In the article List data structure in Python, you have learned about List. This article will explore some operations on List data structure in Python.
1. Traverse each element in the List
1.1. Using for loop
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
# Using for loop
for i in my_list:
print(i, end=' ')Result
2 goc hoc it 8 1 3 51.2. Using for loop and index of List
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
# getting length of my_list
length = len(my_list)
# Iterating the index
for i in range(length):
print(my_list[i], end=' ')Result
2 goc hoc it 8 1 3 51.3. Using a while loop
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
# getting length of my_list
length = len(my_list)
i = 0
# Iterating using while loop
while i < length:
print(my_list[i], end=' ')
i += 1Result
2 goc hoc it 8 1 3 51.4. Using for loop and enumerate() function
Returns an object including all the index and value of corresponding elements in the List.
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
# Using enumerate()
for i, val in enumerate(my_list):
print ('index ', i, " - value ", val, sep='')Result
index 0 - value 2
index 1 - value goc
index 2 - value hoc
index 3 - value it
index 4 - value 8
index 5 - value 1
index 6 - value 3
index 7 - value 52. Creating a List with List Comprehension
List comprehension also provides a shorter syntax to create a new List based on the values of another List.
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
#Creating new_my_list include items which are string in my_list
new_my_list = [x for x in my_list if type(x)==str]
print(new_my_list)Result
['goc', 'hoc', 'it']Syntax of List Comprehension
new_my_list = [expression for item in iterable if condition == True]Example of expression in List Comprehension
Expression is the element being processed of the new List about to be created.
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
# Set the values in newlist to upper case
newlist = [x.upper() for x in fruits]
print("newlist with upper case:", newlist)
# Set all values in newlist to 'hello'
newlist = ['hello' for x in fruits]
print("newlist with 'hello':", newlist)Result
newlist with upper case: ['APPLE', 'BANANA', 'CHERRY', 'KIWI', 'MANGO']
newlist with 'hello': ['hello', 'hello', 'hello', 'hello', 'hello']Example of iterable in List Comprehension
Iterable can be any iterable object like a list, tuple, set, etc. For example, we can use the function range() to create an iterable object.
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
# Creating newlist with range()
newlist = [x for x in range(10)]
print("newlist in range(10):", newlist)
# Creating newlist accept only numbers lower than 5
newlist = [x for x in range(10) if x < 5]
print("newlist in range(10) with condition:", newlist)Result
newlist in range(10): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
newlist in range(10) with condition: [0, 1, 2, 3, 4]Example of condition in List Comprehension
In List Comprehension, condition is optional, it can be present or not.
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
#Condition with if
# newlist include items which arenot 'apple'
newlist = [x for x in fruits if x != "apple"]
print("newlist with condition:", newlist)
#Condition is optional, can be omitted
# No if statement
newlist = [x for x in fruits]
print("newlist with no condition:", newlist)Result
newlist with condition: ['banana', 'cherry', 'kiwi', 'mango']
newlist with no condition: ['apple', 'banana', 'cherry', 'kiwi', 'mango']3. Sorting Lists in Python
The List object has the sort() function to sort the string and number elements in the List in descending or ascending order. By default, the elements are sorted in ascending order. If the reverse=True argument is passed to the sort() function, it will be sorted in descending order. Example:
char_list = ["banana", "apple", "cherry", "mango", "kiwi"]
num_list = [100, 50, 65, 82, 23]
# Sort by default
char_list.sort()
num_list.sort()
print("char_list_after_default_sort:", char_list)
print("num_list_after_default_sort:", num_list)
# Sort by reverse = True
char_list.sort(reverse = True)
num_list.sort(reverse = True)
print("char_list_after_reverse_sort:", char_list)
print("num_list_after_reverse_sort:", num_list)Result
char_list_after_default_sort: ['apple', 'banana', 'cherry', 'kiwi', 'mango']
num_list_after_default_sort: [23, 50, 65, 82, 100]
char_list_after_reverse_sort: ['mango', 'kiwi', 'cherry', 'banana', 'apple']
num_list_after_reverse_sort: [100, 82, 65, 50, 23]4. Copying Lists in Python
We can use the copy() function or the list() function to copy a List. Example:
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
# copy() function
my_list_1 = my_list.copy()
print("my_list_1:", my_list_1)
# list() function
my_list_2 = list(my_list)
print("my_list_2:", my_list_2)Result
my_list_1: [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
my_list_2: [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]Note: We cannot use the assignment operator to copy 2 Lists like my_list_1 = my_list. Because my_list_1 only refers to the List that my_list refers to. If my_list_1 or my_list changes, it will also cause the other list to change.
Example:
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
my_list_1 = my_list
# Change items in my_list_1
my_list_1[0] = 'hello all'
my_list_1[1] = '9999'
print("my_list changed after my_list_1 changed:", my_list)Result
my_list changed after my_list_1 changed: ['hello all', '9999', 'hoc', 'it', 8, 1, 3, 5]5. Joining Lists in Python
The simplest way to join 2 or more Lists is to use the + operator.
my_list_1 = ['goc', 'hoc', 'it']
my_list_2 = [2, 2, 5, 8]
my_list_3 = ['welcome', '99', 0, 1]
# Concatenate my_list_1, my_list_2, my_list_3
my_list = my_list_1 + my_list_2 + my_list_3
print("my_list:", my_list)Result
my_list: ['goc', 'hoc', 'it', 2, 2, 5, 8, 'welcome', '99', 0, 1]We can use the append() function to join 2 Lists with a for loop.
my_list_1 = ['goc', 'hoc', 'it']
my_list_2 = [2, 2, 5, 8]
# Concatenate my_list_1, my_list_2 with append() and for
for x in my_list_2:
my_list_1.append(x)
print("my_list_1:", my_list_1)Result
my_list_1: ['goc', 'hoc', 'it', 2, 2, 5, 8]We can also use the extend() function to join one List to the end of another List.
my_list_1 = ['goc', 'hoc', 'it']
my_list_2 = [2, 2, 5, 8]
# Concatenate my_list_1, my_list_2 with extend()
my_list_1.extend(my_list_2)
print("my_list_1:", my_list_1)Result
my_list_1: ['goc', 'hoc', 'it', 2, 2, 5, 8]6. List Methods in Python
Python supports many methods to make it easier to manipulate Lists. Some commonly used methods with Lists are described in the table below.
| Method | Description |
|---|---|
| append() | Add an element to the end of the list |
| extend() | Add all the elements of a list to the end of another list |
| insert() | Insert an element into a predetermined position |
| remove() | Remove an element from the list |
| pop() | Remove and return an element at a specified index (by default removes the last element of the list) |
| clear() | Remove all elements from the list |
| index() | Return the first index of the element found in the list |
| count() | Return the number of times the specified element appears in the list |
| sort() | Sort the elements in the list |
| reverse() | Reverse the order of the elements in the list |
| copy() | Copy a list |
Example using List methods
my_list = [2, 'goc', 'hoc', 'it', 8, 1, 3, 5]
# Add 'welcome' to the end of my_list
my_list.append('welcome')
# Output: [2, 'goc', 'hoc', 'it', 8, 1, 3, 5, 'welcome']
print("Append my_list:", my_list)
my_list_extend = ['hello', 'John']
# Extend my_list by my_list_extend
my_list.extend(my_list_extend)
# Output: [2, 'goc', 'hoc', 'it', 8, 1, 3, 5, 'welcome', 'hello', 'John']
print("Extend my_list:", my_list)
# Insert an item to my_list
my_list.insert(1, 'gochocit.com')
# Output: [2, 'gochocit.com', 'goc', 'hoc', 'it', 8, 1, 3, 5, 'welcome', 'hello', 'John']
print("Insert my_list:", my_list)
# Remove items from my_list
my_list.remove(2)
my_list.remove(8)
my_list.remove(5)
my_list.remove('welcome')
# Output: ['gochocit.com', 'goc', 'hoc', 'it', 1, 3, 'hello', 'John']
print("Remove my_list:", my_list)
# Pop item by default from my_list
pop_item = my_list.pop()
# Output: John
print("pop item:", pop_item)
# Output: ['gochocit.com', 'goc', 'hoc', 'it', 1, 3, 'hello']
print("Pop my_list:", my_list)
# Pop item at index 3 from my_list
pop_item = my_list.pop(3)
# it
print("pop item:", pop_item)
# Output: ['gochocit.com', 'goc', 'hoc', 1, 3, 'hello']
print("Pop my_list:", my_list)
# Index item from my_list
index_item = my_list.index('hoc')
# Output: 2
print("Index item:", index_item)
# Count my_list
# Output: 1
print("Size of my_list:", my_list.count('gochocit.com'))
# Clear my_list
my_list.clear()
# Output: []
print("my_list:", my_list)Result
Append my_list: [2, 'goc', 'hoc', 'it', 8, 1, 3, 5, 'welcome']
Extend my_list: [2, 'goc', 'hoc', 'it', 8, 1, 3, 5, 'welcome', 'hello', 'John']
Insert my_list: [2, 'gochocit.com', 'goc', 'hoc', 'it', 8, 1, 3, 5, 'welcome', 'hello', 'John']
Remove my_list: ['gochocit.com', 'goc', 'hoc', 'it', 1, 3, 'hello', 'John']
pop item: John
Pop my_list: ['gochocit.com', 'goc', 'hoc', 'it', 1, 3, 'hello']
pop item: it
Pop my_list: ['gochocit.com', 'goc', 'hoc', 1, 3, 'hello']
Index item: 2
Size of my_list: 1
my_list: []You can refer to more List methods in Python at More on Lists.