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Programming Methodology-Lecture
Instructor (Mehran Sahami) :All righty. Let’s go ahead and get started. A couple of quick announcements before we start today – so hopefully you’re all busy working away on Karel and life is good. Just quick poll – how many people have actually finished Karel already? Oh, yeah. I won’t ask how many people have not yet downloaded Eclipse. There are no handouts today. Getting’ a little breather – no handouts. Don’t worry; you’ll get some more of that next time.
Sections start this week, so hopefully you all should have gotten an email about your section assignment and who your section leader is, so you can actually do Assignment No. 1, the email portion. You should have been able to do the programming portion the whole time. But make sure to go to section this week.
And the other thing is the Tresidder Layer, which every once in awhile you’ve heard me refer to. This is a computer cluster up in Tresidder. Is staffed by one of six helpers like almost continuously around the clock or most of the times at reasonable times when people are working.
So Sunday through Thursday, every day except Friday and Saturday because contrary to popular opinion, computer science people actually do have lives or we actually like to pretend we have lives, but every day from Sunday through Thursday 6:00 p.m. to midnight there will be a staff of helpers on there and actually some of the times there is like two or three or four people there.
And they’re there just dedicated for the 106 classes. They’re not like general consultants. They’re just there to help you work out problems in this class, and they know like what assignments you’re working on, the whole deal. They’re all like, your section leaders and they’re all be exceptionally trained to do this.
The other thing that’s going on, hopefully you should be doing Assignment No. 1. I’ve actually gotten a bunch of Assignment No. 1 emails that have already come in. In the early days, when I got the first few, I actually tried to respond to them all, but then at some point, I just woke up and I like, you know, went to my computer and was like, “Oh, you’ve got mail.’
And it just [inaudible]. So I couldn’t respond to everyone individually. I apologize if I don’t respond to you individually, but I do read them all. I guarantee you that I actually read them all and I look at backgrounds. And just to prove to you that I do, here’s some interesting ones that have come in so far – just to share three.
So there’s someone actually spent their time in Taiwan living in a Buddhist monastery, which I thought was interesting, except for the fact they were actually living there as a monk whom I thought was pretty interesting.
Someone else used a – I wasn’t quite sure on this concept, but maybe I can provide a clarification. There was a vegetarian who only eats low-quality meat, and so he mentioned that as things like burgers and not steaks. And I would qualify that by saying that’s not a vegetarian; that’s called being a grad student.
And last, but not least, there was actually someone in here who’s on the Colbert Report, which I thought was actually pretty interesting. I don’t know in what context, but come talk to me afterwards.
So with that said, any questions about anything before we start? Today we’re actually gonna go over some of the graphic stuff you saw, talk a little bit more about objects and classes and get into variables and values and all kinds of goodies. Any questions?
All righty, then let’s just dive right in. So one of the main topics for today is this thing called a variable. And a variable, you know, like variables come up in mathematics and it’s like, oh X and Y are variables, right and there are these things and we do all these manipulations on variables.
In the computer science world, they’re really friendly, right, and you don’t have to worry about integration or differentiation or you know, those kind of variables. Variables are kind of your friend, and basically all the variable is in the computer science worlds is it’s a box. It’s a box where we stick stuff and the stuff we stick into that box can change.
That’s why we call it a variable because it’s a box that has a variable contents, and you think back you know, in the days of yore in math, and you know, oh yeah, it’s kind of like X can have different values, yeah, it’s basically just like that.
So in computer science, what we think of as a variable, is each variable has three things associated with it. It’s got some name, and that’s just how we refer to that particular box. It has a type, which is something a little bit different in mathematics, but the type basically says what kind of thing does this box store? Some boxes store numbers; some boxes store letters; some boxes will store other things, like little objects in the world. But a type is just what’s stored in that box.
And then there’s a value, and the value, as you can imagine is just what’s in the box. What is the actual thing that’s in there, right? If it stores the number, then it might store, for example, the Value 3 and that’s just the value, and it may have some name associated with it.
And how do we actually name these? There’s actually a rule and it’s not a very complicated rule, but a very simple rule you need to remember for what are valid names for variables in Java. So a valid name, so this is how you actually name these puppies, has to start with a letter or an underscore. So it starts with a letter like one of the alphabetic letters and can be upper or lower case or the underscore character.
There’s this thing called a double and a double is actually some real valued numeric value, right. It’s something like 2.3 is a double or even 2.0 can be a double, okay? Why is this thing called a double – anyone know, as opposed to like a real? Uh-huh?
Student: [Inaudible].
Instructor (Mehran Sahami) :Yeah, there’s this wonderful verb people call the I triple E, which is like the Institute for Electrical and Electronic Engineers and they come up with all these standards for things. Is anyone a member of I Triple E here? No one? Oh, man, join and pay your dues. It’s a good time. I'm not actually a member myself.
But what those folks actually do is they come up with standards for things and one of the standards they came up with is how you represent numbers that are real valued numbers inside a computer, right, because remember a computer only understands ones and zeros, so how do you actually represent a real valued number.
And so there’s a standard, and part of that standard has to do with a precision of the number, how many digits in some sense and a rough approximation you store and double stands for a double precision real number.
And so for the purpose of this class, all real valued numbers that we’re gonna use are just of type double. Okay, there’s a couple of other types that I’ll just mention now and we’ll actually go into them in much more detail in a couple of future classes.
One is called Boolean. And if you’ve ever heard of Boolean logic, this is a logical value, so this is just essentially true or false, and we’ll talk about that in excruciating detail next time, but I’ll just let you know that there’s a type called Boolean.
There’s also a type called care or Car and as you can imagine it’s because we like sort of the first syllable of most things and this is the first syllable of character, okay, and so we’ll also talk about this in a couple of weeks time when we actually get into a some things with characters, but that’s just a character.
It’s a variable; it’s a box, but still is a character. It’s a box that stores an integer, box that stores a real value, box that stores true or false and a box that stores a character. Those would be the different types of them.
So one thing people – so that’s kind of types, at least some of the basic types. And then when you think about value that we actually store in this box, people always get uncomfortable when they see INTs and they see double, right, and they sort of say, “But Meron, like 2 is an integer, right?” And I'm like, “Yeah.” And they’re like, “But you just told me 2.0 is a double, right?” And I'm like, “Yeah, 2.0 is a double.”
And so the natural question is why? Why do we have both these things? How come like all integers aren’t subsumed by the real values, right, you’re kind of the mathematician type and you’re like, “Yeah, there’s like strictly more of these than there are of these. So
why are all these like subsumed in here. Why do we have this integer type,” okay? And the reason we do boils down to a simple question and the simple question you want to ask yourself is how much versus how many, okay?
So if you ask someone, let’s say you, ask me, just so I won’t embarrass you, “How much do you weigh, Meron?”
Student: How much do you weigh, Meron?”
Instructor (Mehran Sahami) :I weigh about, you know, 155.632 pounds, okay? That makes perfect sense, right. If I can put a decimal point, I can put as many numbers after it, or I could just say 156, right and those are both valid kinds of things.
Now you could ask me, “Hey Meron, how much children do you have?” I have 2. children. Does that make any sense to you? Yeah, it’s like we had three until that grisly accident. No, it’s just that there’s sometimes – I know that’s horrible to say. We actually have one, and he’s just fine.
There’s sometimes in the world when you care about counting, and when you care about counting, it doesn’t make sense to have fractional values. Those things are integers. They’re a how many kind of value. When you’re thinking about how much, that’s a double value, and you actually want to keep these distinct because if I ask you what’s the next number after 1, you say –
Student: 2.
Instructor (Mehran Sahami) :2 because you’re thinking integer, and that’s perfectly right and if I say what’s the next number after 1.0, you say –
Student: [Inaudible].
Instructor (Mehran Sahami) :Yeah, you start mumbling and you’re like, “Well, I know it’s not 2.0. That would have been the obvious answer and that’s probably not it. How about 1.0000001, and I'm like, “No you missed a couple of zeros in there. Just keep going. Wait until the end of the quarter and put that 1 and come back and talk to me.”
In real values, there is no next value, okay, so when you care about counting, it doesn’t make sense to use a double. That’s when you want to use an integer, so having these things be a distinct type actually make sense and you’ll kind of see that as we go along, okay.
So the value is basically just what you’re actually gonna put into this box over here and it’s gonna be some of the values that we actually talked about, so let me show you some examples of how we might use variables, what the syntax for variables actually is in Java.
there and this would create X with a value of 3 and Y with a value of 5.2 and you wouldn’t have the general form there, but for example, you could have those declarations inside run and what that means is you have those variables created with some values available to you inside this method called run.
They’re only available to you inside that method called run. They’re not available to you in some other method and we’ll talk about how you get values passed around between different methods in a few days, but for the time being when you have your variables they’re only available in that method. Okay, so any questions about that – variables or values? Uh-huh?
Student: If you have them inside the public run method [inaudible], will that method be able to take that variable?
Instructor (Mehran Sahami) :Well, you won’t be defining another method inside run, right; you’ll be defining another method that’s separate from run. It’s just a separate method. It won’t be available in that separate method. It doesn’t matter if it’s public or private. It just not available there. So this public or private thing that you have here doesn’t affect the visibility of your variables. Your variables are always only visible inside the method in which they’re declared and set up, okay?
Now, one other thing that we did when we sort of, you know, did this equals thing over here, if we wanted to, we could have actually done this in two lines rather than setting up X initial value there, we could have said into X and then we could have set X equals 3 over here and gotten the same effect.
And when we say X equals 3, this equals is not equals in mathematics; it’s actually what’s known as an assignment statement in Java or in most programming languages. And the idea is we’re taking some value over here. That’s what’s on the right-hand side of the equals and assigning it to whatever variable is listed over on the left-hand side of the equals, so we’re assigning that value, okay?
And so the general form for an assignment, let’s just do that over here, is pretty straightforward, you just saw it, is variable and this is the name of the variable. So I’ll squiggle – equals some value or some expression for a value and we’ll talk about expressions in just a second with a semi colon at the end of it, and that’s how you assign a value.
You can assign values multiple times to a variable, to a variable. It’s a variable, the contents change. So you say, “Hey, X equals ,” and then down here somewhere, “Hey X equals 4,” and when you get to this line up until you get to that line from here to here, X will have had the value 3 and when it gets to this line, it’ll stick a 4 in the box. And that’s perfectly fine because X is a variable. As long as everything you stick in there is of the type of which X is declared, you’re okay, okay?
So what that also means is when you’re doing Assignment you can do things that are perfectly fine in programming, but to mathematicians, they just go nuts; they go crazy like veins just burst in your head and you hear these popping noises and people dying in the streets, which is you can say, “Hey, I have some variable called total. Let me create some variable in its total,” which I’ll give some initial value like 10 and that’s a good time.
And then I say, “Hey total equals total plus 1.” And if you’re a mathematician, you look at that and say, “No, man. Total can’t equal total plus 1. That’s just not right.” And then you got this whole philosophical question about, you know, if this is like total is equal to infinity and you’re just like, “No, no, no, that’s not what we’re talking about.”
We’re talking about assignment. This is into equals, this is an assignment. So what it says is evaluate the right-hand side, okay? Total – what was total’s previous value. Go and look it up in the box over here you have some box for total. Its value was 10. It looks it up. It says, “Oh, that was 10.” I'm gonna add 1 to it, that gives me 11. What do I do with that 11? Stick it back in the box named Total. So it just says okay, and it puts an 11 over here and that’s perfectly fine.
So it’s perfectly fine to do something like then where you take the old value for a variable, you do some manipulation on it, you stick it back in the same variable. Okay? So with that said, now that you know about variables, we can go back to a couple of the programs that you saw last time and things ill now begin to make a little bit more sense, so if you come to the computer, remember our little friend, add two integers that you saw last time.
Suddenly the world opens up a little bit and we say, yeah, add two integers is a console program, right. So it’s gonna write some text out to a console somewhere and we have our run method. We print out a line; we know that the method print [inaudible] when we say that, whatever we put inside the double quotes gets printed to the screen so it prints out this program adds two numbers, so we execute that line and after we execute it, it writes it out and it comes to the next line.
Low and behold, what have we done here? We’ve declared a variable named N1. What’s the value for that variable that we’re gonna assign to it? We’re going to call some method. The method we’re gonna call is something called Read INT and this is a method that’s provided for you in console programs. All console programs can do this. The way it works is you give it some text between two double quotes. It writes that text on the screen just like you saw last time and asks for input from the user.
Whatever value the user types in and hits enter, that number is the value that gets what we call return by read INT. It’s the value that [inaudible]. It’s kind of giving it back to you. Do what you want. What are you gonna do with it? You’re gonna assign it to the Box N1, so here’s the box for N1.
So here are some of the objects that you considered last time, right? We had things called, like some classes. You have the G label class and the G –rect class and the G-oval class. These are all these graphics classes like a label [inaudible] or rectangle or an oval. I’ll show you examples of all these things this time.
And these guys were actually all classes that are part of a hierarchy of other classes and the other classes, they’re all part of is G object? So what G-object is a G-object is a class. Don’t let the name fool you. Some people see G-object and they think, “Oh, that must be an object.” No, G-object is a class. It just means a graphics object. And guess what, a rectangle or an oval or a line or a label or all different graphics objects. So what that actually looks like in terms of the hierarchy just like you saw last time, is the classes are represented, kind of like this in the hierarchy, all of these classes are actually G-objects. Okay, they’re all sub classes of G-objects. It means they’re specializations. So any behavior that G-object has, all of these things have, but they might have their own specialized behavior, as well.
Now, the interesting thing about all this is that when you have classes and you create objects, you can store those objects in a variable. You can say, “Give me a variable that is a box,” and the type that that box holds is an object that’s the type of some class, so all classes can actually be used as type means, okay?
Now, that’s kind of funky. Let me show you an example, okay? Here is another program you saw last time. Now, we can just kind of reveal the mystery about variables. We have this program, Hello program that extends the graphics program, right, so it’s gonna draw us some pictures.
Well what are we actually doing on this first line? Hey, we’re doing a declaration and assignments on the first line. What’s the type of our variable? It’s a G-label. So we use the class name as the type. The name is label, so we have a box called label and what that box is gonna hold is any object of the class G-label. Okay, so that’s the type. It’s gonna hold any object.
And the question comes up, how do we get an object of type G label. And here’s the funky part. The way we get an object of type G label is we have to ask for a new G-label.
So we give the name of the class here and class is depending on the class, you know, see some examples of this will take in or give in some initial values for what we refer to as parameters, and these parameters are all separated by commas. But at some initial values, that this object uses to sort of set itself up. It initializes the objects.
So objects rather than just taking one value, can potentially hold multiple values in them. In this case, the G-label is gonna have Hello World as some text that it’s gonna display in some location that’s 100,75.
So we say, “Hey G-label, you’re the class. I don’t have an object of you yet. You’re the class.” When I say, “Give me a new G label, it’s sort of like going to the factor. You sort
of show up at the G-label factory and you say, “Yo, G-label factory. Give me a new object,” and G-label factory is sort of sitting up there and says, “Well, what kind of new object do you want? Yeah, I can give you a new G-label, but how is that initial G label going to look,” and you’re providing these parameters that sort of specify what the initial G-label looks like.
So this line is now created a G-label, okay? And it has some initial, so when we execute that line, it has some initial text that it’s gonna display and somewhere it’s also storing the value is 175 because it knows where that’s gonna be on the screen.
Now, what we’re gonna do is we’re going to tell that object to do some additional stuff. The way we tell an object to do something is we specify the name of the variable for the object, okay. So the object name is label. That’s the name of the variable. And so that’s what we say here. We don’t say G-label. We say the name of the variable label.
Then we have a dot; then we have the name of the method that we’re going to call for that particular object. So it’s a little bit different than Karel’s world, all right. In Karel’s world, every time you call the method, Karel was the one executing that particular method.
Here when you have multiple objects, you need to specify which object you want to execute this method. So you say, “Hey, label, yeah, I'm referring to you, buddy. Set your fonts to be this particular font, san seraph which you saw last time.” It just makes it big. It makes it 36-point font. And then you say, “Hey label, yeah, you, same guy, or gal or it or whatever it may be. Set your color to be red.”
And it says, “Okay, I’ll set my color to be red, and then finally if I want to take this label and stick it up onto my graphics canvas which is actually the program I'm looking at. Right, I’ve done all this stuff and my program’s still bringing out a blank screen. It’s like here you go, nothing going on here. I say add and the name of the object that I want to add, right because now I'm holding this little object called label.
This is my little felt Hello World, and I want to say, “Hey take the felt hello world that’s so cute and just stick it in the world,” so we add it to the graphics program, basically and it shows up in the screen where it knows it’s supposed to show up at a 100,75.
So the general form for this, the people will refer to when you see something like this, you have name of the variable and the method name and the Java-esc names that we give to these, just so we hear other programmers refer to them, you know what they’re talking about, is they say the object is the receiver. So the object here is the one who’s receiving or being told to do something, okay?
So the name of the variable is the receiver and the method that’s being called is called the message and so there’s this whole notion of sending messages, kind of like you could think of programming as I am, right, and in your little I am you have like your friend, Bob, and your friend, Sally and your friend, Dave, and your friend, Label, right. And
left hand corner of the H, okay? And so that’s 175. That’s how things were laid out. Uh- huh
Student: Is it possibly to specify [inaudible] using ratios?
Instructor (Mehran Sahami) :Well, everything’s in terms of pixels. There’s some stuff you’ll see later on in the class where we actually specify some things like in polar coordinates if you’re familiar with polar coordinates, you’ll get to that later on.
But you want to think of these numbers as just being absolute pixels and you can do some math to figure out where those pixels should actually be. But they’re still just pixels, just way off.
All right, so remember all of these classes are organized in some hierarchy and everything that we talked about, right, G-label, G rect, G-oval and G-line are all G- objects, which means any behavior that a G-object has, all of these puppies have. So let’s start off by thinking about what are the behaviors or methods that are available on G- objects.
So one is set color; you just saw an example of that, right, you specified the object, you say set color and you give it some color and I’ll show you where the colors come from and it sets the color of the specified object to the color you just specified as this thing called a parameter. So the thing that’s inside the parentheses in our formal speak, we refer to as parameter. So methods have parameters. They’re the values that you provide inside the parentheses when you invoke a particular method, okay?
So there’s a single parameter there, color. There’s also something there called set location, which takes an X and Y coordinates. Again X and Y are in pixels and it sets the location of that object to that XY, so this method has two parameters, X and Y. And you might say, “But Meron, when I created the G-label before, didn’t I specify the X and Y location when I told the factory to give me one of these G-label objects?”
Yeah, you did, but you didn’t have to, and it turns out there’s times you don’t want to; you want to just say, “Hey give me a label that’s got the words Hello World in it and later on I’ll figure out where it’s actually gonna go on the screen, so I’ll specify what the X and Y are,” but until you specify the X and Y, it doesn’t know, but you can specify after you’ve created the object, is the important thing.
And move, and here’s the funky thing. You can actually tell an object, like you have your little, you know, Hello World somewhere on the screen and it’s kind of like the furniture idea of objects. You’re like, “Yeah, I don’t like how it looks there. Move it a little to the right. And so there’s a move method and its coordinates if you’re sort of a Calculus person or DX and DY, and if you’re not a Calculus person, don’t worry, you don’t need to be a Calculus. This is the closest we get to Calculus in this class. Rock on.
DX and DY just means how many pixels in the X direction and the Y direction should you move this object from its previous location and these can be positive or negative, because you can actually have something negative say in the Y direction and it’ll move it up on the screen.
So it’s just the offset or how much you want to, so think of it as the difference in X and the difference in Y, how much do you want to change where it moved. Uh-huh, question?
Student: Do you use a set location XY and then [inaudible]?
Instructor (Mehran Sahami) :Can you do set location and so you –
Student: [Inaudible] later define X and Y.
Instructor (Mehran Sahami) :No, if you want to say set location XY, you’re referring to two variables X and Y, so there’s variables X and Y and each have already been declared and have values.
Student: [Inaudible].
Instructor (Mehran Sahami) :Yeah, so anything that’s a parameter here, you can actually use a variable rather than an actual value, like an actual number, and we’ll see some examples of that as we go along. [Inaudible]. So where do all these colors come from? It turns out these colors come from a standard library that someone in the Java world wrote that’s called the Java AWT package, so if you want to use these colors at the top of the program, and I’ll show you and example, that should say, importJava.AWT.star and these are all the colors, so they’re all the name color, dot and then what the color actually is.
So you just saw an example where we used colored dot red to make Hello World red, but there’s all these different colors and they’re all in your book, so you don’t need to scribble them down hurriedly, but you know, different shades or black or grey or white and you know, magenta and sienna if you’re sort of a color photography sort of person, but there’s a whole set of colors and you can just go to town on them. Okay.
So all G objects, all of these things respond to these methods because these guys are all objects, so any of those three methods will work on any of these objects, of objects of any of these types. But there are some additional behaviors, there are some additional methods that some of the specialized classes, the sub classes actually implement.
So G-label has some additional things that it does beyond just what a G-Object in general would do. And the big one, well first of all, it’s got what we refer to as the constructor. You can think of the constructor and I'm gonna do a little violence to the term, but you can think of the constructor as a factory, okay? What the constructor says is, “Hey, I want a new one of these guys,” so I use the word new and then I specify the constructor, okay?
Okay. So that’s the basic idea is upper left-hand corner and then width and height of the rectangle.
Similarly, for oval – ovals are kind of funky because you look at this and you’re like, “Hey I have an X and Y and I have a height. I didn’t like –width and height like ovals. I thought an oval was defined by like two [inaudible] and that whole thing where you have a string. Did you ever do that – you stick the two like nails in the board and you put the string around it and you draw anyone? There’s like two people – yeah, like sorry.
Do it; go get a piece of wood, stick two nails in it, put some string around it and go get a pencil, something you can draw ovals and you can draw a 1 and if you’re like 6 years old, you’ll draw like 1,000, and if you’re 18, and draw one, you’re like, “Yeah, that was just stupid.”
But the basic idea behind an oval is the reason why we specify this way is you specify sort of an imaginary rectangle for the oval and the oval shows up as an oval that just happens to touch the sides of the rectangle, which is how we sort of specify how wide and broad it is.
So think imaginary rectangle and sort of the four ends of the oval would sort of be touching it, and I’ll show you that momentarily. G line is very straightforward. G-line, the way we specify line just like you know, the days of yore with Euclid, line is defined by two points, right, that’s an infinite line, but we can think of where it ends at the two points. So we specify an initial XY location and an ending XY location and it will just draw a line between those two points, pretty straightforward. Uh-huh?
Student: [Inaudible] is the set location defined as the lower left corner of your object, and the G-rectangle [inaudible] left corner?
Instructor (Mehran Sahami) :Well, for set location is the lower left corner of for textural objects, and then it becomes different for rectangles and ovals and stuff like that, yeah. So it’s a slight variation because that’s just because we’re trying to deal with, you know, texts and rectangles symmetrically and it’s tough to do, okay.
There’s also some additional methods that are just shared by G rect and G-oval. They don’t actually apply to lines or to labels for that sense, rectangles and ovals can be filled, so they can either be an outline. You can either have a rectangle that looks just like this which is an outline or it can be filled in which means it’s just a big solid chunk of stuff. So you said if it’s filled, you say this is either true or false with the actual word true or false.
So if you’ve set it to true, then when you draw it on there, you say, “Hey, put this thing up on my canvas.” By adding it to the canvas, it will show up filled in; it will show up solid. And if this is set to false, it just shows up as the outline of the object, whether or not that’s an oval or a rectangle.
And almost done here. There’s also a notion of a set fill color, and you might be wondering, you’re like, “But Meron, you told me that set color. Here’s where things get a little funky, you told me that set color is something that was defined for a G-object, so if set color is defined over here, doesn’t that mean that a G-rect and a G oval already can set that color?”
Yes, that’s true. “So what’s the set fill color all about?” And this is a slight variation. When you set color for something, you set the color for the whole thing if that’s solid or if it’s an outline, you’re setting the color for the whole thing.
When you set fill color, you are just setting the color for this internal portion. So imagine like this was your rectangle, what color used to fill it in can actually be different than what color the line is. So if you want the line, say, to be black and you want the fill to be red, you can actually do that by setting the fill color to be red and then the outline is still whatever color the actual outline was if you want to do that.
Okay, so if the color – if the interior is fill, so the set fill has to be true, then you can set the fill color to be different. That’s a minor thing, but it’s kind of fun if you’re drawing stuff, okay? So any questions about that? Oh, wait – we have a question over there, uh- huh.
Student: [Inaudible].
Instructor (Mehran Sahami) :Oh, in the place of this word fill?
Student: Yeah.
Instructor (Mehran Sahami) :It’s either the word true or the word false. So if you say true, set filled is true which means it will be a solid thing; it will be filled in. Uh-huh?
Student: If your when you’re using the [inaudible], you specify a [inaudible], what is the why would you use the set location [inaudible].
Instructor (Mehran Sahami) :Maybe you wanna do some animation. Say like you have like your bunny in the world and he’s gonna want to move along.
Student: [Inaudible].
Instructor (Mehran Sahami) :Well, it’s set somewhere to begin with and then later on, guess what you’re actually gonna be doing a program that involves animation. Rock on. And you will move things around. So that’s where it comes from. All righty.
So one last thing that might be useful to you, sometimes you want to say, center something in the graphics window or draw something in the bottom, how do you know how big the graphics window actually is.
knew how that got there. This should all be familiar to you now. We got a new G-label. We had some variable called label that we assigned it in. We set its font. We set its color to red. Because we set its color to red, that’s why we need www.Java.awt because we’re using colors.
We set its color to red and we said, “Hey, I got Hello World, throw it up on the screen,” so we add label. You give it the name of the variable and there it appears.
Over here we say, “Hey G-rect. I want a new rectangle that’s upper left-hand coordinate is 10,10 and it’s size is gonna be 50,50, which means it is a square. It’s not filled in. I haven’t specified its color right,” so if you don’t say set fill, it’s not automatically filled in. If you don’t specify the color to automatically black and then I add it and then you get that little box all the way up in the upper left-hand corner.
Then, I say, “Hey, you know what, I want some other G-rect because it’s cooler.” It’s gonna have colors and it’s gonna be bigger and better and it’s gonna be nationwide, and so what it’s gonna do is I'm gonna have rect 2 which is another variable of Type G-rect.
It’s perfectly fine for me to have multiple objects of the same type which they knew G- rect that’s upper left-hand coordinates is 300,75 and whose size is honking – it’s 200 by 100, so it’s big long thing, and it’s gonna be filled in and I'm gonna set its fill color to be red, so the whole thing is red, throw it up there and so what I get is that big, rectangle. Just ignore the big oval one in front of it for now; I get this big red rectangle up there.
And I say, “Well, that’s kind of cool, but I want to see what this oval thing is all about. So Hey, Meron, make me an oval.” And I'm like, “All right, well, G-oval will make you an oval,” and the dimensions of the oval, its upper left-hand coordinate and its size are exactly the same as the rectangle, and the reason for doing that is to show you the oval in relation to the rectangle that you specified.
It’s set filled is true and it’s set fill color, not its set color, but its set fill color is set to be green, which means the internal dividend will be green, the outline of the oval will still be black, because we did not set its color and then we added and there you get that green oval and you can notice its four ends touch the same four ends as this rectangle and if you look real closely there’s actually a green line that demarcates the oval and then the middle fill is actually green.
It might be a little hard to see, but that’s the case. Then we also have a line, so I will just call this my funky line, and my funky line is a line that starts at 100,150 and it extends to the location 200,175 and there is my funky line. Yeah, it’s a funky line.
All right, and then I add another thing and you’re like, “Dude where’s my line.” And so Dude where is my line is a line that should cut across the entire graphics window because it starts at 00, well not the entire graphics window. It starts at 00 in the upper left-hand corner and goes to 100,100, and this is a common error which is why I how it to you.
You look up there and you’re like, “But Meron, there’s only one line,” and that’s your funky line or my funky line as the case may be. Where is Dude where is my line? And it’s not up there. Why is it not up there?
Student: [Inaudible].
Instructor (Mehran Sahami) :Yeah. You did not add it to the graphics content, okay. Common error – people will go ahead and create something and add all the colors and set all the sides and be like, “Ah, I'm rocking,” and then they’ll run their program and nothing shows up and they start tearing their hair out and beating their section leader and it’s a bad time, and they’re sitting there contemplating life in jail and then they realize, “I just should have added it to the canvas, all right.”
But you can’t give that as an excuse to the judge. All right. So any questions about that? All right. Let me very quickly – I need to push on real quickly. So I want to tell you a little bit about expressions before we finish up and we’ll do more expressions, but you need to know some basic stuff on expressions.
So all an expression is is you already saw one. And expression is something like INT total, equals N1 plus N2. This is an expression. It’s just some mathematical expression that tells us how things kind of relate to each other.
And basically all an expression is it’s a bunch of terms and these terms can be either individual variables or they can be constant values, like they ca be like the Number 2 or 3 or they could be some method called like Read INT that actually gets a value from the user, for example, and there’s some operators between them, like the addition operator, and so when you take some terms and you put operators between them to combine the terms that’s called an expression, all right.
Fairly straightforward stuff – you’ve probably seen these. There’s a bunch of operators you should know about. The operators are things like plus, minus, times, which is actually a star or asterisk, which is how we refer to times. It’s not as X – division and then strangely enough, this thing’s that looks like percent and it’s not the percentage operator, although you might think that. This is what’s known as the remainder operator, okay.
So these puppies generally work like you would think, add two numbers, subtract two numbers. There’s also a uniary minus. The uniary minus is the funky mathematical way of saying negative sign. So you could actually say like equals negative N1 plus N2. That’s like a negative minus. That’s perfectly fine. Okay, or you can use that same thing, you know, as subtraction if it’s between two terms.
Multiplication, division, the thing about division that is funky is it depends on how division is used, okay and I’ll tell you a little bit more about that next time, but I want to talk about remainder right now.
