View source for Flip, rot90, and roll a matrix

== The goal ==
Questions to [mailto:davrot@uni-bremen.de David Rotermund]

== [https://numpy.org/doc/stable/reference/generated/numpy.flip.html numpy.flip] ==
<syntaxhighlight lang="python">numpy.flip(m, axis=None)</syntaxhighlight><blockquote>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.</blockquote>

=== 1d ===
<syntaxhighlight lang="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]</syntaxhighlight>

=== 2d ===
<syntaxhighlight lang="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))</syntaxhighlight>Output:<syntaxhighlight lang="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]]</syntaxhighlight>

== [https://numpy.org/doc/stable/reference/generated/numpy.rot90.html numpy.rot90] ==
<syntaxhighlight lang="python">numpy.rot90(m, k=1, axes=(0, 1))</syntaxhighlight><blockquote>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.</blockquote><syntaxhighlight lang="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()</syntaxhighlight>Output<syntaxhighlight lang="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]]</syntaxhighlight>

== [https://numpy.org/doc/stable/reference/generated/numpy.roll.html numpy.roll] ==
<syntaxhighlight lang="python">numpy.roll(a, shift, axis=None)</syntaxhighlight><blockquote>Roll array elements along a given axis.

Elements that roll beyond the last position are re-introduced at the first.</blockquote><syntaxhighlight lang="python">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))</syntaxhighlight>Output:<syntaxhighlight lang="python">[[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]]</syntaxhighlight><syntaxhighlight lang="python">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))</syntaxhighlight>Output:<syntaxhighlight lang="python">[[0 1 2]
 [3 4 5]]

[[3 4 5]
 [0 1 2]]

[[0 1 2]
 [3 4 5]]</syntaxhighlight><syntaxhighlight lang="python">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))</syntaxhighlight>Output:<syntaxhighlight lang="python">[[0 1 2]
 [3 4 5]]

[[2 0 1]
 [5 3 4]]

[[1 2 0]
 [4 5 3]]

[[0 1 2]
 [3 4 5]]</syntaxhighlight>