>> All right, welcome back to CS-50. This is the start of week 1, so it's this week that we dive into actual programming with a modern programming language and begin to tease apart some of the basic building blocks that frankly will carry us through the whole semester. More on that in a moment as to what that means, but quickly one, you do have a printout today of all the source code that we'll use today and also on Friday. You'll find that they lines are numbered, so if you've ever heard of a language called basic that's actually a language from yesteryear where you actually as the human how to type line 10, line 20, line 30, and then you wrote your code to the right of this. So that was kind of a humanities clever approach to leaving room in case in the future you wanted to add some more lines of code. You could put it between lines 11 and 19, didn't really scale particularly well. These are just printouts, line numbers, so you won't be writing those numbers there but I'll often refer to them. Office hours have been in progress. If you would like to come hang out in either the Quad or Quincy Hall dining hall this evening or next evening, do drop by. The teaching fellows will be there with the CAs to just chit chat about the course, help you with your scratch projects or just generally hang out and spend the evening in more of a communal environment. And then sections, we will have the sectioning process officially begin tonight around midnight, so check CS50.net's home page and we will have an announcement there as to how you section. But so that we have some breathing room and we don't start off the semester on too rushed a foot, what we're going to do is not start in intimate sections where 12 to 15 students and one CF this coming week, we're going to start those in two weeks time and this first week, so this coming Monday Tuesday Wednesday we're going to have super sections led by pairs of TFs. There's just going to be larger sections for the first week that you're welcome to attend any of those. The locations will be announced, so go to CS50.net/sections to see the current Google calendar for that. And then finally we do have a bunch of sensor boards if you would like to take on the hacker edition of PSET-0, but do realize that will always be the case we expect, we encourage almost everyone in the course to pursue the standard edition, so by no means should you feel like you are falling behind or cutting corners by just doing the standard edition that's meant for most folks. So with that said, we spent and a number of you have spent already some time playing with scratch and we actually have kept an eye, partly just out of curiosity, on what kinds of submissions have been coming in already. Now, not to worry if you think it's a little premature for -- little bug sometimes, the mouse stops responding so see if the screen starts responding again. There we go. All right. So I went ahead and dug up this so if only for curiosity's sake, only about 10 percent of the class has submitted already. So here too don't worry that you're already behind given how quickly some folks have started submitting. About 30 people submitted. We found this is slightly statistically interesting. So as might be expected, most of you, many of you are running Windows. What was kind of interesting was that neon of you have gone out and got and installed this brand new shiny operating system by Apple, Snow Leopard, and we'll see if some of those numbers change. Or maybe someone back there, so we'll see when those numbers come in. Some of these questions you'll find in PSET-0 survey are really just for statistical purposes. We'll try to do some fun things with the data throughout the semester. We don't fundamentally need to know what city you were born in, but it's kind of interesting if we can later visualize that with something like a Google map. So don't be too thrown off by those. If you're worried that everyone in this room has an i-Phone, very much not the case. Most of you have, or most of you that have submitted already have a normal mobile phone, which is perfect because it actually means the time Willy Chow CA and myself has spent on shuttle boy voice of late will actually get used. It's kind of a retro approach to making an application using touchtone prompt, but indeed if you call 617-BUG-CS50 that service is now live. You can get out of here quickly to the Quad or to matter after class where it operates 24/7 as well. And coming soon are going to be a whole bunch of other features. So right now it's just option 1 for shuttle boy, but coming soon will be an option for wakeup calls, which we suspect will be kind of useful around mid-term time, exam time. We're mildly nervous because I think this could backfire completely if we only discover bugs the morning you have midterms but we'll try to avoid that. Coming soon too will be dining hall menus. If you want to find out what is for lunch today, what is for dinner today, thanks to a former teaching fellow, Kato Yuchiama [assumed spelling], we'll have an option online there as well, and then soon you'll be able to find out when the next CS-50 office hours actually are. Well, people tend to like where they live. This is a little slide that gives you a sense of where PSETs have been submitted from. Freshmen are kind of slacking it seems thus far, although Holeworthy and Matthews seem to have some decent contingencies. And then here among the houses what's striking is that the only house that is apparently doing other things this week has been Leverett, which ironically is one of the largest houses and last year the data was completely the opposite. We had more students from Leverett house than any so perhaps we're now paying that price. So just random fun facts here from CS-50. So let's now dive in. So this week and beyond is largely about this language called C and when you start playing with C you're playing with a real, a traditional programming language. so PSET 1, which will be realized this Friday on the course's website, will have you tackle something that may be unfamiliar to many of you, this operating system called Linux [assumed spelling], and also this language called C and we'll spend much of the semester on C. What's fun I think particularly about this language is that unlike Java and unlike scripting languages that some of you might know, PHP, Python, buzzwords like this, it's really low level. It's just shy of something called assembly language, which we'll glance at later in the semester, and by that I mean it really gives you find-grained control over where stuff is going in memory, like where in RAM are you actually using strings and numbers and databases and stuff like that. And it allows you at the same time to do really bad things. So many of these exploits that you read about if you read these kinds of publications or websites that involve websites being hacked over servers being taken over by hackers and so forth, to this day a lot of those compromises are either the result of just stupid mistakes by humans or sometimes more subtle mistakes made because the website or the program, whatever it is, was written in C or C++ or some related languages as well because with this power comes as they say great responsibility. You can do a lot of damage to your own data, to your own security by coding improperly and not being mindful of certain security issues. And what's actually fun, I think, as a result, about talking about this language for the next several weeks is you really understand just some societal issues like that, like why did this website get hacked? How could someone possibly do that and then what are more intelligent solutions to some of the problems we ourselves will see. So scratch meets C. so hopefully, even if you haven't sat down to do PSET 0 yet, which is fine, still plenty of time, you hopefully got a sense last week of what some of the basic building blocks are of programming. There's like a statement, do this, whether it's say something, think something, move somewhere. There's conditions. Do this if the following is true. There's variables if you need to keep around a piece of data, and then we saw some slightly more sophisticated topics like threads, multiples scripts and events, this ability to send messages from one script to another while all these same building blocks exist in C as they do in most languages. And even though what you're going to start writing for PSET 1 onward is going to look more like the below and less like the top, it's really the same ideas. And if you take one important point away from today, realize that you will, when you first sit down to program, maybe for the very first time and most likely with C for the first time, you will make some stupid mistake and you'll stress over some meaningless detail like why is my code not working, why is not compiling, and odds are it's going to boil down to something stupid like see that semicolon on line 4 there, like you'll accidentally have omitted that or you will have left off a single quote somewhere or double quote. And frankly this is one of the reasons we start off the course just last week with scratch, right? You don't have to worry about this uninteresting minutiae but realize now that we're going to be programming real computers and we have to be ever more precise and it's this level of precision, this level of nitpickiness that actually does matter. Because you'll find that a computer will do literally what you tell it to do, and if you don't say semicolon there in this particular language, you're not telling the computer that this is the end of the statement, and you're making things ambiguous. So ambiguity tends to be bad in programs as we will see. So what are these things going to start looking like today? Well, a quick teaser, a statement that last week looked like this, oh, hi world, with the little purple block and C onward is going to look a little more cryptic but at the tense of the day the same exact idea, Print F, which means a formatted printing mechanism, Ohi kind of a world and then backslash N, I think we said last week, what does that mean? New line. So you have to be even ever so explicit about where you want the enter key to be hit and /N represents the act of hitting enter on the keyboard. So that's what the statement's going to start looking like shortly, a bouillon expression, so this was something that could evaluate to true or false 1 or 0 on or off, however you want to view the world. Well now what used to look like orange and green up top there is instead going to look a little more high school math, but parenthesis X less than Y, closed parenthesis. And then if you want to join these things together we're going to see things like ampersand ampersand, just happens to be the nomenclature in C for the bouillon idea of and. And the notion of or, if you want to say this or this, it's not going to be or it's going to be a vertical bar a vertical bar. And it's stupid things like this too, where we'll inevitably get an FAQ this semester like where is the vertical bar on my keyboard? This is not a character many of you usually use, it's usually shift and above the enter key on the typical keyboard that would vary. So again don't get hung up early in the semester on stupid details like this because they are stupid and they're intellectually uninteresting so just accept the fact that this is what at least initially some languages are like. Well, what about more interesting constructs, conditions. If X is less than Y last week we wanted to say something. Alt we wanted to say something else. Well, what looks like this nested puzzle piece construction at left this week onward is going to look like the stuff at right. And again it's getting a little messier but it's still basic building blocks, if space open parenthesis, X less than Y close parentheses, curly braces, essentially represent what scratch does graphically by embracing puzzle pieces similarly to embrace chunks or lines of code with these curly braces. Print up is again formatted print. We've got some familiar quotes now, backslash and semicolon. So already this is old hat. Else if now we have a second branch here, else, a third branch, and notice again the symmetry throughout this code. If we open a curly brace we close it and things seem to be done symmetrically. So that will become important as well. So loops, we saw things like forever and repeat some number of times. Well, these two look a little weird at first glance but at the top right there we have while 1 and this is kind of a stupid loop in a sense because while 1, well what does that mean? Well, 1 is synonymous so far as we've seen with the notion of true. So while true what does this suggest is going to happen with that loop at top right? It's going to go forever, right because true is true, it's not going to change. One is not a variable, it's not X, it's not Y. It's not foo it's not far. It's not some placeholder, it is literally a value of truth so this is an intentional infinite loop. Now probably not the best use of an infinite loop here unless you literally want to write a program that when double-clicked or run and a prompt just says ohi ad nauseum forever, literally until the cord is pulled from the computer. So sometimes we'll see infinitive loops are good but infinite loops if you can somehow get out of them and the only way to get out of this is to do the equivalent in Linux will see of force quitting that particular program. And to be clear, these are not real programs, just as the left-hand blocks don't have that when green flag clicked block which started a script last week, these are not complete programs, these are just snippets thus far. We'll see what we need to wrap them in to make them real programs. Now bottom right's a little more cryptic still but it's a pattern you're going to get very familiar with because it's terribly useful. Four int I gets 0. So this says declare variable called I, initialize it to 0. Do the following so long as I is less than 10, so that middle piece there between the two semicolons says it's a condition, so it's similar in spirit to what's in parentheses up above, uninteresting though that parenthetical was, so while I is less than 10, do the following and then you recall from last week what plus plus tends to denote? Increment, so the effect in English of this bottom loop is to do what in layman's terms? Print ohi 10 times. Right. It's not an interesting program, but again what's otherwise cryptic looking syntax just gets a very simple job done. And so that will become now a new building block. Well what about variables? Well, this snippet of scratch code at top declares a variable called counter. It initializes it to zero and then apparently this scratch code forever said the value of counter and this little cartoon bubble on the screen and then it changed counter by one and we said last week that's the equivalent of plus plus. So that loop up there, that scratch construct has the effect of counting from 0 on up. Well, how do we implement that same idea in C? Well, it's going to look a little something like this. To declare a variable in C we shall see will be a relatively simple saying what kind of variable do you want? I want an integer, aka int. what do you want to give its name as? Counter. What do you want to assign it a default value of? Gets 0 so = 0 semicolon. Declares a variable called counter, it says I'm going to put an int inside of this thing, what Int? I'm going to put 0 inside of it initially. Now this loop we saw before while 1 means do this ad infinitum, print F. Now this is a little interesting and we'll look at more of this today and on Friday, percent D, this is what the F is in Print F. When we say Print F it means print a formatted string, well what does it mean to format a string? It means to tell the computer display this value somewhat differently based on what type of value it is. Now this is kind of an uninteresting case because what do we want to print? Just that value of counter, because I want to essentially implement the scratch thing up above, so percent D henceforth, decimal stands for integer. Put in integer here. What integer? Well, put whatever integer is inside of the variable that comes after the comma. So again, basic building blocks. You're certainly welcome to take little notes on things like this but it will recur so frequently that this will very quickly become old hat. And finally counter plus plus, and that simply means increment that counter and do it again. So we have scratch, we have C, functionally they're exactly the same. I think you'll find in week zero and for PSET 0, just a lot more fun to get something up and running using the approach up top, but you're going to run, some of you, into sort of ceilings with scratch, things you just can't really do because it's not expressive enough of language. We'll be able to do much more powerful things with C, much more powerful things with PHP and with java script, so we are on, as you shall see, an upward trajectory. And finally we have this thing called an inventory. What program was it last week for which I used an inventory? Yeah, it was that RPG, the role-playing game where I moved the little guy up, down, left, right and collected fruit and then presented it to the guy at the counter. Well, I was putting fruit into an array, aka a vector, aka a list, although technically we'll see this semester those actually have different useful meanings. In scratch we just have a variable called inventory. We can add a string, like orange, and when I say string, incidentally, this is computer science speak for a sequence of characters that represent a word, a sentence, whatever. So that's going to be represented in C, perhaps most cryptically still is char star inventory, open bracket size closed bracket semicolon. And we won't spend much time on arrays this week, but in C because you're so close to the hardware, so to speak, because you have so much control over RAM and where the computer puts stuff, you have to be more explicit in C than in scratch. You have to say I want an array of what size? Well, you tell the computer by way of this bracket notation how big you want the array to be, so size is actually a variable for some constant value that must be defined somewhere else in my program. It's simply not on screen. But then when you want to put something at a specific location in an array, the IFE [sounds like] location, we'll say, I'll stop using finger quotes today because all these terms are new, I'm sure. So the IFE location in inventory is expressed with inventory bracket I closed bracket gets or equals and then whatever value I want to put in there. and now notice I'm trying even today, this day zero of week 1 to be careful with terms like equals because we'll find that this equals sign that you're familiar with from algebra in most programming languages is not equal, it's not equality, it's assignment. It means take what's on the left and put it inside of the thing on the left. Wait, take what's on the right and put it inside of the thing on the left. And that's why we've seen this pattern array. So this begs an interesting question and invites an interesting bug. How do you compare two values? Is X equal to Y? Well, you can't say X equals Y because that would seem to put Y's value in X, so hopefully there will be a solution to that. So with that said, here comes C. when you write a C program, as I did on the fly on my Mac last week, recall that I opened that like terminal window, it was this little blinking cursor. I quickly whipped up the code, the source code, at top left there, and then I ran a command. Well, trivia what was that command? GCC. So this is a term you will not forget after term's end because you're going to be running it every day that you're tackling problems. That's GCC, is a compiler. A compiler is a program that takes something called source code and turns it into object code. Now what does that mean? Well, nicely enough this course is not about writing stuff like this. There was a time and there were -- there are people still roaming this campus who had to essentially write programs at the level of zeros and ones, or if you've heard of this thing called a punch card, very similar in spirit, and there was a time when the science center was a center of science. You would go into the basement, you would hand in your punch cards and then the person running the mainframe computer in the basement of the science center would essentially run your homework for you overnight, because it would usually take a while, by literally putting a stack of cards somewhere into a machine. So not the most efficient process, and I've talked to current faculty who grew up this way on a campus, not the best time of day to find out that your program is buggy, it has some mistake in it, right? Because that's probably happening at 9 a.m. the next morning when you go back to the center of science and pick up the results of your program and it's just one big error message. So fortunately we've come a long way, and all of your fighting with bugs and implementing cool things will happen much more interactively within seconds in front of your own computer or the lab computer, but the process is essentially still the same. At the end of the day a computer, whether it's a PC or a Mac or whatever you guys own at home, it only understands 0s and 1s. Now the world has done some really clever things with 0s and 1s but you still have to turn your programs into 0s and 1s and a compiler does exactly that. So you'll be writing stuff that looks like this at top left. You'll run that file, which is really just a text file, through a program called a compiler, and that compiler will take care of the task of turning it into 0s and 1s like this. So where do you do this? Well, last week I went ahead and ran this program, so just a quick show of hands then we'll have the data for sure, how many of you have Macs in the room? All right, so that's actually statistically kind of a lot, so it looks like more than half of you in the room this semester, and that's pretty consistent with terms past, what's neat is that all the stuff we're going to start doing this week and beyond, you actually could have been doing on your own computers, your own Macs, this whole time. Now terminal is not a program that's usually in the little dock here as it is in mine because most users don't need to know or care about this little terminal. But you may at least know anecdotally that Mac OS these days is based on an operating system called UNIX. And back in the day UNIX was essentially a command line interface, CLI is the buzzword there, and this means you don't double click on icons to run programs, you don't go to pretty menus that have sub menus and graphical icons and all of this fanciness that developed over the years, you if you want to run a program, you type the name of the program and hit enter. And if you want to influence the behavior of that program, you provide it with commands line argument, words or numbers after the program's name, and then hit enter. If you've ever seen DOS on a computer back home, it's very similar in spirit. Usually a black screen, white text, and you can't do much of interest with it unless you know what's there underneath. So you're not going to write for the most part programs in this course on your own Macs or on your own PCs per se, because it's quite hard for us to standardize the configuration of 300 plus people's laptops. So instead what we do is provide everyone with a uniform environment throughout the semester. Initially this is a cluster of servers called nice.fas.harvard.edu. And this nice cluster does so happen to live in this very idyllic building here. So it's in the basement, unfortunately, the cluster of servers. It's also a bit of a misnomer because NICE new instructional computing environments has been NICE for about 10 years now, and there isn't even an old version so they've kind of painted themselves into a corner here. We're using the same NICE cluster as always. So nice.fas.harvard.edu, it's not a website, it's not something you type into a browser, but it's going to be the name of a server that you access by way of your own Mac or by way of your own PC. Well, what do I mean by that? well, a lot of you these days have college.harvard.edu addresses, but you also have the so-called FAS accounts, and these are the user names and passwords you use to login to the basement of the science center, kiosks and all of that, frankly it will be a few more years, I think, before the university converges on just one account. But for now you have these accounts, though they're of decreasing utility. But for this course at least at the start of the semester we do acquaint you with nice.fas.harvard.edu because what it means is that after the semester you'll actually still have a Linux server environment on which you can run and write code, whether it's for an economic class in the further or a chemistry project or anything where you need the ability to write programs and you're not in CS-50 anymore, you'll at least have this resource for some number for a good amount of time. What we will start using mid-semester, though, and this is the extent of my Photoshop ability, we will use the so-called Cloud. So this is another one of these buzzwords these days, Cloud computing refers to the ability to rent server space somewhere out there on the Internet, you don't know or care who or where the servers are or who is running these servers. So we're essentially going to implement this same idea. CS-50 has its own cluster of servers now in the computer science building called Maxwell Dworkin [sounds like]. We've got a couple of really big Dell servers that have a whole lot of CPUs, 32 gigabytes of RAM inside of them, 6 terabytes of storage space. So we can hold our own against HCS here. We have a couple of those. We have an Apple X Serve and long story short all of this stuff is wired together in such a way that soon this semester you will have your own accounts on cloud.cs50.net, and what this will mean is you'll start using at some point this cluster instead of nice.fas but it's really the same thing. Now what do I mean by cluster? In the basement of the science center, and also in the basement of Maxwell Dworkin, there's a bunch of computers. They're all networked together using Ethernet cables into a thing called a switch. They are exposed publicly on the Internet by way of IP address or these names, nice.fas.harvard.edu, and there is this illusion that when you connect to one of these servers you're connecting to just one, but in fact there's like six computers in this nice.fas cluster, but they've set them up in such a way that you the user don't know or care which particular one you're on because your files are all visible on any one of those machines and all of the commands are identical across the same. So when it comes time to tackle PSET 1, what you're going to do if you're a Mac person is something like this, I just ran the terminal program, I'm going to type a command called SSH. This stands for Secure Shell, and this just means make an encrypted connection between my laptop, wherever I am in the world, and this server. And so what I'm going to do is type SSH.nice let's do Malan at nice.fas.harvard.edu. I'm going to get a prompt then, looks like I am indeed connected to Harvard University server. It's asking me for a password. Fortunately it won't show you on the screen what the password is but now I'm at the so-called blinking prompt that these days no longer blinks but a blinking prompt it once was. So I apparently have three unread messages, which has been some time since I never check this email account anymore. So Malan at nice and then there's this thing in parentheses. Well this is the command line, so to speak, where you can type anything you want. Well, that command was found. And this is where initially we're going to be writing things like C source code and compiling it. So what does that mean exactly? Well I'm going to do the following just to give you a sense of some basics and all of this realize will be repeated in problem set 1 own PDF. So your hand will be held certainly early in the semester so you needn't try to memorize or scribble down all these commands but just to give you a sense of what we're doing here, if I type the command LS, this is list, this means list the files in this directory, this folder, well there's nothing there. So when I'm at this prompt it's equivalent conceptually to being inside of a folder on your own Mac or your own PC, it's just graphically very underwhelming. And it's more mechanical at the keyboard than it is mouse oriented. If I want to make a directory called CS-50 for the duration of the semester, MKDIR is make directory. Back in the day people didn't like to write long commands, pretty reasonable, so it's just MKDIR CS-50 hit enter, nothing seems to happen but if I type LS now for list notice aha, there's a directory as implied by the slash there, that's called CS-50. Well, I can't, unfortunately, it's a little blue on this screen here but that indeed says CS-50 if I type LS. Notice that I can't, and this is kind of a neophyte mistake, can't really double click and nothing really happens there so you have to change directory CD into CS-50 and then hit enter and now my parentheses, this is just a special feature of the way we've configured your accounts, you'll see. This just reminds you where you are, which is nice when you start to have lots of folders open. Now if I type in LS here there's nothing here, so if I want to write my first program I need to open a text editor and the text editor that we'll encourage most students to use is this little program called NANO. If you've ever used notepad on PCs or text edit on Macs, this is like the most user friendly text editor you can find but as such it's also lacking a number of useful features as some of you with more experience will see. So let me go ahead and write hello.c. Notice that I have just a simple window. It's a little cryptic at the bottom, but again this is just stupid stuff that once you know it you kind of never forget. The carrot symbol means hold the control key, so we'll get to that in a second. But let me go each and recreate really quickly what I did last week. So I type this cryptic sequence of characters, include STDIO.h in angle brackets. I then set into main ent arg C char star, delete key is not working, we've got to fix that. Well, that's interesting. Wow. All right, I'm going to do this again without making any mistakes. Standardio.h ent main ent arg C char star, here it comes, arg v bracket parentheses open curly brace 1234 Print F and I'm just going to say hello, world. I'm going to make it look pretty at the end with the backslash n, closed parentheses, semicolon, close. Okay. we're not going to indent properly here we go, closed brace. So that's my first C program, and this is the thing we completely underwhelmed you with last week. But what does this mean I'm going to do now? I'm going to hit control X because down here it said carrot X for exit so now it's saying save modified buffer. This is the un-user friendly way of saying do you want to save your changes? I hit Y for yes. It then checks what name do you want to give this file? Again it's just reminding me what it's already called in case I want to change it. I hit enter and voila, I'm back at the prompt. But if I now type LS what should I see? Hopefully hello.c and this time it's all white because it's not a directory so now I need to run this thing, so I could try doing hello.c or again the newbie mistake of really that's not going to do anything, hit enter, permission denied. But that makes sense because what's inside this file? This is not 0s and 1s clearly, this is just source code. So source code is the human readable form of this stuff but I remember from last week GCC hello.c. Let me go ahead and hit enter now, nothing seems to happen, but wait, LS. Interesting. Again foolishly named default name for a program anything like hello.exe would seem a little more reasonable, but a.out is sort of the standard but we'll be able to override that soon. So a.out has an asterisks which just means I am executable. I have 0s and 1s inside of me. In fact if I get a little adventurous and now open a.out with nano, nano being a text editor, a.out being a binary file, binary meaning just 0s and 1s inside, so that's the program I wrote. Now here's too where you can do very bad things very easily. If you so much as hit the space bar accidentally as you're flailing about in the basement of the science center trying to fix whatever problem you've just created, you just inserted aski characters, random text into what's otherwise a binary file. Why does this look so cryptic? Well, nano is again a text editor. It doesn't know -- it's not supposed to show you 0s and 1s. It's supposed to show you the letters and the numbers and the punctuation that those 0s and 1s represent. So when you see all this crazy stuff on the screen, that's just nano's misinterpretation of what's really 0s and 1s, not necessarily aski letters. Fortunately control X will save me here, save modified buffer, no this time. So I hit N, I go back to the prompt and now I can run a.out just by typing this command here and in fact that's my first repeated C program. So that's it. that's the basic building block of writing a C program. Hopefully we can do much more interesting things than that but that covers the basic workflow. Now just to give you a sense that what we're doing in this course is ultimately pretty much operating system agnostic, here we have Mac, here we have PC, running Mac OS, running Windows, on the Windows world it's not quite as easy as going to a terminal window because Windows does not ship with that little program called SSH with which I created the secure connection, but there's plenty of free tools that do that. One thing you'll see on the course's website is program called putty. So if you're a PC user you run a program like this it's a little more user friendly if overwhelming with all of its options. What I'm simply going to do here is say, you know what, I want to connect to nice.Fas.harvard.edu. so if I were doing this for the first time I would simply do something like that, typing in its host name or its IP address. Now I'm going to go ahead and click -- actually I'm going to load something there so I get the bigger font size. I'm going to click open and now I get this black window again. I'm going to log in as Malan. I get a similar prompt, enter, notice I'm back in here. if I type LS there's CS-50, CD CS-50, aha, there's my a.out program. Same exact thing. So this is actually powerful. Even though I'm on a PC here, a Mac over there, I've accessed the same account so that's one of the upsides of having this central server. You'll be able to access your code anywhere. If you go home on break, if you get sick, if you're just hanging out a friend's you can access your code in this course from any computer on the Internet and you'll find that for better or for worse to be at least useful. So just to give you a sense of what else you can do on a typical Linux server, so the machine I connected to, nice.fas, it's running at Linux, which is again an operating system that's larger text-based but there are goes, graphical user interfaces, and in fact if you headed to the basement of the science center someday as you might for office hours, they've got like 33 percent Linux computers in the basement there. they do have Windows, they do have menus, things you can click, because the world has come up with more user-friendly interfaces to Linux. But you'll get your hands directly this semester with some of the building blocks because frankly when you get comfortable with this stuff you can do so much more, so much more easily and so much more efficiently just by telling the computer precisely what you want. So CD we saw, CP, any guess? CP, copy, kind of the shorthand version of copy, and in fact it is, CP we'll use to copy code this semester. LS is list, MK-Dir is make directory, MV, move, so that's synonymous, we'll see renaming a file, so to rename a file you'll say MV space the original name space the new name, not terribly hard. PWD. Yeah, so print working directory. What this means here is I'm back on the nice.fas server. If I kind of get a little confused sometime and I type PWD it will tell me the full path to where I am on the server, so right now I'm logged in as Malan and Malan does not have access to everything on the server because there's like 20,000 other people with fas accounts. So in fact, fas took the approach years ago, very intelligently, of organizing things hierarchically. So notice there's a directory, the root of the hard drive apparently has a directory in it called NFS, network file system, home, which is where everyone's home directories live. And a home directory is just your personal storage space. M/A just looks like there's a whole hierarchy of names there to keep things clean, slash CS-50, this is the full path to where I am, and realize that I can definitely go poking around if I get curious. This is not a bad thing. If I type CD DOT DOT it doesn't go this way it goes backwards so to speak or higher up in this tree, so now notice I'm back in my so-called home directory. Where am I going to be if I do it once more? Yeah, the directory called A. So with all of the people whose usernames start with MA, which if I do CD it's getting a little curious here. Maybe will this backfire? Maybe if we could put the camera down here for just a moment so that we're not podcasting this to the Internet, these are all of the people on fas.harvard.edu whose names, sorry folks at home can't see this, that begin with MA. If we go back one more directory keep the camera underwhelming down here, there's the whole alphabet. We can see people that start with probably no MZs I'm guessing, CDZ, LS, oh there are. So it's not a full name, it's first letter of the first name, last name. So again you can poke around all you want and see a whole bunch of stuff on this computer, but Linux, like most operating systems, has various permissions and fancy control features so you really can't do bad things. you can't access ,for instance, if I get nosy, let's say home/a/y idida for Jansue's [sounds like] home directory. Let me, I'm forgetting the day, so I'm in it. Thankfully, permission denied. So things are, in fact, configured correctly. So again just random little trivia that ultimately gets useful because just understanding the world in which you're playing I think lets you get yourself out of problems and actually solve more interesting ones. So there are some other commands that we'll see, and again every piece that's pdf will walk you through any of this minutiae. You're not going to have to figure it out or figure out from week 1 what the heck was the command for this. So we're going to see things like GCC, GDB, commands like more and then someone got clever. More is just a program that shows you the contents of a file. Well, someone wanted to be clever and kind of one up that person so they'd called their program less but it does really the same thing. Man for manual, which is something you'll get familiar with as well. So what does it take to write a program, so it's three steps and we've done it twice. Last week and this, one, open up a text editor like nano in a Linux environment and call the file whatever, hi1.c or hello1.c, using the dot C convention for namesake. GCC the name of the file and then you run the thing by simply running the default name. Well, what if we want to do something more interesting, give it a non-stupid name like a.out? So there are again these things called command line arguments or switches with which you can control the behavior of code more interestingly. So if I go back over here into my CS-50 account, my CS-50 directory, here's hello.c. my goal now is to create a more interesting name than a.out, so per that previous slide I say dash 0 for output. Let's call the program hello and then let's say compile this particular program, LS. Now I have a binary, 0s and 1s called hello up. So you'll notice little keystrokes that save you time. If you hit tab it will do auto complete which saves you time. Unfortunately it also reveals all the other commands on the server which start with he, in this case head, hello, help desk, help desk server, help z tags and [inaudible], none of which I care about so hello, enter, same exact program even though it's called something different still. So more interesting but again just a fundamentally uninteresting building block. But your programs are going to get more complicated over time. Most of your programs by semester's end are not going to be this big in a simple text file, they're going to be multiple files. And they're not going to be terribly long each, but we'll see one of the principals we'll preach in the course is design, and good design, and as some of you may have experienced in scratch already, if you're implementing a script that's kind of this big and then you add some more features and then it's this big and then this big and this big and God forbid this wide and wide and wide because you keep adding ifs and its and ifs and its you're probably crossing the boundary at some point of what we'd call good design. If you're scrolling up, down, left right just to see your own program there's probably a better approach. Now for PSET 0, they don't need to worry so much about this because we haven't really told you what good design is, but among the features will be chopping up your code into much more manageable bite-sized pieces, putting it in separate files, putting it in scratch in separate scripts, and you'll see that a command that will facilitate this is called make. So you'll see both of these approaches in PSET 1, simply typing Make and then the name of the program you want to make, in this case, hello. Notice I'm not typing DOT C, I'm saying make hello. Make is a program that's going to infer, well, if this guy wants to make a program called make it's probably based on a file called make.c so if I hit enter it tried to recompile it but it realized -- and this is what's cool about make -- it realized, oh, you haven't changed the program, why should I waste my time regenerated identical 0s and 1s. So in fact it didn't bother. But if I do LS to see my files how do I remove hello and a.out? Yeah, rm, hello, enter, it's going to ask for a sanity check. Remove regular file hello question mark? Yes. RM a.out, yes for that too. So now I just have my source code, make hello enter, so the command's going to look a little more cryptic because we, just to get you started at term start, are configuring these accounts in a way that just makes some things easier and we'll take those training wheels off over time, but it achieves the same results because I now in fact have a program called hello in that directory as well. So what are these programs you can use? So nano is by far the simplest editor you can use at least at term start. On my computer it doesn't support the delete feature but we will fix that so that you don't need to fail the very first time you make a program like I did, but there are other tools out there. Those of you might know programs like VIM and in fact most of the time I'll use a program called VIM just because that's what I was taught in CS-50 years ago. I kind of grew up on it. EMAX is another popular one, but more on this in the future. I mention it now only so that you don't get confused or puzzled when you see this TF used this program, this TF used another. It's just like using pages versus Microsoft Word versus Word Pad, it's all the same kind of stuff in this world as well. So in a moment we're going to actually tease apart what it means to write a more interesting program than hello world. The basic building block for almost any C program is going to start with this construct here, but why don't we go ahead since this is a lot to digest, and take a five minute break first. All right. We are back, so Yuki [assumed spelling] and Jantsu [assumed spelling] informed me that sectioning will indeed debut this evening, go to CS-50.net, take a look at any instructions you'll find on the home page. Check around midnight or certainly by morning, and note this, so per last week's announcements in the syllabus we do offer three tracks of sections, just like First Nights does for our students who are more and less comfortable with music, so we do the same. It's just the idea of being in CS-50, so there's no steadfast rule as to what makes you less comfortable, what makes you more comfortable and what makes you somewhere in between. To be honest, it's a term you can probably just slap on yourself based on just how comfortable you actually are having returned this week to CS-50. You've seen what the percentage break-downs are so we get plenty of folks from each of the buckets, so when you use fas's sectioning tool, realize that because we have these three different types of sections you're going to see a long list of about 30 sections, since we have about 30 teaching fellows and thus 30 sections. and in the comments fields of the sectioning program you'll see if that section slot has been designated for those more comfortable, those less or those somewhere in between. If you're on the fence as to where you belong you cannot go wrong being in the somewhere in between section. That's really for folks that just don't feel themselves at either extreme, but certainly feel free to chat with me or any of the TFs if you're curious as to what this all means or where you belong. Now what does this mean in real terms for sections? Frankly it just means that especially for those less comfortable, it just means you're going to be in a room full of people who feel as apprehensive or as inexperienced as you and it's sort of a safer space in which to put up your hand and ask a question you think is the proverbial dumb question. The sections for those more comfortable are meant to digress intentionally from say the course's syllabus, from the week's problem set and really go off in any interesting technical direction that some of the savvier students might like to explore even though it's not going to help them per se on that particular week's problem set. But again if you don't feel yourself needing that sort of comfort zone here or you don't really think that you'd want to try holding your own with some of those more comfortable then the sweet spot, just like the standard editions for most students is that for those somewhere in between. All right, so let's do something a little more interesting now. I've gone ahead and per the handout you have I have a whole bunch of code already in this account. So I'm going to go back to my screen here and you'll see again commands lli this on PSETs. What I'm going to do for a moment is copy, tilda, CS-50, so henceforth the Tilda character which on most U.S. keyboards is on the top left near the escape key, Tilda CS-50 means go into CS-50's home directory. Now I'm in Malan's home directory at the moment, so go into CS-50 is just like I tried to idida's home directory, so go into CS-50's and then I know because I'm reading this off of a pdf or whatever that there's a pub directory, that there's a source, src, again, nicknames in Linux, source directory, and then I know there's a lectures directory in there and then we're in week 1. So it turns out there is, in fact, a directory called 1 inside of lectures, inside of source, inside of pub, inside of CS-50's home directory and my goal is to copy it from there to here because I, now a student in the class, decide that I want to push ahead, I want to do this after class and play around. I want to copy David's source code into my own account just so I can play. Perfectly fine, notice that DOT represents this directory just as DOT DOT represents the parent directory. So if hit enter this is actually flawed, notice that something bad happened and then if I type LS I didn't get any files. So realize, too, sort of FAQ early on is if you want to copy a directory with CP you need to do it recursively, which is a keyword that we'll revisit in the term, it means to do it not only to that directory but all the files inside and all the subdirectories inside again and again and again until you bottom out. So CP-R means copy the file recursively into my current directory, enter, took a second because there are, in fact, a bunch of files. In fact if I go into my one directory now here are some examples that I've pre-prepared so that we can go off in a known direction here. So one direction we already went in was hi-1.c. but your version, on paper looks a little different from the quick and dirty thing I've been whipping up. there's a lot more purple on the screen here, so all the stuff at top is fundamentally irrelevant to the computer, but just like scratch does if you've discovered it yet, lets you put little comments here and there, sort of sticky notes, so to speak, post-it notes, so does C in most any high-level language. and by high-level language henceforth I mean something that's not 0s and 1s, not something called assembly language. It means ignore all of the following this is useful only to me the human. So in C the means by which you write a comment is to do a forward slash star and that means anything after that on this line or below will be ignored by the compiler, GCC, and the comment stops when you hit the opposite of that, star slash. So the fact that I actually have all of these stars and I've kind of boxed things in is really just some OCD on my part. It's not technical I just wanted to make a nice pretty purple box to box in all of my code. But one key takeaway is that I made sure that this line of stars and the slashes does not exceed the width of my window. So another lesson that's important early on is to make sure your -- the code you start writing is pretty printed. And this just means it actually displays nicely on a typical window. In real terms this means you should never at least for the C part of this course, have a line of text that is longer than 80 characters long. And even that's pushing it, 76 is kind of the geek's rule of thumb as to how long a line should be. How do you know what's 76 or what's 80 characters? Well, right now my window has essentially been expanded to be as long as I wanted it to be, but you get into the habit of knowing when to hit enter to move things onto the next line and nano and programs like it will actually force your lines to wrap if they start to get long. And this is just a matter of good style, because as you could imagine, if I wrote my whole program on just one line it would still work, the computer would not care, but you the humans probably would. In fact, this is hi1.c, let me go ahead and delete all that stuff. I'm going to go ahead and do this, you know, this, I really don't need that there. Let me go ahead and put that there. Right, if I'd given you a printout of this, I mean, rightly so would things start to look pretty damn complicated. Now some of these spaces are important so that you can distinguish what's ints and what's [inaudible], but if I go ahead and save this with control X, hit Y and now go ahead and run GCC of hi1.c, the program thankfully still compiles. I still have eight odd out, so it still works, this version, but it's certainly not very pretty printed and not very legible. So what else is inside of this file? Well, hmm, I didn't really want to do that. Let me go ahead and go back into CS-50, Pub source lectures 1, let me just get a new copy of this. So now it's going to prompt me, wait a minute, overwrite hi1.c, little sanity check, say yes. So now I have the prettier version back. I'm going to start using a program called VI or VIM just because frankly I'm more comfortable with it than nano, even though nano is simpler, do as I say not as I do. Hi1.c, still kind of pretty prints everything and color codes it. And the color is actually a feature that you don't get if you trying writing your first programs with notebad.exe or other similar client side programs. It's just making some inferences about the different roles that some of these keywords play, so I didn't write the word int in green. I didn't write the word char in green, it just knows that that represents the type of a variable. Let me draw David's attention to it with this default color scheme. So what's interesting here? Well, this stuff at the top is all comments, and actually now we get to use a fun little toy. this stuff up here is all comments. All right. So it's fundamentally irrelevant but useful for documenting your code. This things here I'm going to briefly call today a pre-processor directive, which is just a fancy way of saying tell the compiler to go get the contents of some other file that often someone else wrote. So what we don't have here is any kind of specification of what Print F does. Like who wrote Print F? I wrote this program called main, but who wrote Print F? Right, I'm kind of cutting corners here. Well, it turns out that when Linux shipped with the compiler, someone else, some really smart people spent a lot of time getting this very basic important function implemented in their own source code. So by saying sharp include open angle bracket standard io.h closed bracket, that's just telling GCC, you know what, the following code has some functions, some programs that other people wrote. Those programs happen to be inside of a file called standardio.h. please go grab the contents, paste them in here so that I don't have to do it myself. So I'm kind of oversimplifying there because in real terms what we're using here is a library. So a library of code henceforth is just a bunch of code that someone else wrote or maybe you wrote but that you factored out into separate files so that you don't have to copy and paste it into every future program you write. To technically my white lie I just told is that these program Print F these functions, actually as they should really be called, are not actually in that file, standardio.h, inside of that file will eventually see is just a minimal amount of information, essentially a summary of all of the functions in that file because as you might imagine, where is the actual implementation, the actual lines of code that implement Print F? What would the file name probably be called? Standardio.c, so in fact, yeah, there is a file out somewhere on the system called standardio.c that someone else implemented but there's a lot of lines of code in there, a normal human doesn't want to read through it so we have these header files which are called .h files and then c files which have actual code and initially I'm not making any header files myself, we'll get to that eventually but just realize, some of these minor tripping points early on are just about keeping programs relatively clean, so this, to be short, simply says, go get the contents of this file so that I can use functions like Print F and henceforth let me clarify. So I am writing a program, this program is arbitrarily going to be called hello, but I can override that as we've seen with the -o flag from GCC so programs are made up of functions. one or more functions and these are kind of like miniature programs just like your scratch projects are probably made up of multiple scripts. A script is a function so a function is just a piece of code that does something and as you can have multiple scripts in scratch you can have multiple functions in c and just as the default block or script and scratch is that thing called wingreenflag clicked, so in c is the default script, the default function called main. Now why, if its just called main do I need to write something like int and then ints and then RGC, char or this asterisks, this RGV this bracket notation, there's just a lot of crap at the beginning of a program when you're writing it in c, those of you who took app computer science know that there's classfoo public static, voidmain, its even worse in Java and frankly this is just because these languages were not designed with week 0 of a programming class in mind. So again, its stupid stuff, you get past it quickly. For today we're going to wave our hands at details like that but to say for now it's a copy paste thing. To start writing a function you literally write int main and all the rest of that stuff but then the fun part or at least the intellectually interesting part happens in between the curly braces for now. So that's how I would go about printing this simple program. Let me actually take a look at a more interesting version, thought only slightly called high2.c and you have a printout of this. So what's the fundamental difference between version one and this one version two? Hopefully it jumps out at you. yeah, so if bit egotistically this jumps out at you that this program has my name immortalized in it, but how did I do this? Well it looks like I've got this additional line and so David is mentioned but what's going on? well even just using the jargon I started using earlier, string, because it comes before the name of a variable, means what precisely? Yeah, it's the type of variable. So the variable is called main, what's going to be inside of this variable, well its not going to be an int, because David is not an int, it's a string, it's a word, it's a sentence so I'm going to specify that the type of this variable is string, now the equal sign again is the assignment operator, it means put the value of the thing on the right on the inside the thing on the left so the value on the right is itself a string and in c as in many languages if you want to have a literal sequence of characters be treated as one unit, you put them in all double quotes. You can use single quotes but only around single characters, so those of you who know a little of html probably know it doesn't matter if you use single or double, it does matter. In many programming languages including c, so strings are double quotes. All right, then what do I do on the second line? Well, we've not quite seen this syntax but in laymen's term can someone explain what the percent s there represents? String. Right, really not that hard. Percent d happened to represent int, percent s represents string and so the thing after the comma means what do you want to substitute in for that formatting code, the percent s, put the name of the variable. All right, so more interesting, so GCC hi2.c enter, now I'm going to run a.out, ohi David but unfortunately its really not that interesting if your program is completely deterministic and always spits out the same result because really, I mean, there are so many easier ways to just say something like this than this set up so let's take a look at the third version of this which is a little more interesting and finally we get a compelling use for a keyboard, so this program differs for the comments up top in that there's a third line of code here that uses a new function, so we've been using Print F to send output, it turns out that you can also, nicely enough, get input from the user. So not by way of the mouse, at least in this context but by way of the keyboard and what's the new function, just to take an obvious guess here, get string. As the name implies, the function get strings purpose in life is to get a string from the user and do what with the result apparently? Store it. so functions we now see right away, have this ability not only to do something aesthetically, Print F has this so called side effect of displaying something on the screen but functions can also return a value so you can think of a function kind of as a black box that does some work and then can maybe hand you some answers and its up to you, the programmer, to decide if you're going to do anything with that answer. How do you do something with it? well one of the things you could do is keep the answer around. So by having string name on the left that says give me a variable called name, it's a type sting, what are you going to put inside of it? well, whatever the result is. The return value so to speak of the thing on the right so get string returns now a value. How does this all behave in reality? So GCC hi3.c, I'm getting a little tired of a.out, how do I give this a better name? yes, so -o for output, and then let's call it hi3 though I could call it anything I want, oh, oh, did I create a problem there? its kind of a coincidence, so still the same error, yeah, so correct, fancy correct answer so there's a problem here in that get string is a function, yes but it wasn't written by the original authors of the c programming language, in fact just getting input from the user in c is really a pain. You have to use a function among others called scanf and we'll use this eventually, there's other tricks too, but long story short, in week one of a course, especially this course, to complete a bunch of hurdles, you have to jump through as simple as get a character from the keyboard, even in Java, those of you familiar with the scanner class, even that is a pain to actually use I'd say in the first week of a class so notice what did I do in this file? I did one other thing besides mention get string. There's one other key difference outside of the comments. What else is new here? yeah, so I'm not just using the standard io library, it turns out that the cs50 staff a few years ago wrote our own library. And we wrote what that means, two files. One called cs50.h and one called cs50.c and in the latter we wrote a bunch of functions that we thought would be useful to students in the semester namely get int and get string and silly name, but get long long but other functions still that really simplify the process at least at the start of the semester of getting keyboard input from users but unfortunately its not sufficient just to tell GCC, hey make sure you go get a copy of the header file for cs50's library so I know that these functions exist, because I've done that. So one mistake might have been to omit this line together, that would in fact be bad. But that's not the only mistake that I made, I now need to tell cs50's library when you compile this you don't just get away with turning one file, hi3.c into 0's and 1's because you need access to one other file, called what? Yes, so cs50.c so there's a couple solutions here, I could email the core staff, and I could say, hey could I have a copy of cs50.c or they'll be happy to give it because there's no intellectual property there, its just standard c stuff, but that seems kind of wasteful if every time in the semester if you write a program you need to copy the R file, put it in your own directory, kind of bloat your own code, well in fact there's an easier way. You can tell GCC to link against our library and long story short is what this mean, is when GCC is converting hi3.c from source code into object code, from human readable to 0's and 1's it means, oh by the way, go get the library, -l, called cs50 and merge the two at the very last minute so that the file you get out hi3 or a.out contains 0's and 1's from both of those files. So again, another silly detail that's very easy to forget but just to kind of drill it into your minds if you ever see this semester, a message about undefined references, probably means you forgot something simple like that library that you wanted to use, -lcs50. there's others, -lm gives you a math library. There's -lcrypt which gives you a cryptography library and stuff like that, but it did work this time. when I said go ahead and link in this file, I did in fact get hi3 so if I run hi3 state your name, we'll say Jensue, well hi Jensue, well what about Yuki? Okay so now I have, still kind of overwhelming right? not anything to write home about but at least it's a dynamic program that's starting to do a little bit more so just to give you a sense now of some formalities, one I've used here, Print F, a couple of times, I've used it pretty simply to use things like strings and ints but we can do more powerful things with it and in fact you'll have to get used to sort of a new sort of a world where people write things somewhat cryptically sometimes, this is an excerpt from the man page, the manual page for Print F and the way you, the programmer know, how to use a function is often frankly, by rtfm. Just read the manual. And actually look up what the instructions are, Google it later if you'd like. For those less comfortable and less familiar with the speak. So what you'll find on the courses website throughout the semester under resources we have a bunch of links we think you'll find useful and one of them is this wonderful site called cppreference.com. the only downside of it means cpp means c plus plus so the author of the site actually made it fairly c plus plus specific so under resources on the courses web sit we have our own copy of this website because he open sourced it to everyone so if I actually go to cs50.net/resources what you'll see up here under the link to c is a bunch of things, most of which we won't need just yet, but if I go to reference, this is a wonderful useful sight and this is one of the reasons that you don't really need a book for a course like this because so much good stuff is available on line. If I know that I'm doing something with io, that sounds cryptic, but it just means input output print get stuff like that, notice if I scroll down here, clearly we're only scratching the surface today of what C can do and I was interested in what Print F can do and here's just fairly user friendly documentation of the other codes you can use so we saw already, let's see, percent d and there it is at the top for signed integers, things that may or may not have negative sign in front, percents for a string of characters but there's some other stuff here too that we may or may not use in this semester, but what's nice about this site is it give you sample code, it tells you what library, what header file you must include at the top of your program and then it tells you what the functions signature is, what is sort of the canonical form of this thing so you can begin to understand how you use it and again you'll get much more acclimated to this over the course of the semester but we can do more interesting things with this. So just as there's  for new line, there's  for carriage return, so if you think back to retro typewriters when you hit enter on an old fashioned typewriter, not only did the paper move this way, the little typing head moved this way so a carriage return moves the curser all the curser all the way back to the left, a new line moves it down one line so this is another sort of stupid thing from religious battles past so in the Windows world, most text files end with what's called a carriage return and a line feed. Crlf, which means  in the Linux world people were much more conservative, they just used new lines and  in many systems, Macs and, actually not so much Mac sometimes, so in Linux computers  means yes go to the next line and go all the way to the left so I'm just kind of grimacing at some of these details because these things too, are little things you trip over early on. they're fundamentally not that interesting but you get used to little trivia or minutia like this. But this does beg the question, if  means new line,  means carriage return, how do you just write a literal  Well the answer is there but this is a common pattern, what is it?  so unfortunately a little weird, you kind of have to have a corner case like that if you want a literal \ and a string you escape the \ but this notion of escaping is something we'll see quite a bit. Well, it turns out we can do stuff with not just ints not just strings, there are chars for individual characters, doubles for floating point values and float for floating point values and all of these have some different meaning, in fact let me go ahead to an example that's not just text space now, but just to demonstrate, again, to underwhelm, at first because we can definitely one up ourselves easily that math is not hard and c. so here's a program that assigns a variable called x, I'll type integer the value of 1; variable called y type is int, assign it a value of 2 right, new syntax, not interesting. Finally I declare a third variable called z its also going to store an int, its going to store the sum of x and y, looks pretty good but let me go ahead and let me run GCC of math1.c okay, interesting. So it turns out GCC, especially the way we have it configured on nice.faf can also be a bit anal and a little cautious on your behalf saying you know what, odds are you screwed up here. right? its saying it very nicely, warning unused variable z but that's telling you that you did something, you're not using it, probably wasn't a very productive program you just wrote, but let's try it. let's be stubborn and let's run a.out enter, hmm. Is it not working? Its actually working perfectly. Right, the math is correct, what is in z at this point, oh, missed it. the problem is what? Simple answer. I'm not doing anything with it, yes I created 3 variables, all of them at some point in time, do in fact have numbers like 1 2 and hopefully 3 inside of them but if you don't do anything with it, sort of the tree falling in the woods right? no one knows that the answer you just computed was three unless you print it, save it, do something with it, so hopefully there's a solution here. so let's take a look at the second, slightly fixed version of this, so how do I print out the result? Well again, let's just go into our little tool kit that's going to start being filled with more function still, Print F quote unquotes the quotes are important, the percent means display this as an integer and then I pass in z and so, okay, so now not only have I written a correct program, I'm actually doing something productive with it, a.out, now gives -- hmm, still a little weird, its not incorrect but what's that explanation for this weirdness. Right, so there's just no new line, so let me go in here and I'm going to move over here, add between the quotes but after the percent let me recompile, another little Linux trick, if you get tired of typing the same commands over and over again, you can either hit up and that will scroll through the commands you previously typed or you can hit ! aka bang and type the first letter or so of the last command that starts with that sequence and hit enter and if I haven't forgotten some other g command I wrote, it should remember, aha, it was gccmath2.c, again little things you pick up over time. and if I run a.out, okay slightly prettier, slightly prettier. So what else can we do here, well there's going to be other data types still but there's also this format strings so let's see if we can't do a little bit of something that's more interesting underneath the hood so it turns out we have format strings, but there's also this approach here, let me go back to the code. This is a program, again, that you have a print out of, called size of, and here's where we finally scratch the surface of actual design issues. Actual questions that you are interested to answer because they really will affect the correctness of your program and what you can do with it. so this program, its not very long, its about half comments and half actual code, notice that at the top here I've declared some variables in advance, so this is kind of interesting. Even though thus far I've very efficiently, very analy been declaring a variable on the left and then doing something with it on the right, all in one line, you don't need to do that. If you know you're going to need a variable, you can declare the variable earlier in your program and then use it later and you'll learn over time when this is and isn't appropriate, but here I've declared three types of values. A char, a double, a float and an int. a float is a floating point value a real number, something with a decimal point, what's a double perhaps? Any ideas? So its twice something, its actually a floating point value that's twice as big potentially. So it turns out that in this language and in some others, there are actually pre defined meanings for how many bits are used to represent a data type, so we have chars or 8 bits on a typical modern computer with intel inside, a double is, and let's come back to that. A float is 32 bits, now that's pretty big, because if an int is also 32 bits, what did we say last week, the biggest number you can represent with 32 bits is, like 4 billion. Now wait a minute, if you're omitting negative numbers so if its negative numbers also then its like negative two billion, to positive 2 billion roughly but a total of 4 billion so floating point values also use 32 bits but they use them to have some numbers before the decimal point, some after so you can't, it's a trade off between how big a number you can represent as a floating point value versus how many points you can have after the decimal point but a double now is not 32 bits, right, I'm kind of beyond the scope of my arms here, its 64 bits and that's pretty damn big so you'll have some discretion there so what does it mean when I run this down here, Print F all right, so print an int and a new line, but what do you want to print? Well there's this operator in c, that's only occasionally useful, today its marginally enlightening, tell me the size of the variable c. tell me the size of the variable d, f, and i so let me go ahead and run a GCC on sizeof.c now I'm going to go ahead and run a.out, wait a minute, those are not in fact values like 32 and 64. what am I seeing? Yeah, I'm seeing bytes. That's kind of interesting, okay, so that's fine, let me take a look at one other thing. I saw on the screen a moment ago that there's this mention of longs so let me say long is an l and let me print out its length here, so long and I think its l, hopefully, %, let's see, GCC sizeofc, oops not l, dammit, what is it? is it also d? yes all right. if I run this now what do you think a long is going to be? So int was 4, also 4, so again, sort of a historical artifact, when I mentioned long longs before, so if you want an int that's bigger than 32 bits, you want a 64 bit int, its not a long, because a long is the same as an int, it's a long long that you want. So again, sort of random c trivia. But I thought we'd end with this. Here's a crazy looking program, I loaded this off the internet, written by a really, either a really brilliant person or a person with really a lot of free time. so this is an obfuscated C program and you are entering a community that you don't have to stay in in perpetuity of people who like writing programs like this, or have the ability to do it and there are contests that people run for obfuscated c contests whereby you challenge people to write the most confusing looking program you could possibly come up with that no human should be able to understand and then you challenge the audience, the other geeks in the room, what that program actually does, do you have any idea what this is going to do? We will not be writing things like this. This is an exercise in the opposite of good design, but its kind of a powerful thing, that if we run the compiler here, the warnings I'm going to ignore for now, that's fine. A.out, I'll give you the results of your first obfuscated c program which is, that's what that does. We'll see you on Friday. 

==== Transcribed by Automatic Sync Technologies ====