Python List Slicing

In Python, list slicing is a common practice and it is the most used technique for programmers to solve efficient problems. Consider a python list, In-order to access a range of elements in a list, you need to slice a list. One way to do this is to use the simple slicing operator i.e. colon(:)

With this operator, one can specify where to start the slicing, where to end, and specify the step. List slicing returns a new list from the existing list.

Syntax:

Lst[ Initial : End : IndexJump ]

If Lst is a list, then the above expression returns the portion of the list from index Initial to index End, at a step size IndexJump.
Indexing

1. Positive Indexes
Below is a simple program, to display a whole list using slicing.

# Initialize list
Lst = [50, 70, 30, 20, 90, 10, 50]
 # Display list
print(Lst[::])

Output:
[50, 70, 30, 20, 90, 10, 50]
The above diagram illustrates a list Lst with its index values and elements.
2. Negative Indexes
Now, let us look at the below diagram which illustrates a list along with its negative indexes.


Index -1 represents the last element and -n represents the first element of the list(considering n as the length of the list). Lists can also be manipulated using negative indexes also.

           # Initialize list
      Lst = [50, 70, 30, 20, 90, 10, 50]
 
      # Display list
      print(Lst[-7::1])

Output:
[50, 70, 30, 20, 90, 10, 50]
The above program displays the whole list using the negative index in list slicing.
3. Slicing
As mentioned earlier list slicing is a common practice in Python and can be used both with positive indexes as well as negative indexes. The below diagram illustrates the technique of list slicing:


The below program transforms the above illustration into python code:



 # Initialize list
Lst = [50, 70, 30, 20, 90, 10, 50]
 
# Display list
print(Lst[1:5])
Output:
[70, 30, 20, 90]
Below are some examples which depict the use of list slicing in Python:
Example 1:
# Initialize list
List = [1, 2, 3, 4, 5, 6, 7, 8, 9]
 
# Show original list
print("\nOriginal List:\n", List)
 
print("\nSliced Lists: ")
 
# Display sliced list
print(List[3:9:2])
 
# Display sliced list
print(List[::2])
 
# Display sliced list
print(List[::])

Output:
Original List:
 [1, 2, 3, 4, 5, 6, 7, 8, 9]

Sliced Lists: 
[4, 6, 8]
[1, 3, 5, 7, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
Leaving any argument like Initial, End or IndexJump blank will lead to the use of default values i.e 0 as Initial, length of list as End and 1 as IndexJump.
Example 2:
# Initialize list
List = ['Geeks', 4, 'geeks !']
 
# Show original list
print("\nOriginal List:\n", List)
 
print("\nSliced Lists: ")
 
# Display sliced list
print(List[::-1])
 
# Display sliced list
print(List[::-3])
 
# Display sliced list
print(List[:1:-2])

OuA reversed list can be generated by using a negative integer as the IndexJump argument. Leaving the Initial and End as blank. We need to choose the Initial and End value according to a reversed list if the IndexJump value is negative. 
Example 3:tput:
Original List:
 ['Geeks', 4, 'geeks !']

Sliced Lists: 
['geeks !', 4, 'Geeks']
['geeks !']
['geeks !']
# Initialize list
List = [-999, 'G4G', 1706256, '^_^', 3.1496]
 
# Show original list
print("\nOriginal List:\n", List)
 
print("\nSliced Lists: ")
 
# Display sliced list
print(List[10::2])
 
# Display sliced list
print(List[1:1:1])
 
# Display sliced list
print(List[-1:-1:-1])
 
# Display sliced list
print(List[:0:])

Output:
Original List:
 [-999, 'G4G', 1706256, '^_^', 3.1496]

Sliced Lists: 
[]
[]
[]
[]
If some slicing expressions are made that do not make sense or are incomputable then empty lists are generated.
Example 4:
# Initialize list
List = [-999, 'G4G', 1706256, 3.1496, '^_^']
 
# Show original list
print("\nOriginal List:\n", List)
 
 
print("\nSliced Lists: ")
 
# Modified List
List[2:4] = ['Geeks', 'for', 'Geeks', '!']
 
# Display sliced list
print(List)
 
# Modified List
List[:6] = []
 
# Display sliced list
print(List)
Output:
Original List:
 [-999, 'G4G', 1706256, 3.1496, '^_^']

Sliced Lists: 
[-999, 'G4G', 'Geeks', 'for', 'Geeks', '!', '^_^']
['^_^']