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# NumPy | diagonal method

schedule Aug 12, 2023
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NumPy's `diagonal(~)` method returns the diagonal of the input array.

NOTE

From NumPy 1.9 onwards, `diagonal(~)` returns a read-only view of the input array. Modifying this view will result in an error. If you want to modify it, use NumPy's `copy(~)` method.

The NumPy team is considering to change the behavior from read-only to both read-and-write in future versions.

# Parameters

1. `a` | `array-like`

The input array.

2. `offset`link | `int` | `optional`

If offset is positive, the next `offset` diagonal on the top will be returned instead. If negative, then diagonal on the bottom will be returned instead. By default, `offset=0`.

3. `axis1` | `int` | `optional`

The first axis from which to extract the diagonal. By default, axis1=0.

4. `axis2` | `int` | `optional`

The second axis from which to extract the diagonal. By default, axis2=0.

NOTE

You only need to consider the parameters axis1 and axis2 only when dealing with arrays that have 3 or more dimensions.

# Return value

A NumPy array that contains the diagonals of the input array.

# Examples

Consider the following 2D array:

``` a = np.array([[1,2,3],[4,5,6],[7,8,9]])a array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) ```

To get the main diagonal:

``` np.diagonal(a) array([1, 5, 9]) ```

## Offset

To get the diagonal with offset 1:

``` np.diagonal(a, offset=1) array([2, 6]) ```

To get the diagonal with offset -2:

``` np.diagonal(a, offset=-2) array([7]) ```
Published by Isshin Inada
Edited by 0 others
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