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Python | Ways to add row/columns in numpy array

  • Last Updated : 10 Sep, 2020

Given numpy array, the task is to add rows/columns basis on requirements to numpy array. Let’s see a few examples of this problem.
Method #1: Using np.hstack() method 
 

Python3




# Python code to demonstrate
# adding columns in numpy array
 
import numpy as np
 
ini_array = np.array([[1, 2, 3], [45, 4, 7], [9, 6, 10]])
 
# printing initial array
print("initial_array : ", str(ini_array));
 
# Array to be added as column
column_to_be_added = np.array([1, 2, 3])
 
# Adding column to numpy array
result = np.hstack((ini_array, np.atleast_2d(column_to_be_added).T))
 
# printing result
print ("resultant array", str(result))

Output: 
 

initial_array :  [[ 1  2  3]
 [45  4  7]
 [ 9  6 10]]
resultant array [[ 1  2  3  1]
 [45  4  7  2]
 [ 9  6 10  3]]

Method #2: Using column_stack() method 
 

Python3




# python code to demonstrate
# adding columns in numpy array
 
import numpy as np
 
ini_array = np.array([[1, 2, 3], [45, 4, 7], [9, 6, 10]])
 
# printing initial array
print("initial_array : ", str(ini_array));
 
# Array to be added as column
column_to_be_added = np.array([1, 2, 3])
 
# Adding column to numpy array
result = np.column_stack((ini_array, column_to_be_added))
 
# printing result
print ("resultant array", str(result))

Output: 
 

initial_array :  [[ 1  2  3]
 [45  4  7]
 [ 9  6 10]]
resultant array [[ 1  2  3  1]
 [45  4  7  2]
 [ 9  6 10  3]]

Method #3: Using np.vstack() method 
 

Python3




# python code to demonstrate
# adding rows in numpy array
 
import numpy as np
 
ini_array = np.array([[1, 2, 3], [45, 4, 7], [9, 6, 10]])
 
# printing initial array
print("initial_array : ", str(ini_array));
 
# Array to be added as row
row_to_be_added = np.array([1, 2, 3])
 
# Adding row to numpy array
result = np.vstack ((ini_array, row_to_be_added) )
 
# printing result
print ("resultant array", str(result))

Output: 
 

initial_array :  [[ 1  2  3]
 [45  4  7]
 [ 9  6 10]]
resultant array [[ 1  2  3]
 [45  4  7]
 [ 9  6 10]
 [ 1  2  3]]

Sometimes we have an empty array and we need to append rows in it. Numpy provides the function to append a row to an empty Numpy array using numpy.append() function.

Syntax : numpy.append(arr, values, axis=None)

Case 1: Adding new rows to an empty 2-D array

Python3




# importing Numpy package
import numpy as np  
 
# creating an empty 2d array of int type
empt_array = np.empty((0,2), int)
print("Empty array:")
print(empt_array)
 
# adding two new rows to empt_array
# using np.append()
empt_array = np.append(empt_array, np.array([[10,20]]), axis=0)
empt_array = np.append(empt_array, np.array([[40,50]]), axis=0)
 
print("\nNow array is:")
print(empt_array)
Empty array:
[]

Now array is:
[[10 20]
[40 50]]

Case 2: Adding new rows to an empty 3-D array

Python3




# importing Numpy package
import numpy as np  
 
# creating an empty 3d array of int type
empt_array = np.empty((0,3), int)
print("Empty array:")
print(empt_array)
 
# adding three new rows to empt_array
# using np.append()
empt_array = np.append(empt_array, np.array([[10,20,40]]), axis=0)
empt_array = np.append(empt_array, np.array([[40,50,55]]), axis=0)
empt_array = np.append(empt_array, np.array([[40,50,55]]), axis=0)
 
print("\nNow array is:")
print(empt_array)
Empty array:
[]

Now array is:
[[10 20 40]
 [40 50 55]
 [40 50 55]]

Case 3: Adding new rows to an empty 4-D array

Python3




# importing Numpy package
import numpy as np  
 
# creating an empty 4d array of int type
empt_array = np.empty((0,4), int)
print("Empty array:")
print(empt_array)
 
# adding four new rows to empt_array
# using np.append()
empt_array = np.append(empt_array, np.array([[100,200,400,888]]), axis=0)
empt_array = np.append(empt_array, np.array([[405,500,550,558]]), axis=0)
empt_array = np.append(empt_array, np.array([[404,505,555,145]]), axis=0)
empt_array = np.append(empt_array, np.array([[44,55,550,150]]), axis=0)
 
print("\nNow array is:")
print(empt_array)
Empty array:
[]

Now array is:
[[100 200 400 888]
 [405 500 550 558]
 [404 505 555 145]
 [ 44  55 550 150]]

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