Sorting coordinates array

[Maarten van Reeuwijk]

I'm a newbie to Python, so sorry for this maybe trivial question. I have a
numpy array with coordinates, which I want to sort, for example first on
z-coordinate, then x and lastly y-coordinate. So an array like:

[[0, 0, 0], [0, 2, 1], [1, 1, 0], [0, 1, 1]]

should look after sorting like

[[0, 0, 0], [1, 1, 0], [0, 1, 1], [0, 2, 1]]

I tried the sort command, but that command mangles my coordinate pairs. Also
converting the array to a list doesn't seem to improve the results. How
should I tackle this problem?

TIA, Maarten

--
=============== =============== =============== =============== =======
Maarten van Reeuwijk Heat and Fluid Sciences
Phd student dept. of Multiscale Physics
www.ws.tn.tudelft.nl Delft University of Technology

[Ville Vainio]

Re: Sorting coordinates array

Maarten van Reeuwijk <maarten@remove _this_ws.tn.tud elft.nl> writes:
[color=blue]
> I'm a newbie to Python, so sorry for this maybe trivial question. I have a
> numpy array with coordinates, which I want to sort, for example first on
> z-coordinate, then x and lastly y-coordinate. So an array like:[/color]

Implement a comparison function:
[color=blue][color=green][color=darkred]
>>> help([].sort)[/color][/color][/color]
Help on built-in function sort:

sort(...)
L.sort(cmpfunc= None) -- stable sort *IN PLACE*; cmpfunc(x, y) -> -1, 0, 1


Alternatively, you could take a look at DSU pattern (Decorate, Sort,
Undecorate):

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52234

--
Ville Vainio http://www.students.tut.fi/~vainio24

[Peter Otten]

Re: Sorting coordinates array

Maarten van Reeuwijk wrote:
[color=blue]
> I'm a newbie to Python, so sorry for this maybe trivial question. I have a
> numpy array with coordinates, which I want to sort, for example first on
> z-coordinate, then x and lastly y-coordinate. So an array like:
>
> [[0, 0, 0], [0, 2, 1], [1, 1, 0], [0, 1, 1]]
>
> should look after sorting like
>
> [[0, 0, 0], [1, 1, 0], [0, 1, 1], [0, 2, 1]]
>
> I tried the sort command, but that command mangles my coordinate pairs.
> Also converting the array to a list doesn't seem to improve the results.
> How should I tackle this problem?[/color]
[color=blue][color=green][color=darkred]
>>> import Numeric
>>> a = Numeric.array([[0, 0, 0], [0, 2, 1], [1, 1, 0], [0, 1, 1]])
>>> lst = a.tolist()
>>> lst.sort(lambda x, y: cmp(x[::-1], y[::-1]))
>>> lst[/color][/color][/color]
[[0, 0, 0], [1, 1, 0], [0, 1, 1], [0, 2, 1]][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

You can pass an arbitrary comparison function to list.sort(), I just compare
reversed copies of the inner lists.
This is a highly unoptimized version, but at least seems to do the job.

Peter

[Peter Otten]

Re: Sorting coordinates array

Peter Otten wrote:
[color=blue]
> This is a highly unoptimized version, but at least seems to do the job.[/color]

Slightly better:
[color=blue][color=green][color=darkred]
>>> lst = a.tolist()
>>> lst[/color][/color][/color]
[[0, 0, 0], [0, 2, 1], [1, 1, 0], [0, 1, 1]][color=blue][color=green][color=darkred]
>>> for item in lst: item.reverse()[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> lst.sort()
>>> for item in lst: item.reverse()[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> lst[/color][/color][/color]
[[0, 0, 0], [1, 1, 0], [0, 1, 1], [0, 2, 1]][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Peter