437 lines
10 KiB
Markdown
437 lines
10 KiB
Markdown
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# Python 列表备忘单
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> 原文：<https://www.pythonforbeginners.com/lists/python-lists-cheat-sheet-2>
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## 什么是列表？
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Python 列表用于存储数据集合。Python 可以将多个值赋给一个列表，这在处理大量数据时非常方便。
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列表可以保存任何类型的数据，包括整数、字符串，甚至其他列表。列表是动态的，可以更改。使用特殊的方法，我们可以在 Python 列表中添加或删除项目。
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列表中的元素是有索引的，每个元素在列表的顺序中都有明确的位置。与 Python 字符串不同，列表的内容是可以改变的。
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## 列表创建
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Python 列表是用方括号写的。列表中的元素用逗号分隔。稍后，我们将看到如何添加和删除元素。
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```py
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# a list for days of the work week
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weekdays = ["Monday","Tuesday","Wednesday","Thursday","Friday"]
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# an empty list just waiting to do something
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empty_list = []
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# lists can hold data of different types
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mix_list = ["one","two",1,2]
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```
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## 查找列表的长度
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使用 **len()** 方法计算列表的长度。这个方法将返回列表中元素的总数。
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```py
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nums = [0,1,2,3,4,5,6,7,8,9]
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# print the total number of items in the list
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print("Length of the list: ", len(nums))
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```
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**输出**
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```py
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Length of the list: 10
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```
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## 追加列表
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我们可以使用 **append()** 方法向列表中添加条目。新元素将出现在列表的末尾。
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```py
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# a list of popular car manufacturers
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car_brands = ["BMW","Ford","Toyota","GM","Honda","Chevrolet"]
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# add to a list with append()
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car_brands.append("Tesla")
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```
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## 列表插入
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在上面的例子中，我们看到我们可以将项目添加到列表的末尾。如果我们想把一些东西放在开头，甚至中间呢？
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用 **insert()** 方法，我们可以指定在列表中的什么地方添加一个新元素。
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```py
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letters = ['B','C','D','E','F','G']
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letters.insert(0,'A') # add element 'A' at the first index
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print(letters)
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```
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**输出**
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```py
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['A', 'B', 'C', 'D', 'E', 'F', 'G']
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```
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**列表插入语法:**
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```py
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my_list.insert(x,y) # this will insert y before x
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```
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```py
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# an example of inserting an element into the third position in a list
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top_five = ["The Beatles","Marvin Gaye","Gorillaz","Cat Power"]
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top_five.insert(2, "Prince")
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print(top_five)
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```
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**输出**
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```py
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['The Beatles', 'Marvin Gaye', 'Prince', 'Nirvana', 'Cat Power']
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```
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## 从列表中删除元素
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从列表中删除一个元素，使用 **remove()** 方法。这个方法将找到列表中第一个出现的条目并删除它。
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```py
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# a basic to do list
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to_do = ["dishes","laundry","dusting","feed the dog"]
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# we already fed Fido!
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to_do.remove("feed the dog")
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print("Things to do: ", to_do)
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# remove the first 3 in the list
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nums = [1,2,3,3,4,5]
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nums.remove(3)
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print(nums)
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```
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**输出**
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```py
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Things to do: ['dishes', 'laundry', 'dusting']
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[1, 2, 3, 4, 5]
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```
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建议阅读:[如何用 Python 制作聊天 app？](https://codinginfinite.com/python-chat-application-tutorial-source-code/)
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## 扩展列表
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Python 提供了一种用 **extend()** 方法连接列表的方法。使用这种方法，一个列表的元素将被添加到另一个列表的*末端*。
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```py
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# we need a list of items to send to the movers
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furniture = ["bed","chair","bookcase"]
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# add additional elements with extend()
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furniture.extend(["couch","desk","coffee table"])
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print(furniture)
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```
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**输出**
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```py
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['bed', 'chair', 'bookcase', 'couch', 'desk', 'coffee table']
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```
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## 使用 pop()删除元素
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除了 remove()，我们还可以使用 **pop()** 方法从列表中移除元素。使用 pop()方法移除特定索引处的元素。
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```py
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nums = [1,2,3,4]
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nums.pop(1)
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print(nums)
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```
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**输出**
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```py
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[1, 3, 4]
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```
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位于索引 1 的元素已被移除。如果我们不向 pop()传递索引，它将从列表中删除最后一项。
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```py
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# generate a list of numbers 1-10
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nums = [x for x in range(1,11)]
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# pop the last element off the list
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nums.pop()
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print(nums)
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```
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**输出**
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```py
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[1, 2, 3, 4, 5, 6, 7, 8, 9]
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```
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## 关键词
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在处理列表时，有几个 Python 关键字很方便。关键字中的**可以用来检查一个项目是否在列表中。**
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在中使用**的语法如下:**
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```py
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list_item in list
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```
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下面是一个使用关键字中的**来确定列表是否包含特定字符串的示例:**
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```py
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the_beatles = ["John","Paul","George","Ringo"]
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print("Was John in the Beatles? ","John" in the_beatles)
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```
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**输出**
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```py
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Was John in the Beatles? True
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```
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另一个有用的关键词是**不是**。通过使用 not，我们可以确定字符串中是否缺少某个元素。
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```py
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print("So Yoko wasn't a member of the Beatles? ","Yoko" not in the_beatles)
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```
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**输出**
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```py
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So Yoko wasn't a member of the Beatles? True
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```
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## 反转列表
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在 Python 中反转列表最简单的方法是使用 **reverse()** 方法。该方法对列表进行重新排序，使最后一个元素成为第一个元素，反之亦然。
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或者，我们可以使用 Python 切片符号反向遍历列表。
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```py
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superheroes = ["Batman", "The Black Panther", "Iron Man"]
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# use slice notation to traverse the list in reverse
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for hero_name in superheroes[::-1]:
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print(hero_name)
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# use the reverse method to reverse a list in place
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superheroes.reverse()
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print(superheroes)
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```
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**输出**
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```py
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Iron Man
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The Black Panther
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Batman
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['Iron Man', 'The Black Panther', 'Batman']
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```
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## 列表排序
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使用 Python 的 **sort()** 方法对列表中的元素重新排序。默认情况下，sort()将重新排列列表，以便它包含的项目按升序排列。例如，对数字列表使用 sort 会将数字从最小到最大排序。
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```py
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nums = [100,2003,1997,3,-9,1]
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nums.sort()
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print(nums)
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```
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**输出**
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```py
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[-9, 1, 3, 100, 1997, 2003]
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```
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或者，在字符串上使用 sort()将把项目按字母顺序排列。
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```py
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alphabet = ['B','C','A']
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alphabet.sort()
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print(alphabet)
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```
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**输出**
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```py
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['A', 'B', 'C']
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```
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如果需要保持原来的列表不变，选择 **sorted()** 方法。sorted()方法返回一个新的列表，保持原来的列表不变。
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```py
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nums = [7,2,42,99,77]
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# sorted will return a new list
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print("Modified list:", sorted(nums))
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print("Original list: ", nums)
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```
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**输出**
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```py
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Modified list: [2, 7, 42, 77, 99]
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Original list: [7, 2, 42, 99, 77]
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```
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## 列表索引
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使用*索引*来引用列表中的项目。索引代表项目在列表中出现的顺序。
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列表中的第一项位于索引 0 处。第二个在索引 1，依此类推。
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```py
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villains = ["Shredder","Darth Vader","The Joker"]
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print(villains[0])
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print(villains[1])
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print(villains[2])
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```
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**输出**
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```py
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Shredder
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Darth Vader
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The Joker
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```
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与 Python 字符串不同，列表是可以改变的。例如，我们可以使用 Python 来交换列表中第一项和第三项的内容。
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```py
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# swamp the first and third items of the list
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temp = villains[2]
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villains[2] = villains[0]
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villains[0] = temp
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```
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然而，有一种更简单的方法来淹没 Python 中的列表项。
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```py
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# swap list items with the power of Python!
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villains[0],villains[2]=villains[2],villains[0]
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```
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## 限幅
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Python 切片允许我们从一个列表中检索多个项目。切片的符号是期望范围的开始和结束之间的冒号。
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**语法**:
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```py
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my_list[start:end:step]
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```
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对于一个给定的列表，切片符号查找起始索引和结束索引。这告诉 Python 我们要寻找的项目的范围。
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可选地，我们可以指定遍历列表的**步骤**。该步骤告诉 Python 如何遍历列表。例如，我们可以提供一个负数来反向遍历列表。
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```py
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rainbow = ['red','orange','yellow','green','blue','indigo','violet']
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print(rainbow[1]) # get the second item in the list
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print(rainbow[:1]) # get items at indexes 0 and 1
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print(rainbow[1:3]) # items at index 1 and 2
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print(rainbow[:-1]) # all items excluding the last
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```
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**输出**
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```py
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orange
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['red']
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['orange', 'yellow']
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['red', 'orange', 'yellow', 'green', 'blue', 'indigo']
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```
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## 循环和列表
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因为 Python 中的列表是有索引的，所以我们可以使用循环来遍历它们的元素。
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```py
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# a list of random numbers in ascending order
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nums = [2,4,7,8,9,10,11,12,13,15,16,17]
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# a list of prime numbers
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primes = [2,3,5,7,11,13,17]
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# loop through a Python list
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for num in nums:
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if num in primes:
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print(num,end=" ")
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
**输出**
|
|||
|
|
|
|||
|
|
```py
|
|||
|
|
2 7 11 13 17
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## 列出方法
|
|||
|
|
|
|||
|
|
我们已经看到了 Python 列表方法的例子，比如 reverse()和 sort()。不幸的是，这篇文章没有足够的篇幅来涵盖它们，但是我们提供了一个您应该知道的列表，并描述了它们的作用。
|
|||
|
|
|
|||
|
|
* **Append():** 在列表末尾添加一个新项目。
|
|||
|
|
* **Count()** :返回列表中项目的总数。
|
|||
|
|
* Clear(): 从列表中删除所有项目。
|
|||
|
|
* **Extend():** 将一个列表的元素连接到另一个列表的末尾。
|
|||
|
|
* **Index():** 查找列表中某项的索引。
|
|||
|
|
* 将一个条目添加到列表中给定的索引处。
|
|||
|
|
* 从列表中删除最后一项。
|
|||
|
|
* 从列表中删除一个特定的项目。
|
|||
|
|
* **Reverse():** 从最后一项到第一项对列表进行重新排序。
|
|||
|
|
* **Sort():** 对列表进行升序排序。
|
|||
|
|
|
|||
|
|
## 例子
|
|||
|
|
|
|||
|
|
让我们以一些在 Python 中使用列表和列表方法的例子来结束本文。
|
|||
|
|
|
|||
|
|
#### 示例 1:计算数字列表中所有项目的总和
|
|||
|
|
|
|||
|
|
```py
|
|||
|
|
nums = [98,62,77,84,89]
|
|||
|
|
|
|||
|
|
total = 0
|
|||
|
|
for i in range(len(nums)):
|
|||
|
|
total += nums[i]
|
|||
|
|
|
|||
|
|
print("Total: ", total)
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
**输出**
|
|||
|
|
|
|||
|
|
```py
|
|||
|
|
Total: 410
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
#### 示例 2:计算一组数字的平均值
|
|||
|
|
|
|||
|
|
```py
|
|||
|
|
# find the average for a list of numbers
|
|||
|
|
nums = [20,22,1.5,2,7,5.2,99]
|
|||
|
|
|
|||
|
|
total = 0
|
|||
|
|
i = 0
|
|||
|
|
while i < len(nums):
|
|||
|
|
total = total + nums[i]
|
|||
|
|
i = i + 1
|
|||
|
|
|
|||
|
|
average = total/len(nums)
|
|||
|
|
print("The average to 2 decimal places: {:.2f}".format(average))
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
**输出**
|
|||
|
|
|
|||
|
|
```py
|
|||
|
|
The average to 2 decimal places: 22.39
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## 相关职位
|
|||
|
|
|
|||
|
|
* [学习 Python 中的列表理解](https://www.pythonforbeginners.com/basics/list-comprehensions-in-python)
|
|||
|
|
* [探索 Python 字符串串联](https://www.pythonforbeginners.com/concatenation/string-concatenation-and-formatting-in-python)
|
|||
|
|
* [反转列表和字符串](https://www.pythonforbeginners.com/code-snippets-source-code/reverse-loop-on-a-list)
|