How To Add Elements to an Array in Python

Introduction

Python doesn’t have a built-in array data type, however, there are modules you can use to work with arrays. This article describes how to add to an array using the array and the NumPy modules. The array module is useful when you need to create an array of integers and floating-point numbers. The NumPy module is useful when you need to do mathematical operations on an array.

In many cases, you can use List to create arrays because List provides flexibility, such as mixed data types, and still has all the characteristics of an array. Learn more about lists in Python.

Adding Elements to an Array Using the Array Module

With the array module, you can concatenate, or join, arrays using the + operator and you can add elements to an array using the append(), extend(), and insert() methods.

The following example demonstrates how to create a new array object by joining two arrays:

import array

# create array objects, of type integer
arr1 = array.array('i', [1, 2, 3])
arr2 = array.array('i', [4, 5, 6])

# print the arrays
print("arr1 is:", arr1) 
print("arr2 is:", arr2)

# create a new array that contains all of the elements of both arrays 
# and print the result
arr3 = arr1 + arr2
print("After arr3 = arr1 + arr2, arr3 is:", arr3)

The output is:

Output
arr1 is: array('i', [1, 2, 3])
arr2 is: array('i', [4, 5, 6])
After arr3 = arr1 + arr2, arr3 is: array('i', [1, 2, 3, 4, 5, 6])

The preceding example creates a new array that contains all the elements of the the given arrays.

The following example demonstrates how to add to an array using the append(), extend(), and insert() methods:

import array

# create array objects, of type integer
arr1 = array.array('i', [1, 2, 3])
arr2 = array.array('i', [4, 5, 6])

# print the arrays
print("arr1 is:", arr1) 
print("arr2 is:", arr2)

# append an integer to an array and print the result
arr1.append(4)
print("\nAfter arr1.append(4), arr1 is:", arr1)

# extend an array by appending another array of the same type 
# and print the result
arr1.extend(arr2)
print("\nAfter arr1.extend(arr2), arr1 is:", arr1)

# insert an integer before index position 0 and print the result
arr1.insert(0, 10)
print("\nAfter arr1.insert(0, 10), arr1 is:", arr1)

The output is:

Output
arr1 is: array('i', [1, 2, 3])
arr2 is: array('i', [4, 5, 6])

After arr1.append(4), arr1 is: array('i', [1, 2, 3, 4])

After arr1.extend(arr2), arr1 is: array('i', [1, 2, 3, 4, 4, 5, 6])

After arr1.insert(0, 10), arr1 is: array('i', [10, 1, 2, 3, 4, 4, 5, 6])

In the preceding example, each method is called on the arr1 array object and modifies the original object.

Adding Elements to a NumPy Array

With the NumPy module, you can use the NumPy append() and insert() functions to add elements to an array.

The numpy.append() function uses the numpy.concatenate() function in the background. You can use numpy.concatenate() to join a sequence of arrays along an existing axis. Learn more about array manipulation routines in the NumPy documentation.

The examples in this section use 2-dimensional (2D) arrays to highlight how the functions manipulate arrays depending on the axis value you provide.

Appending to an Array using numpy.append()

NumPy arrays can be described by dimension and shape. When you append values or arrays to multi-dimensional arrays, the array or values being appended need to be the same shape, excluding along the given axis.

To understand the shape of a 2D array, consider rows and columns. array([[1, 2], [3, 4]]) has a 2, 2 shape equivalent to 2 rows and 2 columns, while array([[10, 20, 30], [40, 50, 60]]) has a 2, 3 shape equivalent to 2 rows and 3 columns.

Test this concept using the Python interactive console.

First, import the NumPy module, then create some arrays and check their shape.

Import NumPy, then create and print np_arr1:

import numpy as np
np_arr1 = np.array([[1, 2], [3, 4]])
print(np_arr1)

Output
[[1 2]
 [3 4]]

Check the shape of np_arr1:

np_arr1.shape

Output
(2, 2)

Create and print another array, np_arr2:

np_arr2 = np.array([[10, 20, 30], [40, 50, 60]])
print(np_arr2)

Output
[[10 20 30]
 [40 50 60]]

Check the shape of np_arr2:

np_arr2.shape

Output
(2, 3)

Then try appending arrays along the different axes. You can append an array with a shape of 2, 3 to an array with a shape of 2, 2 along axis 1, but not along axis 0.

Append np_arr2 to np_arr1 along axis 0, or by row:

np.append(np_arr1, np_arr2, axis=0)

You get a ValueError:

Output
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<__array_function__ internals>", line 5, in append
  File "/Users/digitalocean/opt/anaconda3/lib/python3.9/site-packages/numpy/lib/function_base.py", line 4817, in append
    return concatenate((arr, values), axis=axis)
  File "<__array_function__ internals>", line 5, in concatenate
ValueError: all the input array dimensions for the concatenation axis must match exactly, but along dimension 1, the array at index 0 has size 2 and the array at index 1 has size 3

You can’t append an array with rows that are three columns wide to an array with rows that are only two columns wide.

Append np_arr2 to np_arr1 along axis 1, or by column:

np.append(np_arr1, np_arr2, axis=1)

The output is:

Output
array([[ 1,  2, 10, 20, 30],
       [ 3,  4, 40, 50, 60]])

You can append an array with columns that are two rows high to another array with columns that are two rows high.

The following example demonstrates how to add elements to a NumPy array using the numpy.append() function:

import numpy as np

# create 2D array objects (integers)
np_arr1 = np.array([[1, 2], [3, 4]])
np_arr2 = np.array([[10, 20], [30, 40]])

# print the arrays
print("np_arr1 is:\n", np_arr1)
print("np_arr2 is:\n", np_arr2)

# append an array to the end of another array and print the result
# both arrays are flattened before appending
append_axis_none = np.append(np_arr1, np_arr2, axis=None)
print("append_axis_none is:\n", append_axis_none)

# append an array to the end of another array along axis 0 (append rows)
# and print the result
append_axis_0 = np.append(np_arr1, np_arr2, axis=0)
print("append_axis_0 is:\n", append_axis_0)

# append an array to the end of another array along axis 1 (append columns)
# and print the result
append_axis_1 = np.append(np_arr1, np_arr2, axis=1)
print("append_axis_1 is:\n", append_axis_1)

The output is:

Output
np_arr1 is:
 [[1 2]
 [3 4]]
np_arr2 is:
 [[10 20]
 [30 40]]
append_axis_none is:
 [ 1  2  3  4 10 20 30 40]
append_axis_0 is:
 [[ 1  2]
 [ 3  4]
 [10 20]
 [30 40]]
append_axis_1 is:
 [[ 1  2 10 20]
 [ 3  4 30 40]]

The preceding example shows how the numpy.append() function works for each axis of the 2D array and how the shape of the resulting array changes. When the axis is 0, the array is appended by row. When the axis is 1, the array is appended by column.

Adding to an Array using numpy.insert()

The numpy.insert() function inserts an array or values into another array before the given index, along an axis, and returns a new array.

Unlike the numpy.append() function, if the axis is not provided or is specified as None, the numpy.insert() function flattens only the first array, and does not flatten the values or array to be inserted. You’ll get a ValueError if you attempt to insert a 2D array into a 2D array without specifying an axis.

The following example demonstrates how to insert elements into an array using the numpy.insert() function:

import numpy as np

# create array objects (integers)
np_arr1 = np.array([[1, 2], [4, 5]])
np_arr2 = np.array([[10, 20], [30, 40]])
np_arr3 = np.array([100, 200, 300])

# print the arrays
print("np_arr1 is:\n", np_arr1)
print("np_arr2 is:\n", np_arr2)
print("np_arr3 is:\n", np_arr3)

# insert a 1D array into a 2D array and then print the result
# the original array is flattened before insertion
insert_axis_none = np.insert(np_arr1, 1, np_arr3, axis=None)
print("insert_axis_none is:\n", insert_axis_none)

# insert an array into another array by row
# and print the result
insert_axis_0 = np.insert(np_arr1, 1, np_arr2, axis=0)
print("insert_axis_0 is:\n", insert_axis_0)

# insert an array into another array by column
# and print the result
insert_axis_1 = np.insert(np_arr1, 1, np_arr2, axis=1)
print("insert_axis_1 is:\n", insert_axis_1)

The output is:

Output
np_arr1 is:
 [[1 2]
 [4 5]]
np_arr2 is:
 [[10 20]
 [30 40]]
insert_axis_none is:
 [  1 100 200 300   2   4   5]
insert_axis_0 is:
 [[ 1  2]
 [10 20]
 [30 40]
 [ 4  5]]
insert_axis_1 is:
 [[ 1 10 30  2]
 [ 4 20 40  5]]

In the preceding example, when you inserted a 2D array into another 2D array along axis 1, each array within np_arr2 was inserted as a separate column into np_arr1. If you want to insert the whole 2D array into another 2D array, include square brackets around the obj parameter index value to indicate that the whole array should be inserted before that position. Without the square brackets, numpy.insert() stacks the arrays in sequence as columns before the given index.

The following example shows the output with and without square brackets around the obj (index) parameter:

import numpy as np

# create 2D array objects (integers)
np_arr1 = np.array([[1, 2], [3, 4]])
np_arr2 = np.array([[10, 20], [30, 40]])

# print the arrays
print("np_arr1 is:\n", np_arr1)
print("np_arr2 is:\n", np_arr2)

# insert an array, column by column, into another array
# and print the result
insert_axis_1 = np.insert(np_arr1, 1, np_arr2, axis=1)
print("insert_axis_1 is:\n", insert_axis_1)

# insert a whole array into another array by column
# and print the result
insert_index_axis_1 = np.insert(np_arr1, [1], np_arr2, axis=1)
print("insert_index_axis_1 is:\n", insert_index_axis_1)

The output is:

Output
np_arr1 is:
 [[1 2]
 [3 4]]
np_arr2 is:
 [[10 20]
 [30 40]]
insert_axis_1 is:
 [[ 1 10 30  2]
 [ 3 20 40  4]]
insert_index_axis_1 is:
 [[ 1 10 20  2]
 [ 3 30 40  4]]

The preceding example shows how numpy.insert() inserts columns into an array depending on the index notation.

Conclusion

In this article you added elements to arrays using the array and NumPy modules. Continue your learning with more NumPy tutorials and Python tutorials.