Python notes (6): definition and use of lists

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Hello everyone , I am a A bowl week , One doesn't want to be drunk ( Internal volume ) The front end of the . If you are lucky enough to get your favor , I'm very lucky ~

Define and use lists

stay Python in , A list is a data structure composed of a series of elements in a specific order , In other words, variables of list type can Store multiple data , And Can be repeated .

Definition list

1. Use [] Literal syntax defines variables , Multiple elements in the list use commas , Segmentation , The sample code is as follows ：

   list1 = ["Hello", " A bowl week ", " Hello "]
   list2 = [1, 2, 3, 4, 5]
   print(list1)  # ['Hello', ' A bowl week ', ' Hello ']
   print(list2)  # [1, 2, 3, 4,5]
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2. Use Python Built in list Program other sequences into a list , The sample code is as follows ：

   list1 = list(range(10))
   list2 = list("hello")
   print(list1)  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
   print(list2)  # ['h', 'e', 'l', 'l', 'o']
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A list is a variable data type , That is, the elements of the list can be modified , This is significantly different from string , After modifying the string type , Will return a new string

Access the values in the list

If you access a value in the list , Use the subscript index to access the values in the list , Like a string, use square brackets to intercept characters , The sample code is as follows ：

list1 = ["Hello", " A bowl week ", " Hello "]
#  Index of the list 
print(list1[1])  #  A bowl week 
#  Slice of the list 
print(list1[1:3])  # [' A bowl week ', ' Hello ']
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List operators

List and string types are the same , It also supports splicing 、 repeat 、 Member operation and other operations , The sample code is as follows ：

list1 = ["Hello"]
list2 = ["World"]
list3 = [1, 2, 3, 4, 5]
list4 = list(range(1, 6))

list5 = list1 + list2  # ['Hello', 'World']
print(list5)

list6 = list1 * 3  # ['Hello', 'Hello', 'Hello']
list7 = list3 * 2  # [1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
print(list6)
print(list7)

print("W" in list1)  # False
print("W" in list2)  # False

#  List comparison operation 
#  The equality ratio of two lists is whether the elements at the corresponding index position are equal 
print(list3 == list4)  # True
list8 = list(range(1, 7))
print(list3 == list8)  # False
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Traversal of list elements

Traversing a list is the same as traversing a string , The sample code is as follows ：

list1 = ["H", "e", "l", "l", "o"]
#  Method 1
for index in range(len(list1)):
    print(list1[index])
#  Method 2
for ch in list1:
    print(ch)
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List method

Add and remove elements

Go straight to the code

list1 = ["cute", "beautiful", " A bowl week "]

# append() Add elements to the end of the list 
list1.append("lovely")
print(list1)  # ['cute', 'beautiful', ' A bowl week ', 'lovely']

# insert() Inserts an element at the specified index position in the list 
list1.insert(2, "prefect")
print(list1)  # ['cute', 'beautiful', 'prefect', ' A bowl week ', 'lovely']

# remove() Deletes the specified element 
list1.remove("lovely")
print(list1)  # ['cute', 'beautiful', 'prefect', ' A bowl week ']

# pop() Delete the element at the specified index location 
list1.pop(2)
print(list1)  # ['cute', 'beautiful', ' A bowl week ']

# clear() Empty the list of elements 
list1.clear()
print(list1)  # []
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stay Python Can also be used in del Keyword to delete a list element , Be similar to pop, Sample code ↓

list1 = ["cute", "beautiful", " Sweet "]

del list1[1]
print(list1) # ['cute', ' Sweet ']

#  Delete the entire list 
del list1
print(list1) # NameError: name 'list1' is not defined
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Element position and number

Use index() To find the location of the element , Use count() To count the number of occurrences of elements

list1 = ["beautiful", "cute", "beautiful",
         'prefect', "beautiful", " A bowl week ", 'lovely']
#  lookup "beautiful" First occurrence 
print(list1.index("beautiful"))  # 0
#  Find... After the fourth element "beautiful" The location of the last occurrence 
print(list1.index("beautiful", 3))  # 4

#  Statistics "beautiful" Number of occurrences 
print(list1.count("beautiful"))  # 3
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Element sorting and inversion

Use sort() Method can realize the sorting of list elements , and reverse() Method can realize the inversion of elements , Sample code ↓

list1 = ["cute", "beautiful", " A bowl week "]
list2 = list(range(10))

#  Sort 
list1.sort()
print(list1)  # ['beautiful', 'cute', ' A bowl week ']

#  reverse 
list1.reverse()
print(list1)  # [' A bowl week ', 'cute', 'beautiful']

list2.reverse()
print(list2)  # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

#  The previous operation is to modify the original list , If you don't let the original data be destroyed, you can use copy() Make a backup 
list3 = list2.copy()
list3.sort()
print(list2)  # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
print(list3)  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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summary

Method	describe	Parameters
append()	Add a new object at the end of the list	Objects added to the end of the list .
insert()	Used to insert the specified object into the specified location of the list	index -- object obj Index location to insert . obj -- To insert an object in the list .
remove()	Function to remove the first match of a value in the list .	Objects to be removed from the list .
pop()	Function to remove an element from the list （ Default last element ）	Optional parameters , To remove the index value of a list element , Cannot exceed the total length of the list , The default is index=-1, Delete the last list value .
clear()	Function to clear the list	nothing
index()	Used to find the index position of the first match of a value from the list .	x-- Objects found . start-- Optional , The starting point of the search . end-- Optional , End of search .
count()	Used to count the number of times an element appears in the list .	Statistical objects in the list .
sort()	Used to sort the original list	key -- It's basically a comparison element reverse -- Sort rule ,reverse = True Descending , reverse = False Ascending （ Default ）
reverse()	For elements in the reverse list	nothing
copy()	Used to copy lists	nothing

The generative expression of the list

requirement ： For the string 123 And string ABC Create a list of Cartesian Products , The sample code is as follows ：

Original method

a = "123"
b = "ABC"
list1 = []
for x in a:
     for y in b:
          list1.append(x + y)
print(list1) # ['1A', '1B', '1C', '2A', '2B', '2C', '3A', '3B', '3C']
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Generate column method

a = "123"
b = "ABC"
list1 = [x + y for x in a for y in b]
print(list1) # ['1A', '1B', '1C', '2A', '2B', '2C', '3A', '3B', '3C']
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This method not only has a small amount of code , And the performance is better than ordinary for Circulation and append Additional way

nested list

Because the variables in the list can store multiple data types , When there is a list in the list , This is called the nesting of lists , The sample code is as follows ：

list1 = [["cute", "beautiful", " A bowl week "], "cute", "beautiful", " A bowl week "]

print(list1[0])  # ['cute', 'beautiful', ' A bowl week ']
print(list1[1])  # cute

#  The one you want to see nested cute You need to use multiple index values 
print(list1[0][0])  # cute
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The same is true no matter how many nested