numpy.append() in Python

numpy.append() function is used to add new values at end of existing NumPy array. This is useful when we have to add more elements or rows in existing numpy array. It can also combine two arrays into a bigger one.

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

• array: Input array.
• values: The values to append the input array. If axis is specified the shape of values must match the dimensions of the input array along the specified axis. Otherwise values will be flattened before appending.
• axis: along which we want to insert the values. By default array is flattened.

1. Appending 1D Arrays

In this example, we append two 1D arrays without specifying an axis. By default the arrays are flattened before appending.

import numpy as geek

arr1 = geek.arange(5)
print("1D arr1 :", arr1)
print("Shape:", arr1.shape)

arr2 = geek.arange(8, 12)
print("1D arr2 :", arr2)
print("Shape :", arr2.shape)

arr3 = geek.append(arr1, arr2)
print("Appended arr3:", arr3)

Output : 

From the above output we can see that the numpy.append() function does not modify the original arrays (arr1 and arr2). Instead it creates a new array (arr3) with the combined elements.

2. Appending 2D Arrays with Axis Specification

When working with multi-dimensional arrays you can specify the axis parameter to control how the values are appended.

import numpy as geek

arr1 = geek.arange(8).reshape(2, 4)
print("2D arr1 :", arr1)
print("Shape :", arr1.shape)


arr2 = geek.arange(8, 16).reshape(2, 4)
print("2D arr2:", arr2)
print("Shape :", arr2.shape)

arr3 = geek.append(arr1, arr2)
print("Appended arr3 by flattened :", arr3)

arr3 = geek.append(arr1, arr2, axis = 0)
print("Appended arr3 with axis 0 :", arr3)

arr3 = geek.append(arr1, arr2, axis = 1)
print("Appended arr3 with axis 1 :", arr3)

Output : 

• When no axis is given both arrays are flattened into a single long 1D array and then joined.
• When axis=0 rows from the second array are added below the first.
• With axis=1columns are added to the right side of the first array.

numpy.append() function is used to extend a array and it creates a new array rather than modifying the original one.