>> DAVID J. MALAN: All right.

>> 戴維·J·馬蘭：好吧。

This is CS50.

這是CS50。

And this is the start of week 5.

這是5週的開始。

And as you may have noticed, some of the material

正如你可能已經注意到， 一些材料

is getting a little more complex, the little denser.

越來越多一點 複雜，小更密。

>> And it's very easy, especially if you've been in the habit for some time,

>> 而且它是很容易的，尤其是當 你已經在習慣了一段時間，

to be trying to scribble down most anything we do, we're saying in class.

是為了要塗下最 什麼我們做什麼，我們說在課堂上。

But realize, that is not perhaps the ideal pedagogical approach

但要意識到，這也許不是 理想的教學方法

to learning this kind of material, and material more generally.

學習這種物質， 而材料更普遍。

And so we are pleased to announce that CS50's own Gheng

因此，我們很高興 宣布，CS50自己Gheng

Gong has begun to prepare a canonical set of notes

龔已開始準備 一個規範的集合票據

for the course, the hope of which is that, one, these

用於操作過程中，希望 其是，一個，這些

not only serve as a reference and a resource

不僅作為 參考和資源

for reviewing material and going back through material that might have

審查材料中去 回通過材料可能具有的

escaped you the first time around, but also so that your heads can be more

逃脫你在第一時間左右，但 也讓你的頭可以更

up than down, when it comes time to lecture,

向上比下降，當它 談到時間來講課，

so that you might engage more thoughtfully, as

所以，你可能搞 更周到，為

opposed to more scribbly.

相對於更雜亂。

>> With that said, what you'll find on the website is such documents as this.

>> 雖這麼說，你會發現 該網站是這樣的文件，因為這。

And notice, at top left, there's not only a table of contents,

並請注意，在左上角，有 不僅表的內容，

but also time codes that will immediately jump you

而且時間碼的 會立即跳到你

to the appropriate part in the video online.

到相應的部分 在視頻網上。

And what Chang here has done is, essentially, documented

什麼常在這裡做了 在本質上，記載

what happened in this particular lecture.

發生了什麼事在這 特別講座。

And many of the lectures are already online now with this URL.

和許多的講座是 已經在線使用這個網址。

And we'll continue to post the remainder of those by the end of this week,

我們將繼續張貼其餘 那些在本週末結束，

so do take advantage of that resource.

所以不要採取資源優勢。

>> So without further ado, we started to peel back

>> 因此，事不宜遲， 我們開始剝離回

the layer that has been string for some time.

已經作為層 串持續一段時間。

And what did we say a string actually is last week?

並沒有說什麼一個字符串 上週居然是？

So char star.

因此，焦炭的明星。

And char star, well, what did that really mean?

和CHAR星，那麼， 這樣做究竟意味著什麼？

Well, all this time, if we've been calling a function,

那麼，這一切的時候，如果我們已經 被調用的函數，

like getString, and storing the so-called return

喜歡的getString和存儲 所謂返回

value of getString in a variable-- it's called

的getString的價值 變量 - 這就是所謂的

s type string-- we've been writing the line of code up there above.

S型string--我們已經寫 這行代碼在那裡上面。

And it's only when I see my handwriting magnified here

而且它是只有當我看到我的 在這裡手寫放大

do I realize just how atrocious this is.

我意識到多麼惡劣，這是。

>> However, let's assume that, on the right-hand side

>> 但是，讓我們假設， 在右手側

is, nonetheless, a reasonable depiction of what's

是，但是，合理的 一個什麼樣的描述

been going on all this time with getString.

一直在進行這一切 時間的getString。

getString, of course, gets a string.

的getString，當然，得到​​的字符串。

But what does that really mean?

但是，這究竟意味著什麼？

It means it gets a chunk of memory from the operating system

這意味著它得到了一大塊 來自操作系統的內存

by calling a function, called malloc.

通過調用一個函數，稱為malloc的。

But more on that later.

但後​​來更多。

And then it populates that chunk of memory

然後填充 該內存塊

with the letters the user has typed in, followed by, of course,

以字母的用戶具有 當然鍵入，緊接著，，

a null character, or backslash zero at the very end.

一個空字符或反斜杠 零在最後。

>> Meanwhile, on the left-hand side of this story, all this time,

>> 同時，在左手側 這個故事，這一切的時候，

we've been declaring a variable, like s.

我們已經聲明了一個變量，如第

And that variable is what now will start calling a pointer.

而這個變量就是現在 將開始調用的指針。

It's not a box inside of which we put the string, Daven, per se,

這不是這裡面一箱 我們把串，Daven，本身

but rather we put in that square box on the left what exactly?

而是我們把在這方 箱體左側究竟是什麼？

Yeah?

是嗎？

>> AUDIENCE: The address of where it's located in memory.

>> 聽眾：的地址 它位於內存中。

>> DAVID J. MALAN: Exactly.

>> 戴維·J·馬蘭：沒錯。

The address of where Daven is located in memory.

Daven的所在地址 位於存儲器。

And not where all of Daven is located, per se, but specifically the address

不，所有Daven的所在， 本身，而是專門的地址

of what?

什麼？

Yeah?

是嗎？

>> AUDIENCE: First character.

>> 聽眾：第一個字符。

>> DAVID J. MALAN: The first character in Daven, which, in this case,

>> 戴維·J·馬蘭：第一個字符 在Daven，其中，在這種情況下，

I proposed was arbitrarily and unrealistically 1, Ox1,

我提出了任意 和不切實際1，OX1，

which just means the hexadecimal number of 1.

它的意思是， 1的十六進制數。

But it's probably going to be a much bigger number

但是，它可能會 是一個更大的數字

that we might draw with a 0x as a prefix,

我們可能會得出 以0X作為前綴，

representing a hexadecimal character.

代表一個十六進制字符。

And because we don't need to know where the rest of the characters of Daven

而且，由於我們並不需要知道在哪裡 Daven的字符的其餘部分

are, because of what simple design decision that was made many years ago?

是因為，有什麼簡單的設計 決定這是多年前？

Yeah?

是嗎？

>> AUDIENCE: Backslash 0.

>> 聽眾：反斜杠0。

DAVID J. MALAN: Yeah, exactly.

戴維·J·馬蘭：是的，沒錯。

The backslash 0 allows you, albeit in linear time, to traverse the string,

反斜線0可以讓你，儘管在 線性時間，來遍歷字符串

walk from left to right, with a for loop, or a while

由左到右行走， 用for循環，或同時

loop, or something like that, and determine, oh, here

環或類似的東西 這一點，決定了哦，這裡

is the end of this particular string.

是這個特定字符串的結尾。

So with just the address at the beginning of a string,

所以只用在地址 字符串的開始，

we can access the entirety of it, because all this while,

我們可以訪問的全部 它，因為這一切的同時，

a string has just been a char star.

一個字符串，剛剛一個char明星。

>> So it's certainly fine to continue using the CS50 library and this abstraction,

>> 因此，這的確精緻繼續使用 在CS50庫，而且這樣的抽象，

so to speak, but we'll begin to see exactly

可以這麼說，但我們會 首先，看看到底

what's been going on underneath this whole time.

什麼是怎麼回事 下面這整個時間。

So you may recall this example, too, from last time, compare 0,

所以，你可能還記得這個例子中， 同樣，從去年的時候，比較0，

which didn't actually compare.

這實際上並沒有進行比較。

But we began to solve this.

但是，我們開始解決這個問題。

>> But as perhaps a refresher, might I interest someone

>> 但是作為也許是複習， 我可能感興趣的人

in a pink elephant today, also made by Chang?

粉紅色的大象的今天， 還長呢？

How about you in front? [INAUDIBLE].

怎麼樣，你在前面？ [聽不清]。

Come on up.

上來吧。

>> And in the meantime, as you come up, let's

>> 並且在此期間， 你上來，讓我們

consider for just a moment what this code was actually doing.

考慮了一會兒就好了什麼 這段代碼實際上做的事情。

It's declaring two variables up top, s and t, and calling getString.

它聲明兩個變量了 頂，S和T，並調用的getString。

This isn't a very user-friendly program, because it doesn't tell you what to do.

這不是一個非常用戶友好的程序， 因為它不會告訴你的事。

But let's just assume we're focusing on the juicy part.

但是，讓我們姑且我們 重點是多汁的一部分。

And then we do, if s equals equals t, it should say printf,

然後我們做的，如果s等於 等於T，應該說printf的，

you typed the same thing.

您鍵入同樣的事情。

Hello.

你好。

What's your name?

你叫什麼名字？

>> JANELLE: Janelle.

>> 詹妮爾：詹妮爾。

DAVID J. MALAN: Janelle, nice to meet you.

戴維·J·馬蘭：詹妮爾， 很高興認識你。

So your challenge at hand for this elephant

所以，你在挑戰 手這頭大象

is to first draw us a picture of what's being represented in those first two

是第一個吸引我們的是什麼圖片 被代表的前兩個

lines.

線。

So s and t might be represented how on the screen?

因此，S和T可能是 顯示在畫面上如何？

And you can just draw it with your finger on this big screen.

而且你可以繪製與 將手指放在這個大屏幕上。

>> So there's two halves to each side of that equation.

>> 因此，有兩部分，以 該方程的每一側。

So there's s on the left, and then getString on the right.

所以有S上的左側，並且 然後GetString的右邊。

And then there's t on the left, and then getString on the right.

再有就是噸左邊， 然後GetString的右邊。

So how might we begin drawing a picture that

那麼我們如何開始 繪製的畫面，

represents what's going on here in memory, would you say?

表示這是怎麼回事 這裡的記憶，你會說什麼？

And let me let you explain what you're doing as you go.

讓我讓你解釋 你在做什麼，當您去。

>> JANELLE: OK.

>> 詹妮爾：好的。

Well, first, it would be asking you to get the input string.

嗯，首先，它會問 你得到輸入字符串。

And it would store-- oh, sorry.

它會store--哦，對不起。

DAVID J. MALAN: OK.

戴維·J·馬蘭：確定。

Good.

好。

And this is called what?

而這個叫什麼？

Oh, OK.

哦，好吧。

Keep going.

繼續前進。

I didn't mean to interrupt.

我不是故意要打斷。

JANELLE: Sorry.

詹妮爾：對不起。

So it would input it into the address of-- not sure.

因此，將其輸入到 地址of--不能確定。

I can't exactly remember the number, but I believe it was starting with 0.

我不記得確切的數字， 但我相信這是從0開始。

>> DAVID J. MALAN: That's all right, because I made the numbers up,

>> 戴維·J·馬蘭：沒關係， 因為我做的數字了，

so there's no right answer.

所以沒有正確的答案。

>> JANELLE: Starting with the 0 arc.

>> 詹妮爾：用0起弧。

>> DAVID J. MALAN: OK, so element 0.

>> 戴維·J·馬蘭：好了，元素0。

Sure.

當然。

>> JANELLE: And then if was like just a two-letter--

>> 詹妮爾：然後如果是 像剛才兩個letter--

>> DAVID J. MALAN: OK, back to you.

>> 戴維·J·馬蘭：好了，還給你。

>> JANELLE: So element 0, and then element 1 or element 2.

>> 詹妮爾：所以元素0， 然後元件1或元件2。

DAVID J. MALAN: And which piece of the picture are you drawing right now?

戴維·J·馬蘭：而且這一塊 圖片是你畫的權利嗎？

The call to getString?

給GetString的調用？

Or the declaration of s?

或s的聲明？

>> JANELLE: The declaration of s, I believe.

>> 詹妮爾：報關 的S，我相信。

Oh, the getString, because it would be inputted into each [? area. ?]

哦，對的getString，因為它會 被輸入到每一個[？區。 ？]

>> DAVID J. MALAN: Good.

>> 戴維·J·馬蘭：好。

Exactly.

沒錯。

Even though this effectively returns an array, recall,

儘管這有效地 返回一個數組，調用，

when we get back a string, we can index into that string using 01 and 2.

當我們回到一個字符串，我們可以 索引使用01和2的字符串。

Technically, these are probably represented by individual addresses,

從技術上講，這很可能是 個別地址表示，

but that's fine.

但是這很好。

>> So suppose, if I can just fast forward to where we left off

>> 於是想，如果我可以快 轉發給我們留下了

last time, if one of the strings was g a b e,

最後時刻，如果一個 字符串是克A B E，

backslash 0, thereby representing gabe's input, how might we represent s now?

反斜杠0，從而表示Gabe的 輸入，怎麼可能，我們代表就談到？

If this is the memory that's been returned by getString?

如果是這樣的存儲器那 被送回了的getString？

>> JANELLE: Would it be represented by an arc?

>> 詹妮爾：這將是 由電弧所表示？

>> DAVID J. MALAN: By an arc?

>> 戴維·J·馬蘭：通過電弧？

Well, no.

哦，不。

Let's just say, pictorially, let me just go ahead

遠的不說，形象地， 讓我繼續前進

and propose that, if this is s, this is the return value of getString.

和建議，如果這是秒，這 是的getString的返回值。

And you've drawn this as 0, 1, 2, which is perfectly reasonable, because we

而你畫這是0，1，2，其中 是完全合理的，因為我們

can index into the string, as such.

可以索引到字符串，作為這樣。

But just to be consistent with last time, let me go ahead

但剛需一致 最後一次，讓我先走

and arbitrarily propose that this is address 1, this is address 2,

並隨意提出這 是地址1，這是地址2，

this is address 3, and so forth.

這是地址3，依此類推。

And so, just to be super clear, what's going

因此，剛需超 顯然，這是怎麼回事

to go in s as a result of that first line of code, would you say?

s中去，作為一個結果， 代碼的第一行，你會說什麼？

>> JANELLE: Address 1?

>> 詹妮爾：地址1？

>> DAVID J. MALAN: Exactly.

>> 戴維·J·馬蘭：沒錯。

So address 0x1.

因此，解決為0x1。

And meanwhile, let me go ahead and duplicate much of what you've done

而與此同時，讓我繼續前進， 重複的很多東西，你做

and add my own t here.

在這裡添加自己噸。

If I were to type in gabe again, a second time,

如果我要在加布型 再一次，第二次，

when prompted with getString, where, of course, is gabe going to go?

當用的getString提示，其中， 當然，被加布要去？

Well, presumably--

那麼，presumably--

>> JANELLE: Like on here?

>> 詹妮爾：像在這裡？

DAVID J. MALAN: Yeah.

戴維·J·馬蘭：是的。

JANELLE: Or it's also in the same boxes?

詹妮爾：或者它也是在同一個箱子？

DAVID J. MALAN: Let me propose, yeah, exactly, so in these additional boxes.

戴維·J·馬蘭：我建議，是啊， 準確，因此，在這些附加的盒子。

But what's key now is that, even though I've drawn these pretty close

但是，什麼是現在的關鍵是，即使 雖然我畫這些八九不離十

together-- 0x1, this is 0x2-- in reality,

together--為0x1，這 被0x2--在現實中，

this now might be address 0x10, for instance, and 0x11, and 0x12,

這個現在可能是地址為0x10， 例如，和為0x11，以及0x12的

and so forth.

等等。

And so, if that's the case, what's going to end up here in t?

因此，如果是這樣的話， 這是怎麼回事結束了在這裡噸？

>> JANELLE: 0x10?

>> 詹妮爾：為0x10？

DAVID J. MALAN: Exactly.

戴維·J·馬蘭：沒錯。

So 0x10.

因此，為0x10。

And so now, final question.

所以現在，最後一個問題。

You have, by far, had to work the hardest for an elephant thus far.

你有，到目前為止，已經工作了 最難的大象迄今。

By now, if I pull up the code again, when I do, in line three,

事到如今，如果我拉起碼 再次，當我這樣做，在三線，

if s equals equals t, what am I actually comparing that we've drawn here?

如果s等於等於T，我算什麼實際 相比較，我們在這裡畫？

>> JANELLE: The two addresses?

>> 詹妮爾：這兩個地址？

DAVID J. MALAN: Exactly.

戴維·J·馬蘭：沒錯。

So I'm saying is s equal equal to t?

所以，我要說的是S等於等於t？

In other words, is 1 equal equal to 10?

換句話說，是1等於等於10？

And of course, the obvious answer now is, no.

並且，當然，在 答案很明顯，現在是，沒有。

And so this program is ultimately going to print what, would you say?

所以這個方案最終是 要打印的內容，你會說什麼？

>> JANELLE: Would it be, you typed the same thing?

>> 詹妮爾：它會是這樣， 您鍵入同樣的事情？

>> DAVID J. MALAN: So if s is 1 and t is 10?

>> 戴維·J·馬蘭：所以，如果 s是1和t是10？

>> JANELLE: You typed different things.

>> 詹妮爾：輸入不同的事情。

>> DAVID J. MALAN: Exactly.

>> 戴維·J·馬蘭：沒錯。

You typed different things.

你輸入不同的事情。

All right.

好吧。

So a round of applause, if we could, here.

所以，掌聲雷動， 如果我們能在這裡。

[APPLAUSE]

[掌聲]

That was painful.

這是痛苦的。

I know.

我知道。

Nicely done.

很好地完成。

So now let's see if we can't tease apart what the fix was.

所以，現在讓我們來看看，如果我們不能 梳理出什麼樣的修復程序。

And of course, when we fixed this-- which I'll now represent in green--

當然，當我們固定this-- 現在我將代表green--

we did a couple of enhancements here.

我們做了幾個增強功能在這裡。

First, just as a sanity check, I'm first checking

首先，正如一個理智 檢查一下，我先檢查

if s equals null and t equals null.

如果s等於null和T等於null。

And just to be clear, when might s or t be null in code like this?

而只是要清楚，如果可能 s或t為null，在這樣的代碼？

When might s or t be null.

當可能s或t為null。

Yeah?

是嗎？

>> AUDIENCE: [INAUDIBLE].

>> 聽眾：[聽不清]。

>> DAVID J. MALAN: Exactly.

>> 戴維·J·馬蘭：沒錯。

If the string that the user typed in is way too long

如果字符串用戶 鍵入的是太長時間

to fit into memory, or some weird corner case like that,

要裝入內存，或者一些 奇怪的角落情況下那樣，

getString, as we'll see, literally today, in its documentation,

GetString的，正如我們所看到的，從字面上 今天，它的文檔中，

says it will return null as a special sentinel value,

表示將返回null作為 一個特殊的標記值，

or just sort of a special symbol that means something went wrong.

或者只是有點特殊符號 這意味著出事了。

So we want to check for that, because it turns out

因此，我們要檢查 是，因為事實證明

that null is a very dangerous value.

那空是一個非常危險的價值。

>> Often, if you try to do something with null involving a function-- passing it

>> 通常情況下，如果你嘗試做一些與 空涉及函數 - 它傳遞

as input, for instance-- that function might very will crash and, with it,

作為輸入，對instance--該功能 很可能會崩潰，並用它，

take down your whole program.

記下你的整個程序。

So this third line now is just a sanity check, error checking, if you will.

因此，這第三條線，現在僅僅是一個理智 檢查，檢查錯誤，如果你願意。

That's a good habit now for us to get into any time we

這是一個好習慣，現在的 我們進入任何時候我們

try to use a value that could, potentially, be null.

嘗試使用一個值， 可能，可能，是空的。

>> Now, in the fourth line here, "if strcmp(s, t)," well,

>> 現在，在第四行此處， “如果STRCMP（S，T），”好了，

what's that referring to?

那是什麼指？

Well, we said this was a very succinctly named function for string comparison.

好吧，我們說這是一個非常簡潔 命名函數的字符串比較。

And its purpose in life is to compare its first argument against it second,

及其在生命的目的是比較 其反對的第一個參數第二，

but not in terms of their addresses, as we did unintentionally a moment

但不是在它們的地址而言， 因為我們沒有無意中片刻