#python学习笔记（九）#列表：创建、遍历、方法、函数和解析字符串

目录

1 A list is a sequence

2 Lists are mutable

3  Traversing a list

4  List methods

5 Lists and functions

6  Lists and strings

7 Parsing lines

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1 A list is a sequence

Like a string, a list is a sequence of values. In a string, the values are characters; in a list, they can be any type. The values in list are called elements or sometimes items.

列表是一列值的综合，这些值可以是任意类型，如：

[10, 20, 30, 40] 数值列表
['crunchy frog', 'ram bladder', 'lark vomit'] 字符串列表

['spam', 2.0, 5, [10, 20]] 列表中包含列表

▲采用[ ] 创建列表

>>> cheeses = ['Cheddar', 'Edam', 'Gouda']
>>> numbers = [17, 123]
>>> empty = []
>>> print(cheeses, numbers, empty)
['Cheddar', 'Edam', 'Gouda'] [17, 123] []

2 Lists are mutable

▲列表和字符串一样可以用[ ]取出某个或某些元素，但是列表中的元素是可以被更改的mutable

>>> numbers = [17, 123]
>>> numbers[1] = 5
>>> print(numbers)
[17, 5]

▲由于列表的可变性，导致它和字符串有明显的区别

>>> a = 'banana'
>>> b = 'banana'
>>> a is b
True

>>> a = [1, 2, 3]
>>> b = [1, 2, 3]
>>> a is b
False

上述情况中，a和b虽然内容相同，但由于他们的内容时可以发生变化的，所以a和b是两个对象

▲另一种情况：

>>> a = [1, 2, 3]
>>> b = a  ###将a的值赋给b，此时a和b指的是一个变量，a和b互为别名alias
>>> b is a
True

>>> b[0] = 17
>>> print(a)   ###改变b时，a也发生变化
[17, 2, 3]

▲in，+，*运算符也可以运用于列表 

>>> cheeses = ['Cheddar', 'Edam', 'Gouda']
>>> 'Edam' in cheeses
True
>>> 'Brie' in cheeses
False


>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = a + b
>>> print(c)
[1, 2, 3, 4, 5, 6]


>>> [0] * 4
[0, 0, 0, 0]
>>> [1, 2, 3] * 3
[1, 2, 3, 1, 2, 3, 1, 2, 3]

3  Traversing a list

▲方法一：for循环遍历每个列表元素，迭代变量为列表里的元素

for cheese in cheeses: ###迭代变量为列表里的元素
    print(cheese)

▲方法二：for循环遍历列表的顺序，再用[ ]取出对应位置的值，迭代变量为列表的位置

for i in range(len(numbers)): ###range函数返回一个从0到n-1的列表
    numbers[i] = numbers[i] * 2

4  List methods

方法	描述
append()	在列表的末尾添加一个元素
clear()	删除列表中的所有元素
copy()	返回列表的副本
count()	返回具有指定值的元素数量。
extend()	将列表元素（或任何可迭代的元素）添加到当前列表的末尾
index()	返回具有指定值的第一个元素的索引
insert()	在指定位置添加元素
pop()	删除指定位置的元素
remove()	删除具有指定值的项目
reverse()	颠倒列表的顺序
sort()	对列表进行排序

append adds a new element to the end of a list

append在列表后添加新元素

>>> t = ['a', 'b', 'c']
>>> t.append('d')
>>> print(t)
['a', 'b', 'c', 'd']

extend takes a list as an argument and appends all of the elements

extend在连接两个列表，注意作为参数的那个列表的值是不变的

>>> t1 = ['a', 'b', 'c']
>>> t2 = ['d', 'e']
>>> t1.extend(t2)          ###只改变t1
>>> print(t1)
['a', 'b', 'c', 'd', 'e']
>>> print(t2)
[ 'd', 'e']                ###t2不变

sort arranges the elements of the list from low to high

sort从低到高给列表排序

>>> t = ['d', 'c', 'e', 'b', 'a']
>>> t.sort()
>>> print(t)
['a', 'b', 'c', 'd', 'e']

pop modifies the list and returns the element that was removed. If you don’t provide an index, it deletes and returns the last element.

pop根据index删除列表中对应位置的元素，并返回该元素的值，如果index为空则删除和返回最后一个元素

>>> t = ['a', 'b', 'c']
>>> x = t.pop(1)
>>> print(t)
['a', 'c']
>>> print(x)
b

 If you know the element you want to remove (but not the index), you can use remove.

remove根据元素内容删除元素，且不返回值

>>> t = ['a', 'b', 'c', 'd', 'e', 'f']
>>> del t[1:5]
>>> print(t)
['a', 'f']

补充：del函数也可以删除列表中的元素，根据列表的index删除一个或多个元素

>>> t = ['a', 'b', 'c']
>>> del t[1]
>>> print(t)
['a', 'c']


>>> t = ['a', 'b', 'c', 'd', 'e', 'f']
>>> del t[1:5]
>>> print(t)
['a', 'f']

5 Lists and functions

>>> nums = [3, 41, 12, 9, 74, 15]
>>> print(len(nums))    ###长度
6
>>> print(max(nums))    ###最大值
74
>>> print(min(nums))    ###最小值
3
>>> print(sum(nums))    ###求和
154
>>> print(sum(nums)/len(nums))    ###平均值
25

▲示例：读取数据并求均值

numlist = list() ###创建空list
while (True):
    inp = input('Enter a number: ') ###用户输入
    if inp == 'done': break         ###输入done时结束
    value = float(inp)              ###输入数据转化成浮点数
    numlist.append(value)           ###输入数据添加到list

average = sum(numlist) / len(numlist)  ###求均值
print('Average:', average)

6  Lists and strings

To convert from a string to a list of characters, you can use list

list讲字符串转化成列表，每个元素是一个字符

>>> s = 'spam'
>>> t = list(s)
>>> print(t)
['s', 'p', 'a', 'm']

If you want to break a string into words, you can use the split method

字符串的方法split（无参数时）把字符串按空格分开，分成不同单词

>>> s = 'pining for the fjords'
>>> t = s.split()
>>> print(t)
['pining', 'for', 'the', 'fjords']
>>> print(t[2])
the

 也可以按别的符号进行分割，如：

>>> s = 'spam-spam-spam'
>>> delimiter = '-'
>>> s.split(delimiter)
['spam', 'spam', 'spam']

用join方法将列表合并成字符串：

>>> t = ['pining', 'for', 'the', 'fjords']
>>> delimiter = ' '
>>> delimiter.join(t)
'pining for the fjords'

▲由于list是可变的，list的方法可以直接改变原本list的值，而不是像字符串一样返回一个新的值， 许多list方法的返回值是None

>>> t1 = [1, 2]
>>> t2 = t1.append(3)  ###直接修改t1，而t2不发生变化
>>> print(t1)
[1, 2, 3]
>>> print(t2)
None

7 Parsing lines

▲示例：找出文件中以From开头的句子，并提取星期，如：

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008 提取Sat

fhand = open('mbox-short.txt') ###建立handle

for line in fhand:  ###循环文件
    line = line.rstrip()  ###删除newline
    if not line.startswith('From '): continue  ###判断是否From 开头，否则进行下一个循环
    words = line.split()   ###分隔字符返回列表
    print(words[2])    ###输出第三个元素