identify patterns within an array in python

Question

I've got a question on identifying patterns within an array. I'm working with the following array:

A = [1.0, 1.1, 9.0, 9.2, 0.9, 9.1, 1.0, 1.0, 1.2, 9.2, 8.9, 1.1]

Now, this array is clearly made of elements clustering about ~1 and elements about ~9.

Is there a way to separate these clusters? I.e., to get to something like:

a_1 = [1.0, 1.1, 0.9, 1.0, 1.0, 1.2, 1.1]  # elements around ~1
a_2 = [9.0, 9.2, 9.1, 9.2, 8.9]  # elements around ~9

Thanks a lot. Best.

Sorting sounds like a reasonable first step. Then, determine the mean and distribution? ... Isn't this a Brainstorming kind of question? — Jongware
– Jongware, Commented Jan 19, 2018 at 12:15
Yeah I know, I was thinking about sorting as well, but I was wondering whether there is a function or something to identify clusters within a list or array. If not, I'll proceed by sorting and getting the maximum step among values :) — urgeo
– urgeo, Commented Jan 19, 2018 at 12:18

malioboro · Accepted Answer · 2018-01-19 15:00:05Z

1

You can do that by comparing each element with which is closer. Is it closer to 1 or 9:

a_1 = [i for i in A if abs(i-1)<=abs(i-9)]
a_2 = [i for i in A if abs(i-1)>abs(i-9)]

But of course this is not a general solution for clustering. It only work in this case when you know the center of the cluster (1 and 9).

If you don't know the center of the cluster, I think you should use a clustering algorithm like K-Means

This is a simple K-Means implementation (with k=2 and 100 as limit iteration). You didn't need to know the center of the cluster, it picks randomly at first.

from random import randint

A = [1.0, 1.1, 9.0, 9.2, 0.9, 9.1, 1.0, 1.0, 1.2, 9.2, 8.9, 1.1]

x = A[randint(0,len(A)-1)]
y = A[randint(0,len(A)-1)]
for _ in range(100):
    a_1 = [i for i in A if abs(i-x)<=abs(i-y)]    
    a_2 = [i for i in A if abs(i-x)>abs(i-y)]    
    print(x,y)
    x = sum(a_1)/len(a_1)
    y = sum(a_2)/len(a_2)

print a_1
print a_2

edited Jan 19, 2018 at 15:00

answered Jan 19, 2018 at 12:16

malioboro

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6 Comments

urgeo Over a year ago

Thanks for the answer! However, I don't know the 1 and 9 values a priori. I'd like to identify those values. That's the point of my question

Vikas Periyadath Over a year ago

Instead of abs you can use if round(i) == 1 right ?

malioboro Over a year ago

@VikasDamodar I don't think it works correctly when there are numbers near in the middle, let say 7

malioboro Over a year ago

@urgeo in that case, I think you need a clustering algorithm, may be K-Means

Vikas Periyadath Over a year ago

@urgeo what malioboro said is exactly correct. You need some base for classification of these numbers ..other wise some numbers won't get as listed ex: 5

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