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How to get all 2D diagonals of a 3D NumPy array?
• Last Updated : 02 Sep, 2020

Let’s see the program for getting all 2D diagonals of a 3D NumPy array. So, for this we are using numpy.diagonal() function of NumPy library. This function return specified diagonals from an n-dimensional array.

Syntax: numpy.diagonal(a, axis1, axis2)
Parameters:

• a: represents array from which diagonals has to be taken
• axis1: represents first axis to be taken for 2-d subarray
• axis2: represents second axis to be taken for 2-d subarray

Return: array of diagonal elements.

Now, let’s see an example:

Example 1:

## Python3

 `# Import the numpy package``import` `numpy as np`` ` `# Create 3D-numpy array``# of 4 rows and 4 columns``arr ``=` `np.arange(``3` `*` `4` `*` `4``).reshape(``3``, ``4``, ``4``)`` ` `print``(``"Original 3d array:\n"``, ``      ``arr)`` ` `# Create 2D diagonal array``diag_arr ``=` `np.diagonal(arr, ``                       ``axis1 ``=` `1``,``                       ``axis2 ``=` `2``)`` ` `print``(``"2d diagonal array:\n"``, ``      ``diag_arr)`

Output:

```Original 3d array:
[[[ 0  1  2  3]
[ 4  5  6  7]
[ 8  9 10 11]
[12 13 14 15]]

[[16 17 18 19]
[20 21 22 23]
[24 25 26 27]
[28 29 30 31]]

[[32 33 34 35]
[36 37 38 39]
[40 41 42 43]
[44 45 46 47]]]
2d diagonal array:
[[ 0  5 10 15]
[16 21 26 31]
[32 37 42 47]]

```

Example 2:

## Python3

 `# Import the numpy package``import` `numpy as np`` ` `# Create 3D numpy array``# of 3 rows and 4 columns``arr ``=` `np.arange(``3` `*` `3` `*` `4``).reshape(``3``, ``3``, ``4``)`` ` `print``(``"Original 3d array:\n"``, ``      ``arr)`` ` `# Create 2D diagonal array``diag_arr ``=` `np.diagonal(arr, ``                       ``axis1 ``=` `1``,``                       ``axis2 ``=` `2``)`` ` `print``(``"2d diagonal array:\n"``,``      ``diag_arr)`

Output:

```Original 3d array:
[[[ 0  1  2  3]
[ 4  5  6  7]
[ 8  9 10 11]]

[[12 13 14 15]
[16 17 18 19]
[20 21 22 23]]

[[24 25 26 27]
[28 29 30 31]
[32 33 34 35]]]
2d diagonal array:
[[ 0  5 10]
[12 17 22]
[24 29 34]]

```

Example 3:

## Python3

 `# Import the numpy package``import` `numpy as np`` ` `# Create 3D numpy array``# of 5 rows and 6 columns``arr ``=` `np.arange(``3` `*` `5` `*` `6``).reshape(``3``, ``5``, ``6``)``print``(``"Original 3d array:\n"``,``      ``arr)`` ` `# Create 2D diagonal array``diag_arr ``=` `np.diagonal(arr, ``                       ``axis1 ``=` `1``,``                       ``axis2 ``=` `2``)`` ` `print``(``"2d diagonal array:\n"``,``      ``diag_arr)`

Output:

```Original 3d array:
[[[ 0  1  2  3  4  5]
[ 6  7  8  9 10 11]
[12 13 14 15 16 17]
[18 19 20 21 22 23]
[24 25 26 27 28 29]]

[[30 31 32 33 34 35]
[36 37 38 39 40 41]
[42 43 44 45 46 47]
[48 49 50 51 52 53]
[54 55 56 57 58 59]]

[[60 61 62 63 64 65]
[66 67 68 69 70 71]
[72 73 74 75 76 77]
[78 79 80 81 82 83]
[84 85 86 87 88 89]]]
2d diagonal array:
[[ 0  7 14 21 28]
[30 37 44 51 58]
[60 67 74 81 88]]

```

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