Inversing a twodimensional array in python

Question

I have an array a, which is twodimensional. A contains objects which also contain objects. I want to make sure that a[1,1] becomes a[n,n], a[2,1] becomes a[n-1,n], a[2,2] becomes a[n-1][n-1] etc. I wrote the following code to do this:

tempArray = copy(self.topArea)
for y in range(0,len(tempArray)):
    for x in range(0,len(tempArray[y])):
        self.topArea[y][x] = tempArray[len(tempArray)-1-y][len(tempArray[y])-1-x]

But this achieves litteraly nothing. Deepcopying also does not help: the array does not get inverted.

How can I invert it?

Is this a numpy array, or are you using the wrong word for a list? — Gareth Latty
– Gareth Latty, Commented Jun 12, 2013 at 12:51
Also note that iterating by index is a terrible idea in Python - it is slow, inefficient, inflexible and hard to read. Python is designed to iterate by value, not index. — Gareth Latty
– Gareth Latty, Commented Jun 12, 2013 at 12:53
Can you give a little better definition of inversing? At first, I answered showing how to take the transpose, but now I think that isn't what you want ... — mgilson
– mgilson, Commented Jun 12, 2013 at 12:57

Ashwini Chaudhary · Accepted Answer · 2013-06-12 13:05:43Z

3

Do you want something like:

tempArray = [list(reversed(x)) for x in reversed(self.topArea)]

If everything is lists, you could also do:

tempArray = [x[::-1] for x in reversed(self.topArea)]

for a possible speed boost.

edited Jun 12, 2013 at 13:05

Ashwini Chaudhary

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answered Jun 12, 2013 at 12:52

mgilson

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3 Comments

Sylvain Leroux Over a year ago

tempArray = [x[::-1] for x in self.topArea[::-1]] ?

Ashwini Chaudhary Over a year ago

@SylvainLeroux reversed is memory efficient as it returns an iterator.

mgilson Over a year ago

Also, presumably, reversed should work for more objects. I'm not sure if I've found that to be true in actual practice, but any sequence can be reversed (including xrange which you can't slice)

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