Name: 簡介 - 計算機科學入門 (Introduction - Intro to Computer Science)
Uploaded: 2021-01-14T05:33:21.000Z
Duration: 3 min 6 s

This unit introduces the next big idea we need for a web crawler,

這單元介紹下一個建立網頁蜘蛛所需的重要概念

基礎被動語態

也就是 “結構化資料” (structured data)

And by the end of this unit you will have finished building a working web crawler.

在這單元結束時，你將完成一個網頁爬蟲 (web crawler)

The closest thing we've seen so far to structured data

我們目前見過，最接近 “結構化資料” 的例子就是

is the string data type introduced in Unit 1 and used

第一單元所介紹的字串資料類型 (data type)

它也使用在第二單元的許多程序裡

A string is a kind of structured data, and that's because

we can break it down into its characters.

and we can operate on sub sequences of the string.

我們可以對字串的次序列 (sub sequences) 進行操作

What we could do with strings was somewhat limited, though,

然而，我們能對字串做的事頗為有限

because the only thing we can put in a string is a character.

Today, we're going to introduce the list data type,

今天我們將介紹列表 (list) 資料類型

and lists are much more powerful than strings,

列表 (lists) 比字串的功能更加強大

so whereas for a string, all of the elements had to be characters,

字串中所有的元素都必須是字元

in a list, the elements can be anything we want.

而列表 (list) 的元素則沒有限制

They could be characters. They could be strings.

They could be numbers. They could also be other lists.

可以是數字，也可以是其他列表 (list)

When we created a string, we just put a sequence of characters

創立字串時，我們只需放入一連串的字元

surrounded by either single or double quotes.

and we could store that string in a variable by using an assignment.

我們可以用指定 (assignment) 將字串儲存在變數裡

With a list, instead of using quotes to identify the list

對於列表，我們不以引號來辨識列表 (list)

we use square brackets, and the elements are separated by commas.

我們使用中括號，並且以逗點來分隔列表中的元素

And just like with a string, we can assign the list that we created

和字串一樣，我們也能指定列表給變數

to a variable, so we'll store that list in the variable "p."

With a string, we could use the square brackets

對於字串，我們可以用中括號來選擇元素

to select elements, and when we index element 0,

we'll get the first element of the string, a sequence of that character,

會得到字串的第一個元素，該字元的序列

With lists, we can also use square brackets to access elements,

對於列表，我們也能使用中括號來取得元素

that will evaluate to the first element of p,

which is the string containing the single letter y.

這裡就是含有單個字母 y 的字串

With strings, we saw that we could use the colon inside the square brackets

對於字串，我們可以在中括號裡使用冒號

to select a sub string of more than 1 character.

來選擇長度超過一個字元的部分字串

Here we're selecting from position 2 through position 4.

That will give us the third and fourth characters of the string,

會得到字串的第 3 和第 4 個字元

which is the sub sequence, the string "bb."

對於列表，我們可以進行相同的操作

We can select from position 2 to position 4,

but instead of returning a string, it will return a list

It will give us a list of the third and fourth element

我們得到變數 p 的第 3 、第 4 元素的列表

of the variable p, which is the list that we have here.

The general grammar for constructing a list

is to have a square bracket followed by a list

of any number of expressions where the expressions

後面跟著一系列任何數量的運算式

We could create a list using just 2 brackets,

我們可以只用一對括號創立一個列表

a left bracket and a right bracket, and this would create a list

containing 0 elements, also known as the empty list.

這會建立包含 0 個元素的列表，[br]也稱為空列表 (empty list)

We could create a list containing 1 element.

我們可以建立包含 1 個元素的列表

That would be the square brackets with 1 element between them.

Here we've created a list containing just 1 element,

現在我們建立了包含 1 個元素的列表

Or we could create a list with many elements, as we did in the first example,

我們也可以建立包含許多元素的列表，就像第一個例子

where we have all of the strings separated by commas.