3d Random sampling

Question

I have a set of points xyz8,

I want to randomly get 10% of points.

Then I want to randomly get 10% of the remaining 90%

Then I want to randomly get 10% of the remaining 70%

etc until all points done

How can I go about doing this?

Any advice is hugely appreciated

Suppose you have 1000 elements, the first round you sample 100. In the second round, do you sample 100 from 900 or 90 from 900? — CT Zhu, Sep 14 '13 at 01:50
@CTZhu Oh, now I understand your comment on my question. Obviously I assumed OP meant 10% of the total each time, not of the remainder ... this avoids zeno's paradox, but who knows what the application is. — askewchan, Sep 14 '13 at 03:46

score 1 · Answer 1 · answered Sep 13 '13 at 22:31

1

something like:

import random

l = [1,2,3,4]
random.shuffle(l)
while len(l) > 0:
    choice = l[:len(l) / 10]
    l = l[len(l) / 10:]

answered Sep 13 '13 at 22:31

Tzach

askewchan · Answer 2 · 2013-09-13T23:10:07.493

1

I interpret this as you want to split the points into 10 equal-sized segments. You can simply do this by shuffling them and reshaping the list:

np.random.shuffle(points)
points.shape = (10,-1) + points.shape[1:]

Then you can access the first 10% as points[0], the second as points[1], etc.

This still works for a multidimensional array since shuffle will only shuffle along the first axis.

edited Sep 13 '13 at 23:10

answered Sep 13 '13 at 23:01

askewchan

1

I am not sure this is correct. The result of this implementation would mean the probability of being sampled are the same in each of the 10 10% segments. But in the original question, the first set has p=0.1, the second has p=(1-0.1)*0.1=0.09 and so on. The probability is not equal across all 10 segments. – CT Zhu Sep 14 '13 at 01:27
No, each particle has equal chance of being in each division, regardless of the order of the sampling. Otherwise the probability does not add to 1. – askewchan Sep 14 '13 at 03:13
I have 50 XYZC. I need A random 10% of those numbers eg 5 , I perform a calculation on the C for these 5 numbers , then i need another different 5 numbers, perform a calculation on there C . Then i need another different 5 numbers, perform a calculation on there C. And do this until I have no numbers left. Everytime I do a calculation on the C i need it to change on the total 50XYZC – West1234 Sep 14 '13 at 11:52
My application has to do with a point cloud simplification method im working on – West1234 Sep 14 '13 at 11:53

2 Answers2