字幕列表 影片播放 列印所有字幕 列印翻譯字幕 列印英文字幕 Welcome back for Unit 3. 歡迎回到第三單元 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 which is structured data. 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) in many of the procedures in Unit 2. 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 字串是一種結構化資料 we can break it down into its characters. 是因為我們能將它分解為字元 The string has a sequence of characters, 字串有一連串的字元 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. 因為我們在字串中只能放字元 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. 可以是字元,也可以是字串 They could be numbers. They could also be other lists. 可以是數字,也可以是其他列表 (list) Let's look at an example. 我們來看個例子 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. 用單引號或雙引號括起來 Here's an example of a string, 這是字串的例子 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." 現在我們將列表儲存於變數 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, 當我們指標到元素 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 which is the character "y." 字元 'y' 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 so if we do p[0], 如果執行 p[0] that will evaluate to the first element of p, 就表示要取得 p 的第 1 個元素 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. 這裡我們選擇位置 2 到位置 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." 也就是次序列 'bb' We can do the same thing with lists. 對於列表,我們可以進行相同的操作 We can select from position 2 to position 4, 我們可以選擇位置 2 到位置 4 but instead of returning a string, it will return a list 但得到的不是字串 containing those elements. 而是包含這些元素的列表 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. 就是這個列表 The general grammar for constructing a list 建構列表的通用文法 is to have a square bracket followed by a list 是使用中括號 of any number of expressions where the expressions 後面跟著一系列任何數量的運算式 are separated by commas. 由逗點將其分開 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 左括號和右括號 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. 也就是中括號裡有一個元素 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 which is the number 3. 數字 3 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. 所有的字串被逗點分開
B1 中級 中文 元素 單元 變數 序列 資料 位置 簡介 - 計算機科學入門 (Introduction - Intro to Computer Science) 445 36 羅紹桀 發佈於 2021 年 01 月 14 日 更多分享 分享 收藏 回報 影片單字