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Reverse an axis
{:.no_toc}
* TOC {:toc}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]]
numpy.roll
numpy.roll(a, shift, axis=None)
Roll array elements along a given axis.
Elements that roll beyond the last position are re-introduced at the first.
import numpy as np
a = np.arange(0, 6).reshape((2, 3))
print(a)
print()
print(np.roll(a, shift=1))
print()
print(np.roll(a, shift=2))
print()
print(np.roll(a, shift=3))
print()
print(np.roll(a, shift=4))
Output:
[[0 1 2]
[3 4 5]]
[[5 0 1]
[2 3 4]]
[[4 5 0]
[1 2 3]]
[[3 4 5]
[0 1 2]]
[[2 3 4]
[5 0 1]]
import numpy as np
a = np.arange(0, 6).reshape((2, 3))
print(a)
print()
print(np.roll(a, shift=1, axis=0))
print()
print(np.roll(a, shift=2, axis=0))
Output:
[[0 1 2]
[3 4 5]]
[[3 4 5]
[0 1 2]]
[[0 1 2]
[3 4 5]]
import numpy as np
a = np.arange(0, 6).reshape((2, 3))
print(a)
print()
print(np.roll(a, shift=1, axis=1))
print()
print(np.roll(a, shift=2, axis=1))
print()
print(np.roll(a, shift=3, axis=1))
Output:
[[0 1 2]
[3 4 5]]
[[2 0 1]
[5 3 4]]
[[1 2 0]
[4 5 3]]
[[0 1 2]
[3 4 5]]