Element-wise median and percentiles of arrays with Numeric Python

Question

I am using Numeric Python. Unfortunately, NumPy is not an option. If I have multiple arrays, such as:

a=Numeric.array(([1,2,3],[4,5,6],[7,8,9]))
b=Numeric.array(([9,8,7],[6,5,4],[3,2,1]))
c=Numeric.array(([5,9,1],[5,4,7],[5,2,3]))

How do I return an array that represents the element-wise median of arrays a,b and c?...such as,

array(([5,8,3],[5,5,6],[5,2,3]))

And then looking at a more general situation: Given n number of arrays, how do I find the percentiles of each element? For example, return an array that represents the 30th percentile of 10 arrays. Thank you very much for your help!

What do you mean by "NumPy is not an option"? Isn't numeric python the 'numpy'? — Anthony Kong, Apr 01 '12 at 19:43
I think they're too separate things. import Numeric works for me, but import numpy doesn't. — steelymatt, Apr 01 '12 at 22:13

score 1 · Answer 1 · answered Apr 01 '12 at 20:17

1

Combine your stack of 2-D arrays into one 3-D array, d = Numeric.array([a, b, c]) and then sort on the third dimension. Afterwards, the successive 2-D planes will be rank order so you can extract planes for the low, high, quartiles, percentiles, or median.

answered Apr 01 '12 at 20:17

Raymond Hettinger

216,523
63
388
485

Would you be able to write this out for me? I'd really appreciate it. Thanks. – steelymatt Apr 01 '12 at 21:46
@steelymatt I know this is late but I wrote something similar out - [element-wise median of three 2-d arrays](http://stackoverflow.com/a/13753506/206192) - as an answer to another question. – John Lyon Dec 06 '12 at 22:30

score 0 · Answer 2 · answered Jul 24 '18 at 13:08

Putting Raymond Hettinger's description into python:

a=Numeric.array(([1,2,3],[4,5,6],[7,8,9]))
b=Numeric.array(([9,8,7],[6,5,4],[3,2,1]))
c=Numeric.array(([5,9,1],[5,4,7],[5,2,3]))

d = Numeric.array([a, b, c])
d.sort(axis=0)

Since there are n=3 input matrii so the median would be that of the middle one, the one indexed by one,

print d[n//2]
[[5 8 3]
 [5 5 6]
 [5 2 3]]

And if you had 4 input matrii, you would have to get the mean-elements of d[1] and d[2].

score 0 · Answer 3 · answered Apr 01 '12 at 19:24

Well, I'm not versed in Numeric, but I'll just start with a naive solution and see if we can make it any better.

To get the 30th percentile of list foo let x=0.3, sort the list, and pick the the element at foo[int(len(foo)*x)]

For your data, you want to put it in a matrix, transpose it, sort each row, and get the median of each row.

A matrix in Numeric (just like numpy) is an array with two dimensions.

I think that bar = Numeric.array(a,b,c) would make Array you want, and then you could get the nth column with 'bar[:,n]' if Numeric has the same slicing techniques as Numpy.

foo = sorted(bar[:,n])
foo[int(len(foo)*x)]

I hope that helps you.

Element-wise median and percentiles of arrays with Numeric Python

3 Answers3