Python | Column deletion from list of lists

The problem of removing a row from a list is quite simple and we just need to pop a list out of lists . But there can be a utility where we need to delete a column i.e particular index element of each of the list. This is problem that can occur if we store any database data into containers. Let’s discuss certain ways in which this can be performed. 

Method #1 : Using del + loop In this strategy, we just delete the column element one by one using a loop to iterate the row number at each of the iteration. 

Python3

# Python3 code to demonstrate
# deleting columns of list of lists
# using del + loop

# initializing list
test_list = [[4, 5, 6, 8],
             [2, 7, 10, 9],
             [12, 16, 18, 20]]

# printing original list
print ("The original list is : " + str(test_list))

# using del + loop
# deleting column element of row
for j in test_list:
    del j[1]

# printing result
print ("The modified mesh after column deletion : " +  str(test_list))

Output :

The original list is : [[4, 5, 6, 8], [2, 7, 10, 9], [12, 16, 18, 20]] The modified mesh after column deletion : [[4, 6, 8], [2, 10, 9], [12, 18, 20]]

Time Complexity: O(n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list 

Method #2 : Using pop() + list comprehension We can do this particular task in an easier and shorter way using the list comprehension technique and using the pop function which can be used to remove the list element. 

Python3

# Python3 code to demonstrate
# deleting columns of list of lists
# using pop() + list comprehension

# initializing list
test_list = [[4, 5, 6, 8],
             [2, 7, 10, 9],
             [12, 16, 18, 20]]

# printing original list
print ("The original list is : " + str(test_list))

# using pop() + list comprehension
# deleting column element of row
[j.pop(1) for j in test_list]

# printing result
print ("The modified mesh after column deletion : " +  str(test_list))

Output :

The original list is : [[4, 5, 6, 8], [2, 7, 10, 9], [12, 16, 18, 20]] The modified mesh after column deletion : [[4, 6, 8], [2, 10, 9], [12, 18, 20]]

Method 3 :  using NumPy library. 

 steps for this approach:

Import the NumPy library.
Convert the list of lists to a NumPy array using the np.array() function.
Use the np.delete() function to delete the column of the NumPy array.
Convert the modified NumPy array back to a list of lists using the tolist() method.
Print the modified mesh after column deletion.

Python3

# Python3 code to demonstrate
# deleting columns of list of lists
# using NumPy

# importing NumPy
import numpy as np

# initializing list
test_list = [[4, 5, 6, 8],
             [2, 7, 10, 9],
             [12, 16, 18, 20]]

# printing original list
print("The original list is : " + str(test_list))

# converting list to NumPy array
test_array = np.array(test_list)

# deleting column using np.delete()
modified_array = np.delete(test_array, 1, 1)

# converting NumPy array back to list of lists
modified_list = modified_array.tolist()

# printing result
print("The modified mesh after column deletion : " + str(modified_list))

OUTPUT : 
The original list is : [[4, 5, 6, 8], [2, 7, 10, 9], [12, 16, 18, 20]]
The modified mesh after column deletion : [[4, 6, 8], [2, 10, 9], [12, 18, 20]]

Time Complexity: O(n*m), where n is the number of rows and m is the number of columns in the original list.

Auxiliary Space: O(n*m), where n is the number of rows and m is the number of columns in the original list.