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.
Python3
Lst = [50, 70, 30, 20, 90, 10, 50]
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.
Python3
Lst = [50, 70, 30, 20, 90, 10, 50]
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:
Python3
Lst = [50, 70, 30, 20, 90, 10, 50]
print(Lst[1:5])
Output:
[70, 30, 20, 90]Below are some examples which depict the use of list slicing in Python:
Example 1:
Python3
List = [1, 2, 3, 4, 5, 6, 7, 8, 9]
print("\nOriginal List:\n", List)
print("\nSliced Lists: ")
print(List[3:9:2])
print(List[::2])
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:
Python3
List = ['Geeks', 4, 'geeks !']
print("\nOriginal List:\n", List)
print("\nSliced Lists: ")
print(List[::-1])
print(List[::-3])
print(List[:1:-2])
Output:
Original List: ['Geeks', 4, 'geeks !'] Sliced Lists: ['geeks !', 4, 'Geeks'] ['geeks !'] ['geeks !']A 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:
Python3
List = [-999, 'G4G', 1706256, '^_^', 3.1496]
print("\nOriginal List:\n", List)
print("\nSliced Lists: ")
print(List[10::2])
print(List[1:1:1])
print(List[-1:-1:-1])
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:
Python3
List = [-999, 'G4G', 1706256, 3.1496, '^_^']
print("\nOriginal List:\n", List)
print("\nSliced Lists: ")
List[2:4] = ['Geeks', 'for', 'Geeks', '!']
print(List)
List[:6] = []
print(List)
Output:
Original List: [-999, 'G4G', 1706256, 3.1496, '^_^'] Sliced Lists: [-999, 'G4G', 'Geeks', 'for', 'Geeks', '!', '^_^'] ['^_^']List slicing can be used to modify lists or even delete elements from a list.
Example 5:
Python3
List = [1, 2, 3, 4, 5, 6, 7, 8, 9]
print("\nOriginal List:\n", List)
print("\nSliced Lists: ")
newList = List[:3]+List[7:]
print(newList)
List = List[::2]+List[1::2]
print(List)
Output:
Original List: [1, 2, 3, 4, 5, 6, 7, 8, 9] Sliced Lists: [1, 2, 3, 8, 9] [1, 3, 5, 7, 9, 2, 4, 6, 8]By concatenating sliced lists, a new list can be created or even a pre-existing list can be modified.