Python | Ways to add row/columns in numpy array - GeeksforGeeks

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 arrayprint("initial_array : ", str(ini_array)); # Array to be added as columncolumn_to_be_added = np.array([1, 2, 3]) # Adding column to numpy arrayresult = np.hstack((ini_array, np.atleast_2d(column_to_be_added).T)) # printing resultprint ("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 arrayprint("initial_array : ", str(ini_array)); # Array to be added as columncolumn_to_be_added = np.array([1, 2, 3]) # Adding column to numpy arrayresult = np.column_stack((ini_array, column_to_be_added)) # printing resultprint ("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 arrayprint("initial_array : ", str(ini_array)); # Array to be added as rowrow_to_be_added = np.array([1, 2, 3]) # Adding row to numpy arrayresult = np.vstack ((ini_array, row_to_be_added) ) # printing resultprint ("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 packageimport numpy as np   # creating an empty 2d array of int typeempt_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 packageimport numpy as np   # creating an empty 3d array of int typeempt_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 packageimport numpy as np   # creating an empty 4d array of int typeempt_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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