Share various operating techniques for lists in Python

I am learning python language recently. Basically learned the basic syntax of python. I feel that python's position in data processing is inseparable from its list operation.

I learned the relevant basic operations and took notes here.

'''
Python --version Python 2.7.11
Quote : https://docs.python.org/2/tutorial/datastructures.html#more-on-lists
Add by camel97 2017-04
'''
list.append(x) #在列表的末端添加一个新的元素
Add an item to the end of the list; equivalent to a[len(a):] = [x].

list.extend(L)#Merge the elements in the two lists together

Extend the list by appending all the items in the given list; equivalent to a[ len(a):] = L.

list.insert(i, x)#Insert the element to the specified position (the position is the previous element with index i)
Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

list.remove(x)#Delete the first element with value x in the list (that is, if there are two x in the list, only the first x will be deleted)

Remove the first item from the list whose value is x. It is an error if there is no such item.

list.pop([i])#Remove the item in the list The i-th element and returns this element. If parameter i is not given, the last element in the list will be deleted by default
Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. (The square brackets around the i in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)

list.index(x)#Return the index in the list of the first item whose value is x. It is an error if there is no such item.

list.count(x)#Return the number of times x appears in the list

Return the number of times x appears in the list.

demo:

#-*-coding:utf-8-*-
L = [1,2,3]   #创建 list 
L2 = [4,5,6]
print L
L.append(6)   #添加
print L
L.extend(L2) #合并
print L
L.insert(0,0) #插入
print L
L.remove(6)   #删除
print L
L.pop()     #删除
print L
print L.index(2)#索引
print L.count(2)#计数
L.reverse()　　　#倒序
print L

result:

[1, 2, 3]
[1, 2, 3, 6]
[1, 2, 3, 6, 4, 5, 6]
[0, 1, 2, 3, 6, 4, 5, 6]
[0, 1, 2, 3, 4, 5, 6]
[0, 1, 2, 3, 4, 5]
2
1
[5, 4, 3, 2, 1, 0]

list.sort(cmp=None, key=None, reverse=False)

Sort the items of the list in place (the arguments can be used for sort customization, see sorted() for their explanation).

1. Sort a list. The default is to sort from small to large

L = [2,5,3,7,1]
L.sort()
print L
==>[1, 2, 3, 5, 7]
L = ['a','j','g','b']
L.sort()
print L
==>['a', 'b', 'g', 'j']

2.reverse is a bool value. The default is False. If it is set to True, then the elements in this list will be compared in reverse order (reverse order ) arrangement.

# reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.

L = [2,5,3,7,1]
L.sort(reverse = True)
print L
==>[7, 5, 3, 2, 1]
L = ['a','j','g','b']
L.sort(reverse = True)
print L
==>['j', 'g', 'b', 'a']

3.key is a function that specifies The key to sort, usually a lambda expression or a specified function

#key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower . The default value is None (compare the elements directly).

#-*-coding:utf-8-*-
#创建一个包含 tuple 的 list 其中tuple 中的三个元素代表名字 ， 身高 ， 年龄
students = [('John', 170, 15), ('Tom', 160, 12), ('Dave', 180, 10)]
print students
==>[('John', 170, 15), ('Tom', 160, 12), ('Dave', 180, 10)]
students.sort(key = lambda student:student[0])
print students
==>[('Dave', 180, 10), ('John', 170, 15), ('Tom', 160, 12)]#按名字(首字母)排序
students.sort(key = lambda student:student[1])
print students
==>[('Tom', 160, 12), ('John', 170, 15), ('Dave', 180, 10)]#按身高排序
students.sort(key = lambda student:student[2])
print students
==>[('Dave', 180, 10), ('Tom', 160, 12), ('John', 170, 15)]#按年龄排序

4.cmp ​​is a function that specifies two parameters. It determines the sorting method.

#cmp specifies a custom comparison function of two arguments (iterable elements) which should return a negative, zero or positive number depending on whether the first #argument is considered smaller than, equal to, or larger than the second argument: cmp=lambda x,y: cmp(x.lower(), y.lower()). The default value is None.

#-*-coding:utf-8-*-
students = [('John', 170, 15), ('Tom', 160, 12), ('Dave', 180, 10)]
print students
==>[('John', 170, 15), ('Tom', 160, 12), ('Dave', 180, 10)]
#指定 用第一个字母的大写(ascii码)和第二个字母的小写(ascii码)比较
students.sort(cmp=lambda x,y: cmp(x.upper(), y.lower()),key = lambda student:student[0])
print students
==>[('Dave', 180, 10), ('Tom', 160, 12), ('John', 170, 15)]
#指定 比较两个字母的小写的 ascii 码值
students.sort(cmp=lambda x,y: cmp(x.lower(), y.lower()),key = lambda student:student[0])
print students
==>[('Dave', 180, 10), ('John', 170, 15), ('Tom', 160, 12)]
#cmp(x,y) 是python内建立函数，用于比较2个对象，如果 x < y 返回 -1, 如果 x == y 返回 0, 如果 x > y 返回 1

cmp allows users to customize the size relationship. Usually we think 1 < 2 and a < b.

Now we can customize the function to sort the list according to the specified rules by customizing the size relationship (for example, 2 < a < 1 < b).

This is often useful when we are dealing with some special problems.

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