>> -A triumph of mathematical and mechanical skill

is this great new automatic calculator at Harvard University.

Intricate problems in mathematics put through the machine in coded form

on tape are accurately solved in a minute fraction of the time required

for human calculation.

Designed to expedite all forms of mathematical and scientific research,

the giant mechanical brain will work for the United States Navy until war's end.

>> [END PLAYBACK]

>> DAVID J. MALAN: This, is CS50.

The past was a magical place, but if you've ever

wondered what that device is in Harvard Science Center,

it is indeed part of the Harvard Mark I, and it's

on display there now for the past many years.

And what you'll see embedded in problem set

one is a look behind the scenes at what that device did, and does,

from Professor Harry Lewis, who is in the computer science department here.

So more on that in just a bit.

>> But today we begin to transition from last week to this,

and we'll bring mention of one photograph here

that's actually quite famous.

Does anyone know what this here is on the screen?

At least according to myth.

Yeah.

AUDIENCE: The first virus.

DAVID J. MALAN: So it was supposedly the very first actual bug in a computer.

Now in reality the word bug, for a mistake in a program,

has been with us for some time in the physical world,

and this was one of the first documented times where someone actually

plucked a large moth out of the Mark II computer, put it in the binder,

and then recorded it as the first known bug

to have actually been found physically.

So thus was entered that into today's lore.

Everything from last week is available online.

In particular resources that you might not necessarily notice, especially

if coming here in person.

So one of the teaching fellows every year makes a canonical set of notes,

so you needn't keep your head down, trying

to transcribe most anything that might happen here in class.

So realize that those so-called [? Scribd ?] notes

are online, certainly the slides and source code that we use in class.

We make full text transcripts available as well.

So if you vaguely remember some term, and want to look it up a little

more readily, realize that all of that is searchable online.

As well as walkthroughs as well.

Indeed, today we begin to look at a lower level programming language,

soon to be known to us as C, and it's very easy

to sort of get lost after just a few moments time, and then sort of wonder

how we actually got to some more complex example from the simplest.

And so realize we'll cherry pick during class some

of the representative examples, but realize that so much more is also

available online so you can work through it at your own pace,

however you're most comfortable.

>> But we left off with programs that were just this.

So I click the green flag, and what does this program do?

Says, hello, world.

It makes the cat say hello in his little world

on the top left hand side of Scratch's user interface.

But we started to introduce some terminology last week.

And so, things like this we called what?

What might be the generic term for this?

Yeah.

A method or a statement.

Sure, it's an action.

Do this.

And yet there were other shapes to scratch,

besides these so-called statement, they were also

things like this, which we called what?

Yeah.

So a loop.

And in this case, this thing repeats, obviously, 10 times.

There's a forever block.

There's a couple of others.

And today we're going to see how to translate this to a more modern world,

if you will, even though it's going to appear at first glance

to be much more arcane.

But those were indeed loops.

And then we have things like this, which had this shape,

and essentially asked questions.

And we referred to these as what?

>> AUDIENCE: Boolean expression.

>> DAVID J. MALAN: Yeah.

Boolean expression.

Something that is either true or false, yes or no, one or zero,

however you want to view the world.

But we called those Boolean expressions, and we plug them

into conditions, or branches.

And in the world of Scratch, everything nicely

interlocks in terms of its shape, if it makes logical sense to do so,

but you don't have to stop at just doing one thing conditionally,

you can have a proverbial fork in the road

and do either this or that if the condition is true or not.

And then, as you'll see, if you haven't already,

you can even nest these blocks.

So Scratch grows to fill the shapes that you might want to insert into it,

so you can actually express most any logic that you might want to do.

>> Now a few words of announcement for both here and New Haven.

So sectioning begins today.

The URL for this is on the course's website, for those unfamiliar.

Sections will be an opportunity to get together

in a much more intimate environment than [? Sanders ?] allows,

with 10 to 20 classmates with a teaching fellow at different tracks,

and different paces, if you so choose.

Indeed the course offers different sections

for those less comfortable, more comfortable, and somewhere in between,

and the role of thumb here, is there's no formal definition,

but if you're less comfortable, you kind of know it.

You're a bit intrepid about the idea of stepping foot, perhaps, back

in the classroom, or showing up to an in between section.

If you've been programming since you were six years old, more comfortable,

perhaps to you.

And if it's kind of a toss-up, that's exactly what the in between

is meant to be.

>> Now, in the meantime, for a class this size,

it takes us quite some time to section everyone, so we have so-called,

super sections, coming up this weekend and early next week,

so take a look at the course's website for those that are course wide,

and also filmed for those who cannot make them.

And sections themselves will start in week three of CS50 zero index.

Problem set 0 is already on the course's website.

And realize that you do have nine late days this semester.

So this is an upgrade from last year, when we had five late days.

>> So if you're wondering, we have nine problem sets.

You have nine late days, each of which you can only spend one per week on.

That effectively makes all of the problem sets due Friday,

but the goal is to put a little bit of psychological pressure

on you to try to get things in on Thursday

because as you will soon discover, for better or for worse, most

everything in the software world ends up taking much longer than you anticipate.

So consider it a sort of opportunity to help

yourself to target Thursday, but take comfort in that, yes,

everything is due Friday.

And if you submit them on Thursday, that's wonderful.

There's no bonus points or extra credit for meeting that deadline.

It's meant to be a crutch only.

>> And office hours, both here and in New Haven,

in person, if you'd like to meet with any of our courses

teaching fellows and CAs for assistance.

>> All right.

So now, let's make this more real.

And for a moment it's going to start to get a little more arcane, but very

quickly on Friday and next week we will be

able to do so much more than we could thus far in Scratch.

So we introduced code last week, albeit pictoral.

It was puzzle pieces.

And we formalized that it is something called source code,

but would did look like?

Well, it looked a little something like this.

And, in fact, just to kind of tease where we can go with this.

I'm going to go ahead and open up a window here,

and I'm just going to choose New File.

And I'm going to save this file as something like hello dot c.

More on that choice of words in just a moment.

And this is how someone might go about writing a program.

Include standard I/O dot h, int main void, and then open curly brace, close

curly brace, printf hello comma world, backslash n, semicolon, Save, done.

OK.

I've written my first program, but it's going

to be incredibly underwhelmed because when I actually

run this program, of course, by first writing make hello,

and then dot slash hello, and we'll go through this in much more detail

in a moment, that's all it does.

Doesn't pop open any windows.

It doesn't do anything particularly fancy.

All it does is what I told it to do, but how do we get to that?

And how do we build upon that?

Well, let's introduce a few different concepts here.

>> So one, we're about to start using a program called a compiler.

A compiler exists for Mac OS, Windows, Linux, any number of operating systems,

and we'll soon introduce you to the environment that we will be using,

but a compiler is just a general term for a piece of software

that takes source code and produces object code from it.

In other words, if you have as input your source

code, something like what I just typed, it ultimately produces object code.

And object code is really just a fancy way of saying it outputs

zero's and one's.

In other words, source code, even though it's

going to look a little arcane, and a little less friendly

than Scratch at first glance, it's at least something

that we humans can understand.

It kind of looks and sounds like English even though it's very succinct.

Whereas programming in zero's and one's, as our predecessors had to do,

some years ago, was not a particularly fun challenge.

And yet, this is what the computer still understands.

So compiler gets us from source code to object code.

It's like opening a file with the compiler, and when you save it,

you get those zeros and ones.

So we'll see how to do that in just a moment.

>> But let's now try to provide a bridge between last week and this,

so that even as we get hung up on things like curly braces,

and semicolons, and parentheses, and all of these intellectually interesting

aspects of programming, that very understandably trip people up early on,

realize that all of the ideas today, are the same as last.

So last, we saw this example here, Hello World program.

Well, starting today, if we want to implement that same program,

it's going to look a little something like this.

And we'll explain before long what int, and main, and void, and all of that

is, but at least I see here already the familiar Hello World.

>> Meanwhile, things like function, say hello,

is very simply going to be a very simple statement like this.

Printf hello world, with some quotes, with a backslash,

with a semicolon, but more on that to come.

Loops, meanwhile, this is a loop that, of course, says hello world forever,

and this one's going to look a little weirder than in the last,

but if I say something like, while true, I

can then print out Hello world again and again and again and again.

And then as you can kind of start to infer here,

this convention of using curly braces is kind

of like embracing the indented line of code there.

The printf line of code much like the yellow puzzle piece up above.

So we're not going to say forever, we're going to say while true,

but we'll see why that is in just a bit.

>> Meanwhile, if we have a loop like this, where you repeat something a finite

number times, this one too it's going to take a little getting used

to, but it's going to say for int i gets zero, i less than 10, i++,

and then printf again.

But there's going to be patterns.

And for those of you familiar with AP Computer Science, or Java,

or any number of other languages, almost all of these constructs

thus far should look mostly familiar.

>> So variable.