# NumPy concatenate()

Summary: in this tutorial, you’ll learn how to use the NumPy `concatenate()` function to join elements of two or more arrays into a single array.

## Introduction to the NumPy concatenate() function

The `concatenate()` function allows you to join two or more arrays into a single array. Here’s the basic syntax of the `concatenate()` function:

`np.concatenate((a1,a2,...),axis=0)`

In this syntax, the `concatenate()` function joins the elements of the sequence of arrays (a1, a2, …) into a single array. The arrays in the sequence must have the same shape.

The axis specifies the axis along which the funciton will join the arrays. If the `axis` is `None`, the function will flatten the arrays before joining.

The `concatenate()` function returns the concatenated array.

## NumPy concatenate() function examples

Let’s take some examples of using the `concatenate()` function.

### 1) Using the concatenate() function to join two 1D arrays

The following example uses the `concatenate()` function to join elements of two 1D arrays:

`import numpy as np`

a = np.array([1, 2])
b = np.array([3, 4])

c = np.concatenate((a, b))
print(c)

Output:

`[1 2 3 4]`

In this example, the `concatenate()` function joins the elements in the array a and b into a single array c.

### 2) Using the concatenate() function to join two 2D arrays

The following example uses the `concatenate()` function to join two 2D arrays:

`import numpy as np`

a = np.array([
[1, 2],
[3, 4]
])
b = np.array([
[5, 6],
[7, 8]
])

c = np.concatenate((a, b))
print(c)

Output:

`[[1 2] [3 4] [5 6] [7 8]]`

The output shows that the `concatenate()` function joins two arrays vertically because by default the axis argument is zero.

If the axis is one, the `concatenate()` funciton will join two arrays horizontally. For example:

`import numpy as np`

a = np.array([
[1, 2],
[3, 4]
])
b = np.array([
[5, 6],
[7, 8]
])

c = np.concatenate((a, b), axis=1)
print(c)

Output:

`[[1 2 5 6] [3 4 7 8]]`

## Summary

• Use the numpy `concatenate()` function to join elements of a sequence of arrays into a single array.