# Python NumPy numpy.concatenate() Function

Python NumPy `numpy.concatenate()` function concatenates multiple arrays over a specified axis. It accepts a sequence of arrays as a parameter and joins them into a single array.

## Syntax of `numpy.concatenate()`

``````numpy.concatenate((a1, a2,...),
axis= 0,
out= None)
``````

### Parameters

`a1, a2,...` It is a sequence of an array-like structure. It is the sequence of input arrays to concatenate. The input arrays should have the same shape.
`axis` It is an `integer`. It represents the axis over which the function will concatenate the arrays. Its default value is zero which means that the concatenation of arrays will be row-wise. If it is 1, the concatenation will be column-wise.
`out` It is an N-dimensional array. It shows the final shape of the concatenated arrays. If it is provided then its shape must match with the output concatenated array.

### Return

It returns an N-dimensional array. This array shows the concatenation of input arrays.

## Example Codes: `numpy.concatenate()`

We will concatenate a one-dimensional array first using `numpy.concatenate` function.

``````import numpy as np

a1 = np.array([45, 12, 65, 78, 9, 34, 12, 11, 2, 65, 78, 82, 28, 78])
print ('First array:')
print (a1)
a2 = np.array([89, 34, 56, 87, 90])
print ('Second array:')
print (a2)
outarray = np.concatenate([a1,a2])
print ('Concatenated array:')
print(outarray)
``````

Output:

``````First array:
[45 12 65 78  9 34 12 11  2 65 78 82 28 78]
Second array:
[89 34 56 87 90]
Concatenated array:
[45 12 65 78  9 34 12 11  2 65 78 82 28 78 89 34 56 87 90]
``````

The function has returned a concatenated array. If you want the output array to start with the second array, simply pass the second array first as a parameter.

## Example Codes: `numpy.concatenate()` to Pass a Multi-Dimensional Array

We will pass a multi-dimensional array now.

``````import numpy as np

a1 = np.array([[11, 12], [15, 10]])
print ('First array:')
print (a1)
a2 = np.array([[10, 13], [15, 8]])
print ('Second array:')
print (a2)
a3 = np.array([[11, 5], [34, 78]])
print ('Third array:')
print (a3)
outarray = np.concatenate((a1,a2,a3))
print ('Concatenated array:')
print(outarray)
``````

Output:

``````First array:
[[11 12]
[15 10]]
Second array:
[[10 13]
[15  8]]
Third array:
[[11  5]
[34 78]]
Concatenated array:
[[11 12]
[15 10]
[10 13]
[15  8]
[11  5]
[34 78]]
``````

The output shows that the input arrays are now joined together to form a single array.

## Example Codes: `numpy.concatenate()` to Pass a Multi-Dimensional Array With the `axis` Parameter

Now, we will pass a sequence of multi-dimensional arrays as a parameter with `axis` as 0. The resultant array will show the concatenation row-wise.

``````import numpy as np

a1 = np.array([[11, 12, 5], [15, 6, 10]])
print ('First array:')
print (a1)
a2 = np.array([[10, 8, 13], [12, 15, 8]])
print ('Second array:')
print (a2)
a3 = np.array([[11, 12, 5], [34, 78, 90]])
print ('Third array:')
print (a3)
outarray = np.concatenate((a1,a2,a3))
print ('Concatenated array:')
print(outarray)
``````

Output:

``````First array:
[[11 12  5]
[15  6 10]]
Second array:
[[10  8 13]
[12 15  8]]
Third array:
[[11 12  5]
[34 78 90]]
Concatenated array:
[[11 12  5]
[15  6 10]
[10  8 13]
[12 15  8]
[11 12  5]
[34 78 90]]
``````

Note that the output array shows the concatenation of arrays in an increasing rows manner as the default value for `axis` is 0.

Now we will set the value of `axis` to be 1.

``````import numpy as np

a1 = np.array([[11, 12, 5], [15, 6, 10]])
print ('First array:')
print (a1)
a2 = np.array([[10, 8, 13], [12, 15, 8]])
print ('Second array:')
print (a2)
a3 = np.array([[11, 12, 5], [34, 78, 90]])
print ('Third array:')
print (a3)
outarray = np.concatenate((a1,a2,a3), axis= 1)
print ('Concatenated array:')
print(outarray)
``````

Output:

``````First array:
[[11 12  5]
[15  6 10]]
Second array:
[[10  8 13]
[12 15  8]]
Third array:
[[11 12  5]
[34 78 90]]
Concatenated array:
[[11 12  5 10  8 13 11 12  5]
[15  6 10 12 15  8 34 78 90]]
``````

The output array shows the concatenation in an increasing columns manner.

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