Sorting 2-D Array Python Outputing as array within an array

Question

So I'm trying to sort an array like this by the third item in the array:

[[591 756   3]
 [296 669   7]
 [752 560   2]
 [476 474   0]
 [214 459   1]
 [845 349   6]
 [122 145   4]
 [476 125   5]
 [766 110   8]]

so it would look like

[[476 474   0]
 [214 459   1]
 [752 560   2]
 [591 756   3]
 ....and so on]

This is the code I'm currently using:

sortedanswers = sorted(answithcpoints,key=lambda x:x[2])

but printing sortedanswers would output something like

[array([476, 474,   0]), array([214, 459,   1]), array([752, 560,   2]), array([591, 756,   3]), array([122, 145,   4]), array([476, 125,   5]), array([845, 349,   6]), array([296, 669,   7]), array([766, 110,   8])]

seems to be some sort of array inside the array?

Does anyone know why it's doing this?

Numpy array? You can argsort on the last column and re-index. — cs95
– cs95, Commented Nov 24, 2017 at 3:43
Your desired output is also a structure of arrays within an array; what do you really want? — Reblochon Masque
– Reblochon Masque, Commented Nov 24, 2017 at 3:44

cs95 · Accepted Answer · 2017-11-24 04:28:51Z

2

If you're dealing with numpy arrays, don't bring sorted into this. Call argsort on the last column, and use the sorted indices to re-arrange the array -

arr[arr[:, -1].argsort()]

array([[476, 474,   0],
       [214, 459,   1],
       [752, 560,   2],
       [591, 756,   3],
       [122, 145,   4],
       [476, 125,   5],
       [845, 349,   6],
       [296, 669,   7],
       [766, 110,   8]])

arr[:, -1] grabs the last column from arr

arr[:, -1]
array([3, 7, 2, 0, 1, 6, 4, 5, 8])

arr[:, -1].argsort() - sorts arr's last column based on its indices. The result is an array of sorted indices of x
```
arr[:, -1].argsort()
array([3, 4, 2, 0, 6, 7, 5, 1, 8])
```
arr[arr[:, -1].argsort()] indexes rows in arr based on the contents of the sorted indices from the previous step

edited Nov 24, 2017 at 4:28

answered Nov 24, 2017 at 3:45

cs95

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1 Comment

elliot430 Over a year ago

this worked wonderfully thank you! for education purposes, would you mind explaining the line a little bit? what does array[:,-1] actually mean?

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