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Signed-off-by: David Rotermund <54365609+davrot@users.noreply.github.com>
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# Reverse an axis
{:.no_toc}
<nav markdown="1" class="toc-class">
* TOC
{:toc}
</nav>
## The goal
Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
## [numpy.flip](https://numpy.org/doc/stable/reference/generated/numpy.flip.html)
```python
numpy.flip(m, axis=None)
```
> Reverse the order of elements in an array along the given axis.
>
> The shape of the array is preserved, but the elements are reordered.
>
> **m** : array_like
>
> Input array.
>
> **axis** : None or int or tuple of ints, optional
>
> Axis or axes along which to flip over. The default, axis=None, will flip over all of the axes of the input array. If axis is negative it counts from the last to the first axis.
>
> If axis is a tuple of ints, flipping is performed on all of the axes specified in the tuple.
>
> **out** : array_like
>
> A **view** of m with the entries of axis reversed. Since a view is returned, this operation is done in constant time.
### 1d
```python
import numpy as np
a = np.arange(0, 6)
print(a) # -> [0 1 2 3 4 5]
print()
print(np.flip(a)) # -> [5 4 3 2 1 0]
print()
print(a[::-1]) # -> [5 4 3 2 1 0]
```
### 2d
```python
import numpy as np
a = np.arange(0, 6).reshape((2, 3))
print(a)
print()
print(np.flip(a))
print()
print(np.flip(a, axis=0))
print()
print(np.flip(a, axis=1))
```
Output:
```python
[[0 1 2]
[3 4 5]]
[[5 4 3]
[2 1 0]]
[[3 4 5]
[0 1 2]]
[[2 1 0]
[5 4 3]]
```
## [numpy.rot90](https://numpy.org/doc/stable/reference/generated/numpy.rot90.html)
```python
numpy.rot90(m, k=1, axes=(0, 1))
```
> Rotate an array by 90 degrees in the plane specified by axes.
>
> Rotation direction is from the first towards the second axis. This means for a 2D array with the default k and axes, the rotation will be counterclockwise.
```python
import numpy as np
a = np.arange(0, 6).reshape((2, 3))
print(a)
print()
print(np.rot90(a, k=1))
print()
print(np.rot90(a, k=2))
print()
print(np.rot90(a, k=3))
print()
print(np.rot90(a, k=4))
print()
```
Output
```python
[[0 1 2]
[3 4 5]]
[[2 5]
[1 4]
[0 3]]
[[5 4 3]
[2 1 0]]
[[3 0]
[4 1]
[5 2]]
[[0 1 2]
[3 4 5]]
```