94 lines
2.3 KiB
ReStructuredText
94 lines
2.3 KiB
ReStructuredText
Rotate a Matrix in Python
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#########################
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:author: tyrel
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:category: Tech
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:tags: python
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:status: published
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I've been doing Advent of Code for a few years now, and every year I do it in my favorite language, Python.
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One thing that comes up a lot, is rotating matrices.
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One way to do this, is to use Numpy, using ``np.rot90(mat)``, but not everyone wants to install Numpy just to do one small task.
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I know I don't always.
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The way I always do it, that will support non-square matrixes, is to use zip.
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.. code-block:: python
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>>> matrix = [
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[1,2,3],
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[4,5,6],
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[7,8,9]
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]
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>>> rotated = list(zip(*matrix[::-1]))
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# And result is
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[[7, 4, 1],
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[8, 5, 2],
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[9, 6, 3]]
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We can break this down bit by bit.
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This will copy the list, with a -1 step, resulting in a reverse order
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.. code-block:: python
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>>> matrix[::-1]
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[[7,8,9],
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[4,5,6],
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[1,2,3]]
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Next we need to call zip in order to get the x-th item from each inner list, but first, we need to unpack it. If you'll notice, the unpacked version isn't wrapped with another list, which is what zip needs from us.
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.. code-block:: python
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# Too many lists
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>>> print(matrix[::-1])
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[[7, 8, 9], [4, 5, 6], [1, 2, 3]]
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# Just right
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>>> print(*matrix[::-1])
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[7, 8, 9] [4, 5, 6] [1, 2, 3]
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From there, we can pass this unpacked list of - in our case - three lists, to zip (and in Python 3 this returns a generator, so we need to call list again on it, or just use it)
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.. code-block:: python
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>>> # Again, we get the rotated matrix
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>>> list(zip(*matrix[::-1]))
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[[7, 4, 1],
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[8, 5, 2],
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[9, 6, 3]]
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Notes
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-----
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Small note: If you run this, you will actually get a list of tuples, so you can map those back to a list, if you need to update them for any reason.
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I just wanted square brackets in my examples.
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.. code-block:: python
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# This is just messy looking, so I didn't mention it until now
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>>> list(map(list, zip(*matrix[::-1])))
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As I mentioned, due to using ``zip`` this will work with non-square examples as well.
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.. code-block:: python
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>>> matrix = [
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... [1,2,3,4,5,6,7,8,9],
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... [9,8,7,6,5,4,3,2,1],
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... ]
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>>> print(list(zip(*matrix[::-1])))
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[(9, 1),
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(8, 2),
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(7, 3),
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(6, 4),
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(5, 5),
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(4, 6),
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(3, 7),
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(2, 8),
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(1, 9)]
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