method to reverse row and column orders of a matrix

[videlec]

This ticket implements a method reverse_rows_and_columns

sage: m = matrix(ZZ, 2, 3, range(6))
sage: m.reverse_rows_and_columns()
sage: m
[5 4 3]
[2 1 0]

Needed in #22841.

Note: in numpy they rather use flip (that return a copy) and Python list uses reverse

CC: @kwankyu

Component: linear algebra

Author: Vincent Delecroix

Branch/Commit: e7d56d0

Reviewer: Kwankyu Lee

Issue created by migration from https://trac.sagemath.org/ticket/23050

[videlec]

Commit: 17bbf4e

[videlec]

Branch: u/vdelecroix/23050

[videlec]

New commits:

17bbf4e

23050: reverse_rows_and_columns

[mforets]

comment:2

salut Vincent, i saw your cool function, maybe you can paste this too or similar:

The row-and-column-reversed matrix of `M` is defined as:

.. MATH::

    (i,j) \mapsto M[nrows - i - 1, ncols - j - 1]

In other words, this function flips `M` up to down and left to right.

[videlec]

comment:3

Replying to @mforets:

salut Vincent, i saw your cool function, maybe you can paste this too or similar:

The row-and-column-reversed matrix of `M` is defined as:

.. MATH::

    (i,j) \mapsto M[nrows - i - 1, ncols - j - 1]

In other words, this function flips `M` up to down and left to right.

good idea.

[kwankyu]

comment:4

Looks and works nice. Thank you! Just two minor comments:

1. Docstrings better to be consistent in the three methods. Note the period at the end.

Reverse the orders of rows and columns.

2. You may want to also show the matrices before reversing in some of the examples.

[videlec]

comment:5

What do you think of calling the method simply reverse?

[kwankyu]

comment:6

If someone implements reverse_rows or reverse_columns in future, then reverse will be somewhat unclear. I prefer to stick to the current name.

[videlec]

comment:7

This method already exists and called antitranspose! I propose to keep this ticket to make an inplace version.

[videlec]

comment:8

Sorry, it is different: transpose and antitranspose does inverse rows <-> columns. They are symmetries with respect to diagonals. However, both transpose and antitranspose return a different matrix. It would be weird to have a different behavior for reverse.

I propose to have

• transpose, antitranspose, reverse as methods returning a different matrix
• _transpose_inplace, _antitranspose_inplace and _reverse_inplace that modifies the matrix

[videlec]

comment:9

Note that in numpy, there is method to do reflection that is called flip. What I am trying to achieve with this ticket corresponds to

sage: import numpy as np
sage: a = numpy.array([[0,1],[2,3],[4,5]])
sage: np.flip(np.flip(a,0), 1)
array([[5, 4],
       [3, 2],
       [1, 0]])

And numpy provides shortcuts flipud (up/down) and fliplr (left/right) for flipping with respect to axes 0 and 1.

[sagetrac-git]

Changed commit from 17bbf4e to 39818b2

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

39818b2

23050: reverse_rows_and_columns

[videlec]

comment:11

Needs review again. Transpose and antitranspose are a bit different since the matrix space changes.

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

dbfaeaa

23050: reverse_rows_and_columns

[sagetrac-git]

Changed commit from 39818b2 to dbfaeaa

[kwankyu]

comment:14

Still missing a period at the end:

Reverse the row order and column order of this matrix.

[kwankyu]

comment:15

You may fold this long line

ValueError: matrix is immutable; please change a copy instead (i.e., use copy(M) to change a copy of M).

[sagetrac-git]

Changed commit from dbfaeaa to e7d56d0

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e7d56d0

23050: documentation details

[videlec]

comment:17

done

[kwankyu]

Reviewer: Kwankyu Lee

[vbraun]

Changed branch from u/vdelecroix/23050 to e7d56d0