reversing rows for indexes, python

Question

I have data with (x,y) coordinates that origins in the lefttop corner. I turn into a matrix with indexes starting from 0 that looks like that:

| 0 | 1 | 2 | 3 | 4|
| 5 | 6 | 7 | 8 | 9|
|10 |11 |12 |13 |14|
|15 |16 |17 |18 |19|
|20 |21 |22 |23 |24|

for example (x,y)=(1,4) id = y*5 + x = 21. But i want it to origin in the left bottom so that i obtain a matrix with indexing like that:

| 20| 21| 22| 23| 24|
| 15| 16| 17| 18| 19|
| 10| 11| 12| 13| 14|
| 5 | 6 | 7 | 8 |  9|
| 0 | 1 | 2 | 3 |  4|

so basically reversing the rows. Is there a way to do that in pythonic way? What would be the most computationally efficient way to achieve it otherwise?

Lifu Huang · Accepted Answer · 2016-10-26 03:40:50Z

1

I am not sure about whether "Pythonic" or not, but the most direct and efficient way to me is simply just:

u = [v[5 * (len(v) // 5 - i // 5 - 1) + i % 5] for i in xrange(len(v))]

EDIT:

If you feel uncomfortable about these divisions and modulos, an alternative without any math calculation(but might be less efficient) is:

sum([v[i: i+5] for i in xrange(0, len(v), 5)][::-1], [])

edited Oct 26, 2016 at 3:40

answered Oct 26, 2016 at 3:28

Lifu Huang

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Comments

Eli Sadoff · Accepted Answer · 2016-10-26 03:19:50Z

0

I don't know how pythonic this is, but it works fine

for i in len(v)//2:
      v[i], v[-i] = v[-i], v[i]

answered Oct 26, 2016 at 3:19

Eli Sadoff

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LittleQ · Accepted Answer · 2016-10-26 03:23:18Z

0

Reverse the array by x

>>> M = [[0,1,2],[3,4,5],[6,7,8],[9,10,11]]
>>> M[::-1]
[[9, 10, 11], [6, 7, 8], [3, 4, 5], [0, 1, 2]]

answered Oct 26, 2016 at 3:23

LittleQ

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boboquack · Accepted Answer · 2016-10-26 03:23:40Z

0

Use either list(reversed(x)) or x[::-1] like:

matrix=[[0,1,2,3,4],[5,6,7,8,9],[10,11,12,13,14],[15,16,17,18,19]]
backwards=matrix[::-1]
inverted=list(reversed(matrix))

reversed returns a backwards iterator that list converts to a list, and [::-1] returns a list splice taking the elements backwards.

answered Oct 26, 2016 at 3:23

boboquack

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Zeugma · Accepted Answer · 2016-10-26 03:32:08Z

0

When the shape of your matrix is width, height = 5, 5, the general formula to get the bottom up index from the list index is:

width*(height-index//height-1)+index%height

answered Oct 26, 2016 at 3:32

Zeugma

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