cse 113 introduction to computer programming lecture slides for week 5 monday, september 26 th, 2011...

CSE 113Introduction to

Computer Programming

Lecture slides for Week 5

Monday, September 26th, 2011Instructor: Scott Settembre

University at Buffalo: CSE 113 Instructor: Scott Settembre

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COURSE ADMINISTRATIONSection 1

Monday, Sept. 26th, 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 229F (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 1-3 and plan to finish chapter 4 before your lab, then you are behind the rest of the class.

• Please do the following:– Complete chapter 1-4 in Bell 101.– If you cannot finish up chapter 4 by your lab, then:

• Finish chapters 1-3, AND THEN• Stay up late at night and finish chapter 4 before your lab.• You can DO it!



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Lecture and Lab this Week• Lectures will go over the following:

– Lab Quizzes for Week 3 and 4– Chapter 5 concepts and more formal definitions

• mathematical and boolean expressions• code abstraction• loops• local variables• arrays (or lists of objects)• strings (and string manipulation)• icons, jpg, gif, png file differences (not in book yet)

• 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 lab– Working on your Project 1



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QUIZ ANSWER AND DISCUSSIONSection 2



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Quiz Week 3 and 4 Review

• I will be going over the answers to the Lab Quizzes from week 3 and 4.

• I will answer any questions about the quizzes 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, Sept. 26th, 2011


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CHAPTER 5Section 3



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Mathematical and Boolean expressions

• You will find the need to add, subtract, multiply, divide, do modulus (find the remainder after a division) in arithmetic expressions.

• You will find the need to use the logical operators: AND, OR, and NOT, in boolean expressions.

• Parenthesis are used just like in real math, to ensure precedence.


University at Buffalo: EAS 230 Instructor: Scott Settembre

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Math Expressions

• These are sometimes called “arithmetic expressions” and they use the arithmetic operators:

Thursday, June 16th, 2011

Operator Action Example

+ addition a = 1 + 1; // a == 2

- subtraction b = 2 - 1; // b == 1

* multiplication c = 4 * 4; // c == 16

/ division d = 5 / 2; // if d is type int, then d == 2

% modulo† e = 5 / 2; // e == 1

† Modulo, sometimes referred to as “mod”, returns the remainder from an integer division.


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Code example : using math

private int Sum = 0;private int Average = 0;

Sum = 50 + 80 + 90 + 100 + 100 + 100;Average = Sum / 6;

// or we could have done

Average = (50 + 80 + 90 + 100 + 100 + 100) / 6;Monday, Sept. 26th, 2011

University at Buffalo: EAS 230 Instructor: Scott Settembre

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Parenthesis, “and”, “or”, “not”• Boolean expressions can also include parenthesis for

precedence (just like math) and the “AND”, “OR”, or “NOT” logical operators:– “AND” is “&&”– “OR” is “||”– “NOT” is “!”

• For example:– ( a == 10 && b == 10 ) // true if a and b == 10– ( c < 10 || c > 10 ) // true if c != 10– ( d < 10 && d > 10 ) // always false– ( !(a < b) ) // false, if a is less than b

Tuesday, June 11th, 2011


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Code example : using “&&” (and)private int a = 5;private int b = 10;private int c = 20;

if ( (a < b) && (b < c) ){

// then a is less than c}else{

// then a is not less than c}



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Abstraction

• Talked about abstraction in class– Examine what it means to abstract a class– Show how to use the constructor to make an

abstract class to create a specific object



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More on Wednesday

• Keep working on your projects.

• Finish up to and including chapter 4 by your lab section.

• I will talk more about chapter 5.



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What is “abstraction” for programs?

• Abstraction is a technique used to make code more general.– It makes code general, so that you do not need to

change a class to create something specific.– You will use an abstract class to instantiate a specific

object.

• We have done this before, by shift+left-clicking a class onto the world, but we did not specify how each object was specific.



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Abstract example


“Dog” class

These are all instances of “Dog”, but they are all the same!• They all use the same code.• They have the same values in their instance variables.• They all were created using the code “new Dog()”

new Dog()new Dog()new Dog()


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Abstract example : continued


“Dog” class

• What if we made a Dog constructor that took a parameter list?– This way, we can specify some information when we create a new

object.– Then if we created a new dog, we can specify its name.

new Dog(“Rex”)new Dog(“Woofy”)new Dog(“Fido”)


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What does this look like in code?

public class Dog extends Actor{

private String name;

public Dog(String newName){name = newName;}

}Monday, Sept. 26th, 2011


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Another abstraction example

• Let say we want to make an OceanWorld that has many different types of fish.

– However, we only want to make one java class that covers all fish.

– We do this because we don’t want to make hundreds of different classes to cover all the different types of fish there are.

– All fish swim, and we wouldn’t want to have to write hundreds of different “swim” methods!



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OceanWorld Fish class


public class Fish extends Actor{

public String color = “blue”;public int length = 10;public int speed = 2;

public Fish(){

// Any initializing code – like loading in a picture file}

public void Swim(){

move(speed);}

}


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So to create fish…

• In the OceanWorld constructor, we would see a lot of:

addObject ( new Fish(), 100, 200 );addObject ( new Fish(), 520, 150 );addObject ( new Fish(), 302, 63 );addObject ( new Fish(), 10, 250 );

• And they would all have length == 10, speed == 2, and color == “blue”. How boring…



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OceanWorld Fish class using Abstraction


public class Fish extends Actor{

public String color;public int length;public int speed;

public Fish(String theColor, int theLength, int theSpeed){

color = theColor;length = theLength;speed = theSpeed;// Any other initializing code – like loading in a picture file

}

public void Swim(){

move(speed); // note: This is “speed” from the instance variable, not “theSpeed”}

}


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So to create fish with abstraction…

• In the OceanWorld constructor, we would see a lot of:

addObject ( new Fish(“red”,6,2), 100, 200 );addObject ( new Fish(“blue”,8,5), 520, 150 );addObject ( new Fish(“white”,1,1), 302, 63 );addObject ( new Fish(“blue”,8,7), 10, 250 );

• And they would all have different lengths, colors, or speeds, proving that there really are many fish in the sea!



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Control code : Loops• Besides the if..else statement, loops are the other control code

you will need to learn.

• It allows you to communicate to the computer that it should run a block of code over and over again, until some condition is met.

• Beware! You can make a mistake and have a loop run FOREVER!– This is called an infinite loop and will lock up your machine.– Greenfoot handles this (so your machine will not really lock up), but

other languages (and running your program directly on Java on another machine) may lock it up.



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What is the “while” loop.

• The “while” loop is one form of loop you will get good at.• Here is what it looks like:

while ( some-condition-is-true ){

Do statement #1Do statement #2Do statement #3…Do statement #n

}Monday, Sept. 26th, 2011

Then loop back and check the condition again to see if we need to run the code block again.


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“while” loop example #1

int x = 0;while ( x < 5 ){

x = x + 1;}

// what is the value of x after the loop???// x == 5Monday, Sept. 26th, 2011


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int x = 0;while ( x == 5 ){

x = x + 1;}



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int x = 0;while ( x < 5 ){

x = x + 2;}



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int x = 0;while ( x <= 5 ){

x = x + 1;}



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What is a “local variable”?

• A local variable is a variable that stores data, however, this data (and the variable) goes away after the code block it is in is not being used.

• It also does not need to use the keywords “public” or “private”.

• You can easily set its value with the assignment operator “=“ when you declare it.



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Example of a local variablepublic void Swim(){

if ( !sleeping ){

int counter = 0; // “counter” is the local variablewhile (counter < speed){

move(); // move the fish one grid spacecounter = counter + 1; // increment the counter

} // What is the value of “counter” here?

}// What is the value of “counter” here? *trick question*

}



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Which are local variables and which are instance variables?

public class A extends Z {public int myVar1;private int myVar2;

public A(){

int myVar3 = 100;}

public void B( int myVar4){

int myVar5;}

}


Instance Variables

Local Variables

It is a parameter, but it is also a local variable


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More on Monday

• Keep working on your projects.

• Finish up chapters 1-4, and most of chapter 5.

• Arrays next!


cse 113 introduction to computer programming lecture slides for week 5 monday, september 26 th, 2011...

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