Slicing a list in Python

Slicing is a powerful technique in Python that allows you to extract specific portions of a list with ease. With slicing, you can effortlessly retrieve a subset of elements from a list based on their indices or create new lists by extracting desired segments. This essential feature enables you to manipulate data efficiently, whether you need to access a range of values, reverse a list, or extract every nth element. In this tutorial, we will explore the syntax and various techniques of list slicing, empowering you to manipulate and extract data from lists like a pro. Let's get started and unlock the full potential of list slicing in Python!

Slicing a list in Python :

• Slicing is a process of getting a sub list (a portion of the list) from the original list.
• Slicing process in Python can be used for many purposes : 
  • getting a sub-list from main list
  • updating the main list
  • resizing the list
  • deleting a portion of the list.
• The syntax of slicing a list in python is :  sub_list = original_list[begin: end: step:] 
• The sub_list is extracted based on the values of  begin ,  end  and  step  arguments passed in the square brackets.
  • The list is sliced from the  begin  index up-to the  end  index. 
  • The list is sliced in steps of the  step  argument.
  • All these three arguments are optional.
• If we omit any or all of these arguments, their default values are used for slicing the list :
  • The default value of  begin  argument is 0.
  • The default value of  end  argument is the length of the list.
  • The default value of  step  argument is 1.
• All these three arguments  begin ,  end  and  step  can take positive as well as negative values.
  • Positive arguments slice the list from the first element to the last element.
  • Negative arguments slice the list from the last element to the first element.
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

# Slicing Using values of begin end and steps
sub_list = my_list[2:6:2]
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# sub_list =  [3, 5]



# Slicing Using default values of end and steps
sub_list = my_list[2:]
print("my_list = ", my_list)
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# sub_list =  [3, 4, 5, 6, 7, 8]



# Slicing Using default values of begin and steps
sub_list = my_list[:4:]
print("my_list = ", my_list)
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# sub_list =  [1, 2, 3, 4]



# Slicing Using negative value of begin
sub_list = my_list[-3:]
print("my_list = ", my_list)
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# ub_list =  [6, 7, 8]

List Slicing Operations in Python :

There are a lot of operations that can be performed using list slicing in Python : 

• get the first n-elements of a list in Python
• get the last n-elements of a list in Python
• get every nth element of the list in Python
• reverse a list in Python using slicing
• substitute a part of the list in Python
• resize the list in Python
• delete list element using slicing in Python

How to get the first n-elements of a list in Python :

• The begin argument is omitted to get the first n elements of a list.
• The basic syntax of extracting the first n elements of a list in python is :  sub_list = original_list[:n] 
• We can explicitly write the begin argument as 0 i.e. we can write :  sub_list = original_list[0:n] 
• but in practice it is not needed as default value of begin argument is itself 0.
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

n = 5
sub_list = my_list[:n]
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# sub_list =  [1, 2, 3, 4, 5]

How to get the last n-elements of a list in Python :

• We use negative indexes to get the last n elements of the list.
• The end argument is omitted to get the last n elements of a list.
• The basic syntax of extracting the last n elements of a list in python is :  sub_list = original_list[-n:] 
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

n = 5
sub_list = my_list[-n:]
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# sub_list =  [4, 5, 6, 7, 8]

How to get every nth element of the list in Python

• The step argument is used to get every nth element of the list.
• The begin and end arguments are optional.
• The syntax of getting every nth element is :  sub_list = original_list[begin: end: n] 
• If we omit the begin and end arguments their default values will be used.
• The syntax in that case will be :  sub_list = original_list[::n] 
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

n = 3
sub_list = my_list[::n]
print("sub_list = ", sub_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# sub_list =  [1, 4, 7]

How to reverse a list in Python using slicing :

• We can use negative step argument to reverse a list in Python.
• The syntax of reversing the list is :  reversed_list = original_list[::-1] 
• When we give negative step argument, sub-list is created by including elements starting from the last to the first, hence the list gets reversed.
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

rev_list = my_list[::-1]
print("rev_list = ", rev_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# rev_list =  [8, 7, 6, 5, 4, 3, 2, 1]

How to substitute a part of the list in Python :

• The slicing can be used to change the values of items in the list.
• The syntax of modifying values in list is as follows :  original_list[begin: end] = ["new_item_1", "new_item_2"]  
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

my_list[2:5] = [300, 400, 500]
print("my_list = ", my_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# my_list =  [1, 2, 300, 400, 500, 6, 7, 8]

How to resize the list in Python :

• The slicing can be used to resize the list in python
• The syntax of resizing the list is as follows :  original_list[begin: end] = ["new_item_1", "new_item_2"] 
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

my_list[2:5] = [300, 400, 500, 550, 560]
print("my_list = ", my_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# my_list =  [1, 2, 300, 400, 500, 550, 560, 6, 7, 8]

How to delete list element using slicing in Python :

• We can use the  del  keyword along with list slicing to delete a portion of the original list.
• The syntax of the deleting elements is as follows :  del original_list[begin: end] 
• Code Sample :

my_list = [1, 2, 3, 4, 5, 6, 7, 8]
print("my_list = ", my_list)

del my_list[2:5]
print("my_list = ", my_list)

# Output
# my_list =  [1, 2, 3, 4, 5, 6, 7, 8]
# my_list =  [1, 2, 6, 7, 8]