Does Python/Numpy have a function that does the opposite of argsort, that fills an array with the location of that element in the sorted array?

Asked Sep 06 '18 at 08:33

Active Sep 06 '18 at 08:40

Viewed 296 times

so possort([100,50,200,5,150]) would return

[2,1,4,0,3]

I know argsort would give:

[3,1,0,4,2]

and then I could just do

for i,j in enumerate(a):
    x[j]=i

but that seems clumsy and slow

edited Sep 06 '18 at 08:36

kabanus

asked Sep 06 '18 at 08:33

Pax SpaceGoat

1

Not sure you have your dupe-hammer pointed in the right direction @Divakar. Looks more like he wants, for `idx = np.argsort(arr)`, `possort = idx[idx[idx]]` – Daniel F Sep 06 '18 at 09:30
@DanielF I went with the posted code, with `a` as `[3,1,0,4,2] `. Seems like thats what OP wanted. – Divakar Sep 06 '18 at 09:32
@Divakar The result should be `[2,1,4,0,3]`, I think @DanielF is correct. – kabanus Sep 06 '18 at 09:34
Nah, I'm not just tested it with another data set. I know it's some trick like that though. – Daniel F Sep 06 '18 at 09:35
@kabanus From the second link : `argsort_unique(np.array([3,1,0,4,2]))` is `array([2, 1, 4, 0, 3])`. – Divakar Sep 06 '18 at 09:36
@Divakar ah I see now, thanks for clarifying. – kabanus Sep 06 '18 at 09:37
Now I remember, it's `possort(x) = np.argsort(np.argsort(x))` – Daniel F Sep 06 '18 at 09:40
1

And the actual duplicate is [here](https://stackoverflow.com/questions/35136244/how-can-you-get-the-order-back-after-using-argsort?rq=1) – Daniel F Sep 06 '18 at 09:40
There is [a proposal to add such a function](https://github.com/numpy/numpy/issues/9880) – Eric Sep 06 '18 at 15:55
Thanks, I honestly tried searching for this until I got sick of having to prove I was a human for searching so many times. I suppose its tough since I cant think of a specific term to refer to what I'm looking for as opposed to what argsort gives. the argsort(argsort(x)) magic is a great trick! – Pax SpaceGoat Sep 07 '18 at 02:45

0 Answers0