Rank values in a 2D array

Question

Hi basically I want to input a 2D array and then create a new ranked array with the values from lowest to highest.

The array I am using is an elevation raster converted to a numpyarray. This array has a value for elevation. I only want to sort by elevation. Below is how far I got with the code, but it is not generating what I want!

import arcpy
import numpy

array = arcpy.RasterToNumPyArray(r"C:\Current\SedMod\SedModv3\TestModel.gdb\Smallprepfin_Clip")

order = array.argsort()
ranks = order.argsort()

print ranks

Thanks for any help!

Additional clarification:

I want to sort all the values contained within the array from the highest value to the lowest value but maintain the indices and shape of the array.

i.e the highest value(elevation) would be rank 1 and the lowest rank 100 if there are 100 values!

Answer 1

How about something like this:

>>> a
array([[ 0.76303184,  0.17748702,  0.89365504,  0.07221609],
       [ 0.12267359,  0.56999037,  0.42877407,  0.8875015 ],
       [ 0.38178661,  0.57648393,  0.47056551,  0.03178402],
       [ 0.03606595,  0.93597727,  0.02199706,  0.73906879]])
>>> a.ravel().argsort().argsort().reshape(a.shape)
array([[12,  5, 14,  3],
       [ 4,  9,  7, 13],
       [ 6, 10,  8,  1],
       [ 2, 15,  0, 11]])

Not super happy with this, there is likely a better way.

Answer 2

You need scipy.stats.rankdata for this

from scipy.stats import rankdata
ranks=rankdata(a).reshape(a.shape)

To rank from highest to lowest, invert the input a

ranks=rankdata(a.max()-a).reshape(a.shape)