How do you slice a 2d list in python?

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. 

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