127 lines
2.1 KiB
Markdown
127 lines
2.1 KiB
Markdown
# Reverse an axis
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{:.no_toc}
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<nav markdown="1" class="toc-class">
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* TOC
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{:toc}
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</nav>
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## The goal
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Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
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## [numpy.flip](https://numpy.org/doc/stable/reference/generated/numpy.flip.html)
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```python
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numpy.flip(m, axis=None)
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```
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> Reverse the order of elements in an array along the given axis.
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>
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> The shape of the array is preserved, but the elements are reordered.
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>
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> **m** : array_like
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>
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> Input array.
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>
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> **axis** : None or int or tuple of ints, optional
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>
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> 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.
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>
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> If axis is a tuple of ints, flipping is performed on all of the axes specified in the tuple.
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>
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> **out** : array_like
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>
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> A **view** of m with the entries of axis reversed. Since a view is returned, this operation is done in constant time.
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### 1d
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```python
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import numpy as np
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a = np.arange(0, 6)
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print(a) # -> [0 1 2 3 4 5]
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print()
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print(np.flip(a)) # -> [5 4 3 2 1 0]
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print()
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print(a[::-1]) # -> [5 4 3 2 1 0]
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```
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### 2d
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```python
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import numpy as np
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a = np.arange(0, 6).reshape((2, 3))
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print(a)
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print()
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print(np.flip(a))
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print()
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print(np.flip(a, axis=0))
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print()
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print(np.flip(a, axis=1))
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```
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Output:
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```python
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[[0 1 2]
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[3 4 5]]
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[[5 4 3]
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[2 1 0]]
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[[3 4 5]
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[0 1 2]]
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[[2 1 0]
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[5 4 3]]
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```
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## [numpy.rot90](https://numpy.org/doc/stable/reference/generated/numpy.rot90.html)
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```python
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numpy.rot90(m, k=1, axes=(0, 1))
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```
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> Rotate an array by 90 degrees in the plane specified by axes.
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>
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> 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.
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```python
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import numpy as np
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a = np.arange(0, 6).reshape((2, 3))
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print(a)
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print()
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print(np.rot90(a, k=1))
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print()
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print(np.rot90(a, k=2))
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print()
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print(np.rot90(a, k=3))
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print()
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print(np.rot90(a, k=4))
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print()
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```
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Output
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```python
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[[0 1 2]
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[3 4 5]]
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[[2 5]
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[1 4]
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[0 3]]
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[[5 4 3]
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[2 1 0]]
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[[3 0]
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[4 1]
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[5 2]]
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[[0 1 2]
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[3 4 5]]
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```
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