pytutorial/numpy/reverse_an_axis/README.md

Reverse an axis

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The goal

Questions to David Rotermund

numpy.flip

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

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

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:

[[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

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.

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

[[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]]