Table of Contents

## Problem Formulation

Given two NumPy arrays `a`

and `b`

. How to concatenate both?

## Method 1: np.concatenate()

NumPy’s `concatenate()`

method joins a sequence of arrays along an existing axis. The first couple of comma-separated array arguments are joined. If you use the axis argument, you can specify along which axis the arrays should be joined. For example, `np.concatenate(a, b, axis=0)`

joins arrays along the first axis and `np.concatenate(a, b, axis=None)`

joins the flattened arrays.

You can see how the axis argument changes the semantics of the array concatenation:

import numpy as np a = np.array([[1, 2], [3, 4]]) b = np.array([[5, 6]]) res_1 = np.concatenate((a, b), axis=0) print(res_1) ''' [[1 2] [3 4] [5 6]] ''' res_2 = np.concatenate((a, b.T), axis=1) print(res_2) ''' [[1 2 5] [3 4 6]] ''' res_3 = np.concatenate((a, b), axis=None) print(res_3) ''' [1 2 3 4 5 6] '''

Note that in the second case, we brought the arrays into a fitting shape via transformation `b.T`

so that we can concatenate the array `b`

as a third column vector.

## Method 2: np.append()

NumPy’s `append()`

method appends values to the end of the array. The optional `axis`

argument allows you to append arrays along the specified axis.

The following code shows the same examples as before—just using the `append()`

method instead of the `concatenate()`

method:

import numpy as np a = np.array([[1, 2], [3, 4]]) b = np.array([[5, 6]]) res_1 = np.append(a, b, axis=0) print(res_1) ''' [[1 2] [3 4] [5 6]] ''' res_2 = np.append(a, b.T, axis=1) print(res_2) ''' [[1 2 5] [3 4 6]] ''' res_3 = np.append(a, b, axis=None) print(res_3) ''' [1 2 3 4 5 6] '''

## Method 3: np.vstack()

NumPy’s `vstack()`

method takes a tuple argument and stacks the arrays in sequence vertically (row wise). This is like concatenating along the first axis after reshaping 1-D arrays of shape *(N,)* to *(1,N)*.

The following code shows the vertical stack arising from the array concatenation:

import numpy as np a = np.array([[1, 2], [3, 4]]) b = np.array([[5, 6]]) res_3 = np.vstack((a, b)) print(res_3) ''' [[1 2] [3 4] [5 6]] '''

## Conclusion

This article has demonstrated how to concatenate two arrays in Python using the NumPy library.

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