 Java review - References Equality comparison operator == e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs if (c2 == c3) System.out.println( c2 and c3 are the same card ); When reference variables are compared for equality with ==, what happens?

18 Java review - References Equality comparison operator == e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs if (c2 == c3) // if c2 and c3 are aliases System.out.println( c2 and c3 are the same card ); When reference variables are compared for equality with ==, what happens? If both references refer to the SAME object, then it evaluates to true. That is, if the memory addresses are the same, (if c2 and c3 are aliases for the same object) then true is evaluated. So will the above if condition be true?

19 Java review - References Not equal comparison operator!= e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs if (c2!= c3) // if c2 and c3 are not aliases System.out.println( c2 and c3 are not the same ); When reference variables are compared with!=, what happens? If both references refer to the SAME object, then it evaluates to false. That is, if the memory addresses are the same, (if c2 and c3 are aliases for the same object) then false is evaluated. So will the above if condition be false?

20 Java review - References What if we do the following: e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2; System.out.println( The card is + c1.tostring()); System.out.println( Another card is + c2.tostring());

21 Java review - References What if we do the following: e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2; System.out.println( The card is + c1.tostring()); System.out.println( Another card is + c2.tostring()); Problem with c2.tostring() because c2 DOES NOT refer to an object. Its value is null.

22 Java review - References Card c1 = new Card(5,0); // Five of Diamonds Card c2; System.out.println( The card is + c1.tostring()); System.out.println( Another card is + c2.tostring()); Either c2 needs to be assigned a non-null value from another reference c2 = c1; Or it should be assigned to a newly instantiated object. c2 = new Card(10, 3); // Ten of Spades

23 Java review - comparing Comparing variables/data of primitive types vs. comparing references of class types. ==, <, >, <=, >=,!= vs..equals( ) and.compareto( ) What do these methods return? How do we use them?