sorting – How to sort 2d array by row in python?

sorting – How to sort 2d array by row in python?

How does your 2D array look like?

For example:

>>> a = [
     [12, 18, 6, 3], 
     [ 4,  3, 1, 2], 
     [15,  8, 9, 6]
]
>>> a.sort(key=lambda x: x[1])
>>> a
[[4,  3,  1, 2], 
 [15, 8,  9, 6], 
 [12, 18, 6, 3]]

But I guess you want something like this:

>>> a = [
     [12, 18, 6, 3], 
     [ 4,  3, 1, 2], 
     [15,  8, 9, 6]
]
>>> a = zip(*a)
>>> a.sort(key=lambda x: x[1])
>>> a
[(6,  1,  9), 
 (3,  2,  6), 
 (18, 3,  8), 
 (12, 4, 15)]
>>> a = zip(*a)
>>> a
[(6, 3, 18, 12), 
 (1, 2,  3,  4), 
 (9, 6,  8, 15)
]

Python, per se, has no 2d array — it has (1d) lists as built-ins, and (1d) arrays in standard library module array. There are third-party libraries such as numpy which do provide Python-usable multi-dimensional arrays, but of course youd be mentioning such third party libraries if you were using some of them, rather than just saying in Python, right?-)

So Ill assume that by 2d array you mean a list of lists, such as:

lol = [ range(10), range(2, 12), range(5, 15) ]

or the like — i.e. a list with 3 items, each item being a list with 10 items, and the second row would be the sublist item lol[1]. Yeah, lots of assumptions, but your question is so maddeningly vague that theres no way to avoid making assumptions – edit your Q to clarify with more precision, and an example!, if you dislike people trying to read your mind (and probably failing) as you currently make it impossible to avoid.

So under these assumptions you can sort each of the 3 sublists in the order required to sort the second one, for example:

indices = range(10)
indices.sort(key = lol[1].__getitem__)
for i, sublist in enumerate(lol):
  lol[i] = [sublist[j] for j in indices]

The general approach here is to sort the range of indices, then just use that appropriately sorted range to reorder all the sublists in play.

If you actually have a different problem, there will of course be different solutions;-).

sorting – How to sort 2d array by row in python?

Rather than use lambda x: x[1] you can use operator.itemgetter as the key to the sort or sorted functions. itemgetter(n) creates a function that gets the nth item from a list.

>>> matrix = [ [4,5,6], [1,2,3], [7,0,9]]
>>> from operator import itemgetter
>>> sorted(matrix, key=itemgetter(1))
[[7, 0, 9], [1, 2, 3], [4, 5, 6]]