Ah, my name is David Malin, and I actually took this course myself sophomore year some years ago, but but I almost didn't At the time, I was quite uneasy with the idea, frankly, of taking a computer science course, let alone this course.

And so my freshman year, at least, I really gravitated towards courses and departments with which I was much more familiar.

Computer science was well beyond my comfort zone at the time, and it was really these unfamiliar waters that I wasn't quite ready to shop even my first year.

But sophomore year, I finally got up the nerve to come, like through the door of CS 50 and on Lee, because the professor at the time let me take the course pass fail.

I was that uneasy, and if only I had known at the time what I now know, which is.

The 2/3 of CS 50 students have never taken a computer science course before, so if you are feeling similarly uneasy with the idea of trying something new, or even if you have prior background but are looking to fill in gaps in your knowledge.

If you're particularly self taught, realize you are very much in good company.

And ultimately, what matters in this course is not so much where you end up relative to your classmates, but where you end up relative to yourself when you began.

Which is of course, today.

So what, then, is computer science and what was it that I was so uneasy with?

Well, I dare say we can describe computer science is this?

It's just the process of solving problems and what does it mean to solve a problem?

You've got some input, and the goal is to get some output.

The solution to that particular problem and in between really is computer science, and we'll see what's in this literal black box on the screen as we begin to fill in some of those blanks.

But when we consider ah problem to be solved, we have to all agree from the gecko, especially for using machines.

How are we going to represent these inputs and outputs?

And so one of the first concept we explore in computer science is how you represent information itself and odds are, you probably know, coming into this course that's computers only Speak what language.

So to speak S O'Brien.

Aaron, if you've never heard that term before, Fine as well, But zeros in one somehow.

But how do we get to that point?

Well, even if you kind of know the computer, speak binary, whatever that means.

But you haven't necessarily thought about how that works.

Will consider how most of us learn Tout accounts.

Maybe first on our hands, right.

If I want to count the number of people in the room, I might do 12345 putting up one finger for every person in the room.

So that's what's known as you Neri notation you are, you know, implying one, because I'm just putting up a finger or not to count people in the room.

Of course, I can only count so high on just one hand, but thankfully, we have many symbols in our so called human world of decimal dec, meaning 10.

We have digits zero through nine, and from there we can express any numbers with which we're all familiar.

But computers don't have that many digits.

They only have zeros and ones.

And yet somehow they're able to store not only numbers, but letters and images and videos and sounds, and so much more.

So how does that work?

Well, let's take a quick look at just this to get everyone on the same page as to how you can build the phones in our pockets.

The laptops on our desk today using just zeros and ones.

Well, this are just symbols on the screen here.

What is this, of course, represent 123.

But why?

I mean, these literally are just symbols, or GL ifs on the screen, sort of strokes of a pen that we all ascribe meaning to.

We just see this as 123.

But why is that?

Well, if you're like me, you probably learned way back when to think of the right most digit as being in the ones place with one's Call them.

The middle digit is being in the tens place, and the left most digit is being in the hundreds place.

So how do we get from this pattern of 1 to 3 to the number 123?

Well, it's, um, quick mental math that we all just do instinctively.

It's 100 times one plus two times, 10 times two plus one times three.

That, of course, gives us 100 plus 20 plus three or 123.

So all of us just kind of take that process for granted now.

But it turns out that there's a system at play here, a system such that we can figure out any number in the same way.

So if we've got our one's place tens place in hundreds place, of course, in the decimal world, this is the number one.

This is the number 23456789 Something interesting happens, Of course.

After nine, you sort of carry the one.

And why then does 010?

Or if we ignore the leading zeros as being insignificant?

Why does 10 represent the number?

Well, no, obviously, as 10.

Well, just cause it's 10 times one plus one time zero.

So that's the system we've been using for years, and it turns out that these columns realize are just powers of 10.

So 10 to the zero is one tend to the one is 10 and tend to the two is 100.

And that's how we got ones tens and hundreds.

Place.

Now, in the computer world, you don't have 10 digits.

Zero through nine.

You have just two digits, zero and one.

So we can just make a little tweak to our mental model here, so to speak.

And just now, use two powers of two instead of powers of 10.

So that means the right hand column is gonna be one.

The middle column is gonna be to the left column is gonna be four.

And if we kept going, it be 8 16 32 instead of 1000 10,000, 100,000.

But the idea is exactly the same.

So I propose that computers on Lee speak zeros and ones, of course.

But how do they represent larger numbers than zero and one?

Well, this is representing zero.

This is representing one.

What pattern of symbols on the screen would represent the number we humans know is too.

S 010 So you just spoke binary.

So even if you just generally knew what buying that binder is spoken by computers, it just means that a computer to represent the number we know is to somehow stores a pattern of symbols of 010 How do they represent three 011 for the same reason?

It's two times one plus one times one gives us, of course, three.

And now, just as before when we went 9 to 10.

Now we actually go from 3 to 4 by carrying the one now.

So in binary by meaning to hence, the zero and 1100 is not 100 per se.

It is literally the number we know it's for, and we can keep going on up.

This, of course, now is five.

This would be six.

This would be seven.

And what happens if we want to represent eight s?

So we kind of have to solve this somehow.

We need another digit, and that's fine, right?

In math class, you would just add another place, another column in order to get back the value that you want.

So we need more zeroes or ones otherwise now known as bit.

If you've ever heard of the phrase bit, it just means binary digit and a zero or a one.

It's just something we know is a bit And why is this remain to computers?

Well, it's fancy is our Macs and PCs and phones are today.

Consider.

After all that, at the end of the day, what do you have to do typically with your your laptop, your desktop, your phone these days?

What do you do with the very end of the day s So you charge it right, you plug it into some physical resource.

That is the Onley physical resource into these computers that we use today.

And that's kind of nice, because, of course, the court can either be plugged in or not plugged in.

Or maybe we could call that a one or a zero, True or false, on or off.

Which is to say, if electricity is our only input and there's either yes, electricity or no electricity will.

That actually maps quite nicely to the idea of binary, because we need just two symbols zero and one often on to represent that idea.

Of course, with a single bit, you can't do all that much.

You can only count from zero toe one, and that's it.

So computers tend to use larger numbers of bits to represent information, and they do this ultimately, physically.

So it turns out that with both plug in the wall or even a light bulb or switch thereof, you can represent a zero or one.

But if you want to represent Maur zeros and ones while we just need maybe eight bits at a time.

And if you've ever heard of the expression bite Aah!

Bite is just a term describing eight bits, eight zeros and ones and more useful measure instead.

So given that I think it's time perhaps toe to see if we can't solve the problem ourselves, Perhaps we could get, say, two volunteers.

Come on.

Okay, so one hand here and one hand over here.

Come on down.

If you want to meet me on the other end of the stage.

Come on over here.

Come on down.

What's your name?

Priyanka.

Nice to meet you, David.

Come on over and say if you want to wait right here and what's your name?

Calvin.

David.

Nice to meet you.

Come on over here.

Where?

Crown kiss.

So proud how you raise your hand first.

So you get to choose.

Do you want to go first or second in this little challenge?

ahead.

Okay, So Priyanka is gonna go first if you want to stand over there.

Kalman.

So the challenge ahead here is Could you go ahead and represent for us in binary, using each of these light bulbs and in turn, switches a zeros and ones say the number 50 they might turn one light bulb on representing the 30 two's place might turn a light.

Boban, representing the eights place or total count now is 32 plus not eight plus 16 I think.

Which is going to give us 32 plus 16 which is 48.

And so we get now a round of applause if we could, Priyanka, thanks very much.

Give us just a moment.

So each of these lightbulbs then represents just a switch orbit and inside of your computer.

If you've ever heard the phrase transistor, a transistor is just a tiny little switch and our computers that they have millions or billions of these switches that they use physically to represent information in store values, just like Priyanka did here.

So if a computer were to represent the number 50 it would literally turn on three switches of sorts.

Store a little bit of electricity here, here and here to represent the number 50.

And it would leave off all of the other switches.

The other five.

In this case, if we're using eight bits or one bite Calvin, you raise your hand seconds.

And so we have one other challenge ahead.

Fortunately, these things are magnetic.

So let's take things up a notch.

And if you would Calvin, how about the number 13 if you will?

How would a computer represent the number 13 where each of these light bulbs from one toe 128 represents a bit?

We had, of course, the ones place over here.

The tooth place 48 16 and so forth.

So we can ask the audience.

Should we turn on for instance, this bulb here.

No way to big.

How about this one?

Okay.

And you're in charge as the audience, This one.

Okay, so we have 1248 four gives us eight plus four is 12 and another round of applause.

Thank Thank you.

Got tougher job.

Thanks to you both.

So, at the end of the day, while this is a very large physical incarnation of the notion of minor.

That's all that's going on inside of our computers every day when they represent information.

But we only thus far have the ability to represent numbers.

It seems.

So.

How, then, does a computer allow you to send text messages and e mails and composed documents and so forth?

We need to all agree how we're going to go about representing characters or letters of an alphabet piot, English or something else instead, and any intuition for how, if a computer only has millions or billions of switches that can be turned on and off there by representing numbers, we could possibly go about representing something other than numbers like the letter A sagan by a digit.

So we may be just need to agree.

As a group.

You know what?

Let's all agree that the letter A in the context of a word processing program or a text message or email just needs to be represented by a number, and we all need to agree what that number is.

So maybe super simply, Let's just say a is one B is to see is three and so forth, and you could imagine them having a computer turn on lots of little transistors to represent A and B and C, so long as the software on that computer as well see, knows that it's a word processing program and not, say, a calculator or something that's meant to use numbers alone.

So it turns out that computers don't actually use the number one for a or two.

For B.

It's a little bigger than that.

The world decided decades ago that the Capital Letter A is gonna be represented by the number 65 65 which is to say, if you have a bite of information in the computer and this is the ones, place twos for a 16 30 to 64.

If a computer were to store the capital letter a using eight bits or switches, it would just turn on those to the 64th place and the ones place.

And so when you have actually received a message in a text message or an email with the Capital Letter A.