Name: CS50 2019--閱讀4--指點江山 (CS50 2019 - Lecture 4 - Pointers)
Uploaded: 2021-01-14T10:50:37.000Z
Duration: 1 min 32 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

情態副詞

And in fact, we can visualize this even a little metaphorically.

So for instance, here is, for instance, a mailbox.

Well it's a variable of type int, called n,

So here's the number 50, which is an integer inside of this variable, today,

represented as a mailbox instead of as a locker.

Well suppose that this mailbox over here is not n but suppose this is p.

If this variable p is a pointer to an integer, namely that one over there,

when I open this door, what am I going to find?

Well I'm hoping I find the equivalent of-- we

picked these up at the Coop earlier --the equivalent

of a conceptual pointer saying the number n is over there.

But what specifically, at a lower level, is actually inside this mailbox

DAVID J. MALAN: Yeah, the address, indeed, 123.

So it's sort of like a treasure map if you will.

Oh, I have to go to 123 to get this value.

Oh, the integer in question is indeed 50.

This is the int that happens to be inside of this variable of type int.

This is the address that's a pointer that's in this other variable, p,

but that is conceptually, simply pointing from one variable

to another, thereby giving any sort of conceptual breadcrumbs.

And we'll see-- frankly, in one week --how amazingly powerful it is.

When you can have one piece of memory pointing at another,

pointing at another, pointing at another,

you can start to construct very sophisticated data structures,

as they're called, things like family trees,

and lists, and other data structures that you might have heard of.

Or even if you haven't, these will be the underpinnings next week

of all of today's fanciest algorithms used by,

certainly the Googles, and the Facebooks,

and the Microsofts of the world to manage large data sets.

That's where we're going next week, in terms of application.

AUDIENCE: Does that mean that your memory has to be twice as big?

DAVID J. MALAN: Sorry can you say it once more?

AUDIENCE: Is that to say your memory has to be twice as big to store pointers?

DAVID J. MALAN: Ah, really good question.

Is it the case that your pointers need to be twice as big?

Not necessarily, just, this is the way life is these days.

On most modern Macs and PCs, pointers use 64 bits-- the equivalent of a long,

if you recall that brief discussion in Week 1.

So I deliberately drew my pointer on the screen

I've deliberately drawn my integer n as taking up 4 bytes or 32 bits.

That is convention these days on modern hardware.

Frankly, I could not find a bigger mailbox at Home Depot,

so we went with two identical different colored ones.

So moving from this to something more familiar now, if you will.

Recall that we've been talking about strings for quite some time.

And in fact, most of the interesting programs we've written thus far

involve maybe input from the human and some form of text

But string we said in Week 1 is a bit of a white lie.

I mean, it is the training wheels that I promised

So let's consider what a string actually is now in this new context.

So if we have a string like EMMA here, declared in a variable

called s, and quote unquote, EMMA in all caps, as we've done a couple of times

What does this actually look like inside of the computer?

Well somewhere in my computer's memory there are four, nay, five bytes,

storing E-M-M-A, and then additionally, that null terminating character that

demarcates where the end of the string is.

So that's where EMMA might be represented in the computer's memory.

But recall that the variable in question was s.

And so that's why over the past few weeks

any time you want to manipulate a string, you use its name, like s.

And you can access bracket 0, bracket 1, bracket 2, bracket 3,

to get at the individual characters in that string like EMMA, E-M-M-A,

But of course it's the case, especially per today's revelation, that really,

all of those bytes have their own addresses.

We're not going to care after this week what those addresses are

They're deliberately 1 byte away because remember a string is defined

So let's say for the sake of discussion that EMMA name in memory

Well, what then really is that variable s?

字幕列表影片播放

CS50 2019--閱讀4--指點江山 (CS50 2019 - Lecture 4 - Pointers)

literally

deliberately

instance

character