CSE 113 Introduction to Computer Programming Lecture slides for Week 6 Monday, October 3 rd, 2011 Instructor: Scott Settembre

CSE 113Introduction to

Computer Programming

Lecture slides for Week 6

Monday, October 3rd, 2011Instructor: Scott Settembre

University at Buffalo: CSE 113 Instructor: Scott Settembre

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COURSE ADMINISTRATIONSection 1

Monday, Oct. 3rd, 2011


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For Project Assistance• You can find the up-to-date hours and locations in the

“Contact” section of UBLearns.

• Here are the names, emails, and office hours as of 9/26/2011: (Also come to additional labs if you need help)


Name Email address Office hours

Bich (pronounced “Bic”) Vu [email protected] M (11-11:50am) Bell 329F (12-12:50pm) Bell 329

Troy Koss [email protected] Tues (3-5pm) Bell 329

Scott Settembre [email protected] M (10-11am) Bell 232F (10-11am) Bell 340Also at all times by email AND video conference by appointment.

mailto:[email protected]




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You are behind the class if…

• If you have not completed chapter 1-4, then you are behind the rest of the class.

• Please do the following:– Complete chapter 1-4 in Bell 101.– Chapter 5 must be finished by the end of this

week.



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Lecture and Lab this Week

• Lectures will go over the following:– Lab Quiz for Week 5– Chapter 5 concepts and more formal definitions

• arrays (or lists of objects)• strings (and string manipulation)• icons, jpg, gif, png file differences (not in book yet)

– Starting chapter 6.

• Lab will have you do the following:– Finish completely chapters 1-4 if you haven’t– Start on chapter 5, finish what you can’t before the next week– Work on your Project 1



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QUIZ ANSWER AND DISCUSSIONSection 2



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Quiz Week 5 Review

• I will be going over the answers to the Lab Quiz from week 5.

• I will answer any questions about the quiz you have.

• Each quiz is worth 2% of final grade (the quizzes may have a curve applied if appropriate)



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MIDTERM AND FINAL EXAM DISCUSSION

Section 3



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Midterm exam (15%)

• The midterm exam will be held in lecture on October 21st.

• It will cover chapters 1-6, including the exercises in the chapter that you do in lab.

• It will be a series of true/false, multiple choice, and code examination (running the code in your head), similar to the quizzes that you will have taken.



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Final Exam (25%)

• The final exam will be during class during the last week of class.

• It may span two days (I may give half on one day and half the next day).

• It will consist of questions like the quizzes, as well as some code reading and understanding.

• More on this in November.Monday, Oct. 3rd, 2011


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CHAPTER 5Section 3



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What is an “array”?

• “An array is an object that holds multiple variables. These can be accessed using an index.”

• What is an array???– It is just a numbered list.

• What is an index then???– It is the number that refers to an item on the list.



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array Example

int [] grades;

grades = { 80, 50, 100 };

// the index starts at 0, so…// grades[0] == 80// grades[1] == 50



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array Example 2

private String[] TA = { “Bich”, “Troy” };

// If it was then asked, who is TA[1]?// You then should answer…// “Troy”



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array Example 3

Cars[] obstacles;

obstacles[0] = new Car();obstacles[1] = new Car();

// But what is the computer really doing?



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Behind the “array”

• In general, the arrays you will be using will be arrays of objects.


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1

2

3

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String[] friends = { “Scott”, “Bich”, “Troy”, “Carmen”}

Computer memoryfriends array

“Scott”

“Troy”

“Bich”“Carmen”

[friends array]

???


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Things to know about arrays

• Arrays start with index 0.

• Each item in the array can also be called an element.

• Each item/element points to an object.

• Getting rid of (or clearing) an array does not necessarily get rid of the elements. The objects in the array may still exist.



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array Example 4

int [] grades = { 100, 80, 100, 50 };int x;

x = grades[1] + grades[2] + grades[3];

// What is the value of “x”// If you answered, x == 280, then you forgot that// grades[0] == 100!!!// An index of an array starts at 0!Monday, Oct. 3rd, 2011


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array Example 5

int [] ages = {18, 20, 22, 20, 20}int sum = 0;int average;int index = 0;

while (index <= 4) {sum = sum + ages[index];index = index + 1;

}average = sum / 5;Monday, Oct. 3rd, 2011

Note: The size of the array is 5 elements, but the last index is 4.


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array Example 6

// What is the final result of “sum”?int sum = 0;int [] paychecks = { 180, 190, 180, 230, 220 };int index = 0;

while (index < 4) {index = index + 2;sum = sum + paychecks[index];

}

// If you got sum == 400, you were right.


(index 2 + index 4)


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array Example 6 – with error// What is the final result of “sum”?int sum = 0;int [] paychecks = { 180, 190, 180, 230, 220 };int index = 0;

while (index <= 4) {index = index + 2;sum = sum + paychecks[index];

}

// This is an example of an out-of-bounds// error, trying to access an index of an array.Monday, Oct. 3rd, 2011

The error here is subtle, but this is exactly what trips students up for hours debugging. Note how index will be 4 IN the second loop, but 6 IN the third loop! This will be an error.


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Quick note on the String class

• A “String” is just another type of object.

• It is made up of all the characters between double quotes “”.

• Only tricky thing is that the backslash “\” cannot be used alone. It denotes an “escape” character and is used in various ways.



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Typical StringsString name = “Scott”;

String a = “is A”;String z = “zing!”;String mommy = “ma”;

// You can “concatinate” strings together …// in other words add the strings together.

String myInstructor;

// Homework: What is the value of myInstructor???myInstructor = name + a + mommy + z;Monday, Oct. 3rd, 2011


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Lab this week

• Please make sure you complete chapters 1-5 this week.

• Work on your projects.

• You will be shown how to submit your project.– Includes “zipping” up your project directory– Submitting it through the Assignments page on UBLearns– There is a document on UBLearns – Course Documents

section that shows you how to submitMonday, Oct. 3rd, 2011


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CHAPTER 1-5 SUPPLIMENTSection 4



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There are methods in crab-world that I cannot use in my project? Why?

• Let’s take a look at the crab world class hierarchy:


The code in here, comes with Greenfoot. You cannot see or edit this code, BUT you can subclass the class and use the methods from that class.


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Let’s take a look at the Actor class


OMG, do I have to know what to do with all this now???

No, instead let’s reuse another person’s work!!!


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So if I don’t use Actor, what can I use?

• Let’s look again at the crab-world hierarchy:


Interesting, maybe the Animal subclass has methods I could use and then don’t have to write myself!!!


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I don’t see the Animal classin the java docs?!

• The Animal class is not part of Greenfoot, it is just a subclass of Actor (which is a part of Greenfoot).

• It uses (inherits) methods from Actor, that may be more complicated to understand, and simplifies them for use.

• If you want to take advantage of this already written and debugged code, you have two choices:– You can cut/paste code from “Animal” into your classes.– Or you can sub-class the Animal class into a new class,

thereby inheriting all the hard work someone else has done!Monday, Oct. 3rd, 2011


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Let’s take a look at the Animal class


Oooh, now this is a useful method. I can detect if my “Frogger” hero is overlapping an obstacle. And I don’t need to write any code to use it, I can just inherit it!! Woot!


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ADDITIONAL MATERIALSection 5



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What picture file formats are we supposed to use?

• You can use any of the supported file formats: JPEG, PNG, GIF, BMP, or TIFF

• These should be placed in your project directory in a subdirectory called “images”



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What file type would you use?

• If you are using or creating your own files, you will want to use the PNG file format.– It allows transparency, so that you don’t have a

case of the “blockies”:

– but instead have this:



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How does PNG work then?

• GIF and PNG files use a “transparency color”.– One color value is designated as a transparent pixel.– That color value is usually a color that goes unused,

like magenta.

• The problem is that you may still get “jaggies” around your image, since the edge pixels do not blend with the background perfectly, but at least you won’t get a big white box around your image.
