current position:Home>Python Basics: do you know how to use lists?

Python Basics: do you know how to use lists?

2022-01-29 11:32:34 Yunyun yyds

Little knowledge , Great challenge ! This article is participating in “ A programmer must have a little knowledge ” Creative activities .

list

Python There are four collection data types in :

  • list : Is an ordered and variable ( Modifiable ) Set . Allow duplicate members .
  • Tuples : Is orderly and cannot be changed or modified ( immutable ) Set . Allow duplicate members .
  • Set: It's a disorder 、 An indexed and immutable collection , But we can add new items to the collection . Duplicate members are not allowed .
  • Dictionaries : It's a disorder 、 variable ( Modifiable ) And index collection . There are no duplicate members .

The list is ordered and modifiable ( variable ) A collection of different data types . The list can be empty , You can also have items with different data types .

How to create a list

stay Python in , We can create lists in two ways :

  • Use list built-in functions
#  grammar 
lst  =  list ()
 Copy code 
empty_list  =  list () #  This is an empty list , There are no items in the list 
print ( len ( empty_list )) # 0
 Copy code 
  • Use square brackets ,[]
#  grammar 
lst  = []
 Copy code 
empty_list  = [] #  This is an empty list , There are no items in the list 
print ( len ( empty_list )) # 0
 Copy code 

List with initial values . We use len() To find the length of the list .

fruits = ['banana', 'orange', 'mango', 'lemon']                      #  Fruit list 
vegetables = ['Tomato', 'Potato', 'Cabbage','Onion', 'Carrot']       #  List of vegetables 
animal_products = ['milk', 'meat', 'butter', 'yoghurt']             #  List of animal products 
web_techs = ['HTML', 'CSS', 'JS', 'React','Redux', 'Node', 'MongDB'] #  Network technology  
countries = ['Finland', 'Estonia', 'Denmark', 'Sweden', 'Norway']

# Print the list and its length 
print('Fruits:', fruits)
print('Number of fruits:', len(fruits))
print('Vegetables:', vegetables)
print('Number of vegetables:', len(vegetables))
print('Animal products:',animal_products)
print('Number of animal products:', len(animal_products))
print('Web technologies:', web_techs)
print('Number of web technologies:', len(web_techs))
print('Countries:', countries)
print('Number of countries:', len(countries))
 Copy code 
 Output 
Fruits: ['banana', 'orange', 'mango', 'lemon']
Number of fruits: 4
Vegetables: ['Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot']
Number of vegetables: 5
Animal products: ['milk', 'meat', 'butter', 'yoghurt']
Number of animal products: 4
Web technologies: ['HTML', 'CSS', 'JS', 'React', 'Redux', 'Node', 'MongDB']
Number of web technologies: 7
Countries: ['Finland', 'Estonia', 'Denmark', 'Sweden', 'Norway']
Number of countries: 5
 Copy code 
  • The list can contain items of different data types
 lst  = [ 'Asabeneh' , 250 , True , { 'country' : 'Finland' , 'city' : 'Helsinki' }] #  Contains a list of different data types 
 Copy code 

Use positive index to access list items

We use their indexes to access each item in the list , A list index from 0 Start , The following figure clearly shows where the index starts

 Insert picture description here

fruits = ['banana', 'orange', 'mango', 'lemon']
first_fruit = fruits[0] #  We use its index to access the first item 
print(first_fruit)      #  Banana 
second_fruit = fruits[1]
print(second_fruit)     #  The oranges 
last_fruit = fruits[3]
print(last_fruit) #  lemon 
#  The last index 
last_index = len(fruits) - 1
last_fruit = fruits[last_index]
 Copy code 

Use negative index to access list items

A negative index means starting at the end ,-1 It means the last item ,-2 It means the penultimate term .

 Insert picture description here

fruits = ['banana', 'orange', 'mango', 'lemon']
first_fruit = fruits[-4]
last_fruit = fruits[-1]
second_last = fruits[-2]
print(first_fruit)      # Banana 
print(last_fruit)       #  lemon 
print(second_last)      #  Mango. 
 Copy code 

Unpacking list items

lst  = [ 'item' , 'item2' , 'item3' , 'item4' , 'item5' ]
 first_item , second_item , third_item , * rest  =  lst 
print ( first_item )      # item1 
print ( second_item )     # item2 
print ( third_item )      # item3
print( rest )            # ['item4', 'item5']
 Copy code 
#  First example 
fruits  = [ 'banana' , 'orange' , 'mango' , 'lemon' , 'lime' , 'apple' ]
 first_fruit , second_fruit , third_fruit , * rest  =  lst 
print ( first_fruit )      #  Banana 
print ( second_fruit )     #  Orange 
print ( third_fruit )      #  Mango. 
print ( rest )           # ['lemon','lime','apple'] 
#  The second example of unpacking list 
first , second , third , * rest , tenth  = [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]
print(first)          # 1
print(second)         # 2
print(third)          # 3
print(rest)           # [4,5,6,7,8,9]
print(tenth)          # 10
# The third example is about unpacking list 
countries = ['Germany', 'France','Belgium','Sweden','Denmark','Finland','Norway','Iceland','Estonia']
gr, fr, bg, sw, *scandic, es = countries
print(gr)
print(fr)
print(bg)
print(sw)
print(scandic)
print(es)
 Copy code 

Slice items from the list

  • Positive index : We can start by specifying 、 End and step to specify a series of positive indexes , The return value will be a new list .( The default value for the start = 0, end = len(lst) - 1( Last item ), step = 1)
fruits = ['banana', 'orange', 'mango', 'lemon']
all_fruits = fruits[0:4] #  It returns all the fruit 
#  This will also give the same result as the above one 
all_fruits = fruits[0:] #  If we don't set where to stop, it needs all the rest 
orange_and_mango = fruits[1:3] #  Excluding the first index 
orange_mango_lemon = fruits[1:]
orange_and_lemon = fruits[::2] #  Here we use the third parameter ,step. It will need every  2cnd  term  - ['banana', 'mango']
 Copy code 
  • Negative index : We can start by specifying 、 End and step to specify a series of negative indexes , The return value will be a new list .
fruits = ['banana', 'orange', 'mango', 'lemon']
all_fruits = fruits[-4:] #  It returns all the fruit 
orange_and_mango = fruits[-3:-1] #  It does not include the last index ,['orange', 'mango']
orange_mango_lemon = fruits[-3:] #  This will start from -3 To end ,['orange', 'mango', 'lemon']
reverse_fruits = fruits[::-1] #  A negative step will take the list in reverse order ,['lemon', 'mango', 'orange', 'banana']
 Copy code 

Modify the list

A list is an ordered collection of items that can be changed or modified . Let's modify the fruit list .

fruits = ['banana', 'orange', 'mango', 'lemon']
fruits[0] = 'avocado'
print(fruits)       # ['avocado', 'orange', 'mango', 'lemon']
fruits[1] = 'apple'
print(fruits)       # ['avocado', 'apple', 'mango', 'lemon']
last_index = len(fruits) - 1
fruits[last_index] = 'lime'
print(fruits)        # ['avocado', 'apple', 'mango', 'lime']
 Copy code 

Check the items in the list

Use in Operator to check whether an item is a member of a list . See the example below .

fruits = ['banana', 'orange', 'mango', 'lemon']
does_exist = 'banana' in fruits
print(does_exist)  #  really 
does_exist = 'lime' in fruits
print(does_exist)  #  false 
 Copy code 

Add item to list

To add an item to the end of an existing list , How we use append().

#  grammar 
lst  =  list ()
 lst.append(item)
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.append('apple')
print(fruits)           # ['banana', 'orange', 'mango', 'lemon', 'apple']
fruits.append('lime')   # ['banana', 'orange', 'mango', 'lemon', 'apple', 'lime']
print(fruits)
 Copy code 

Insert item into list

We can use insert() Method inserts a single item at the specified index in the list . Please note that , Other items move right . This plug-in () Method has two parameters : Index and insert items .

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst.insert(index, item)
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.insert(2, 'apple') #  Insert an apple between an orange and a mango 
print(fruits)           # ['banana', 'orange', 'apple', 'mango', 'lemon']
fruits.insert(3, 'lime')   # ['banana', 'orange', 'apple', 'lime', 'mango', 'lemon']
print(fruits)
 Copy code 

Remove items from the list

remove Method to delete the specified item from the list

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst.remove(item)
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon', 'banana']
fruits.remove('banana')
print(fruits)  # ['orange', 'mango', 'lemon', 'banana'] - this method removes the first occurrence of the item in the list
fruits.remove('lemon')
print(fruits)  # ['orange', 'mango', 'banana']
 Copy code 

Use Pop Delete the project Pop up of () Method to delete the specified index ,( Or if the last item in the index is not specified ):

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst . pop ()        #  Last item 
lst.pop(index)
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.pop()
print(fruits)       # ['banana', 'orange', 'mango']

fruits.pop(0)
print(fruits)       # ['orange', 'mango']
 Copy code 

Title Use Del Delete the project

The delete keyword deletes the specified index and it can also be used to delete items within the index range . It can also completely delete the list

#  grammar 
lst  = [ 'item1' , 'item2' ]
 del  lst [ index ] #  There is only one project 
del  lst         #  Completely delete the list 
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon', 'kiwi', 'lime']
del fruits[0]
print(fruits)       # ['orange', 'mango', 'lemon', 'kiwi', 'lime']
del fruits[1]
print(fruits)       # ['orange', 'lemon', 'kiwi', 'lime']
del fruits[1:3]     # #  This will delete entries between the given indexes , Therefore, it will not delete the index as  3  Project !
print(fruits)       # ['orange', 'lime']
del fruits
print(fruits)       #  This should be for :NameError: name “ Fruits ” No definition 
 Copy code 

Clear list items

In a clear () Method to clear the list :

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst.clear()
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.clear()
print(fruits)       # []
 Copy code 

Copy list

You can copy the list by reassigning the list to a new variable in the following ways :list2 = list1. Now? ,list2 It's right list1 References to , We are list2 Any changes made in will also modify the original list2, But in many cases , We don't like to modify the original version , But like to have different copies . One way to avoid these problems is to use copy().

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst_copy  =  lst . copy()
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits_copy = fruits.copy()
print(fruits_copy)       # ['banana', 'orange', 'mango', 'lemon']
 Copy code 

Join the list

stay Python There are many ways to connect or connect two or more lists .

  • Plus operator (+)
#  grammar 
list3  =  list1  +  list2
 Copy code 
positive_numbers = [1, 2, 3, 4, 5]
zero = [0]
negative_numbers = [-5,-4,-3,-2,-1]
integers = negative_numbers + zero + positive_numbers
print(integers) # [-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5]
fruits = ['banana', 'orange', 'mango', 'lemon']
vegetables = ['Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot']
fruits_and_vegetables = fruits + vegetables
print(fruits_and_vegetables ) # ['banana', 'orange', 'mango', 'lemon', 'Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot']
 Copy code 
  • Use extend() Methods the connection extend() Method allows you to append a list to a list . See the example below .
#  grammar 
list1  = [ 'item1' , 'item2' ]
 list2  = [ 'item3' , 'item4' , 'item5' ]
 list1.extend(list2)
 Copy code 
num1  = [ 0 , 1 , 2 , 3 ]
 num2 = [ 4 , 5 , 6 ]
 num1.extend ( num2 )
 print ( 'Numbers:' , num1 ) # Numbers: [0, 1, 2, 3, 4, 5, 6] 
negative_numbers  = [ - 5 , - 4 , - 3 , - 2 , - 1 ]
 positive_numbers  =[ 1 , 2 , 3 , 4 , 5 ]
zero = [ 0 ]

negative_numbers.extend(zero)
negative_numbers.extend(positive_numbers)
print('Integers:', negative_numbers) # Integers: [-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5]
fruits = ['banana', 'orange', 'mango', 'lemon']
vegetables = ['Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot']
fruits.extend(vegetables)
print('Fruits and vegetables:', fruits ) #  Fruits and vegetables : ['banana', 'orange', 'mango', 'lemon', 'Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot']
 Copy code 

Calculate items in the list

Count of () The number of times the item returned by the method is displayed in the list :

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst.count(item)
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
print(fruits.count('orange'))   # 1
ages = [22, 19, 24, 25, 26, 24, 25, 24]
print(ages.count(24))           # 3
 Copy code 

Find the index of the item

The index of () Method returns the index of the item in the list :

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst.index(item)
 Copy code 
Fruits  = [ 'banana' , 'orange' , 'mango' , 'lemon' ]
 print ( fruits . index ( 'orange' ))    # 1
ages  = [ 22 , 19 , 24 , 25 , 26 , 24 , 25 , 24 ]
print(ages.index(24))             #2, First appearance 
 Copy code 

Reverse list

In reverse () Method reverses the order of the list .

#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst.reverse()
 Copy code 
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.reverse()
print(fruits) # ['lemon', 'mango', 'orange', 'banana']
ages = [22, 19, 24, 25, 26, 24, 25, 24]
ages.reverse()
print(ages) # [24, 25, 24, 26, 25, 24, 19, 22]
 Copy code 

Sort list items

To sort the list , We can use sort() Method or sorted() Built in functions , This sort () Method to reorder the list items in ascending order , And modify the original list , If sort() Method parameters reverse be equal to true, It will sort the list in descending order .

  • sort(): This method modifies the original list
#  grammar 
lst  = [ 'item1' , 'item2' ]
 lst . sort ()                 # Ascending 
lst . sort ( reverse = True )     #  Descending 
 Copy code 

Example :

fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.sort()
print(fruits)             #  Sort alphabetically , ['banana', 'lemon', 'mango', 'orange']
fruits.sort(reverse=True)
print(fruits) # ['orange', 'mango', 'lemon', 'banana']
ages = [22, 19, 24, 25, 26, 24, 25, 24]
ages.sort()
print(ages) # [19, 22, 24, 24, 24, 25, 25, 26]

ages.sort(reverse=True)
print(ages) # [26, 25, 25, 24, 24, 24, 22, 19]
 Copy code 
  • sorted(): Return the sequence table without modifying the original list Example :
fruits = ['banana', 'orange', 'mango', 'lemon']
print(sorted(fruits))   # ['banana', 'lemon', 'mango', 'orange']
# Reverse order
fruits = ['banana', 'orange', 'mango', 'lemon']
fruits = sorted(fruits,reverse=True)
print(fruits)     # ['orange', 'mango', 'lemon', 'banana']
 Copy code 

If it helps you in your study, you must keep it , More information about Python Learning experience , Tool installation package , E-book sharing , But I don't know , Pay attention to me , Continuous updating .

copyright notice
author[Yunyun yyds],Please bring the original link to reprint, thank you.
https://en.pythonmana.com/2022/01/202201291132328631.html