Name: CS50 2017 - Lecture 3 - Algorithms
Uploaded: 2018-02-11T09:57:36.000Z
Duration: 1 h 36 min 56 s
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And today, the goal is to be a lot
more algorithmic than code-oriented,

and to really kind of come back to
where we started in lecture zero,

when we talked about computational
thinking, and algorithms.

Because now you have a
few weeks under your belt,

and you have a few new tools, and
hopefully skills, under your belt,

even though odds are,
you're still getting

And so let's look, then,
at this piece of hardware

DAVID MALAN: Yeah, memory,
or RAM, random access memory.

And this happens to be
pretty big on the screen,

but it's actually pretty
small, if you were

to find it in your Mac, or PC, in
your laptop, or some other device.

And so, memory is places you can store
information-- numbers and characters

And recall that if you want to use
this memory, you need to address it.

You need to put something
in particular locations.

And we're going to start doing this
all the more So in particular, there's

four black chips on here, which is
actually where the zeros and ones are

And little gold connectors
you see on the screen,

too-- that just allows
all of those black chips

to intercommunicate, ultimately, and
for the zeros and ones to move around.

But if we zoom in on just
one of these chips of memory,

arbitrarily, and then zoom in here, you
can think of this, wonderfully enough,

Could be any shape, certainly, but
if we think about it as a rectangle,

we can divide this chip of
memory up into a grid, like this.

Completely arbitrary how I've
drawn this, but the fact, now,

that I have a whole
bunch of small boxes--

you can start to think of this
as being one byte, and then

And I coincidentally wrote eight bytes
across, but it could be any number.

And so, you can start to
think about these bytes

as having specific locations,
or numbers, or addresses,

So if I zoom in on the
top of this, you could

start to think of the top
left as being byte number 0,

because computer
scientists generally start

counting at zero, and then byte 1,
and 2, and 3, and 4, and dot dot dot.

And then, just as a refresher, in
a typical Mac or PC these days,

how much RAM, or memory,
would you typically have?

And giga means billion, so
that's four billion bytes.

And maybe you have a little less,
maybe you have a little more,

So we're only scratching the surface
of just how much storage there is,

and how many numbers and
characters you can store.

But the point is that we can address
them, in some way-- zero, one, two,

So this is how we stored
Stelios' name in memory.

Recall, this was an
example last time, where

we needed to put the characters
of his name somewhere.

And so we actually put
the S, and then the T,

and then so forth, starting at
the leftmost location on forward.

It's not sufficient just to put
the letters of his name in memory.

DAVID MALAN: Yeah, the zero command,
or more specifically, the zero byte,

N-U-L. And it's
represented as backslash 0.

Because if you were just
to type a normal zero,

that would technically be a
character from your keyboard.

It looks like a number, but it's
technically a character, a char.

And so backslash zero literally means
8-0 bits in that byte's location.

So now that we have this
ability to represent Stelios

with a sequence of characters,
null-terminated at the end,

we don't really need to worry
about the hardware anymore.

We can abstract away from that
just as we did in week zero,

and not worry about how those
zeros and ones are stored.

We can just trust that
they are, somehow.

And so we can abstract
away, and just start

thinking about it as a sequence,
or a grid, of letters like this.

But that backslash zero
is important, because it

is the only way that this
language we're currently using,

If you didn't have that, it
would-- printf, for instance,

might just keep printing all of the
contents, all four gigabytes, of memory

that your computer has, no matter
what those characters actually are.

And then, of course, you couldn't
distinguish Stelios' name from Maria,

So, let me go ahead and open up the
IDE, just to demonstrate now what

Suppose I wanted to write
a program that allows

me to let a user type
in his or her name,

and then I just want to print
out his or her initials.

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CS50 2017 - Lecture 3 - Algorithms

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ultimately

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process