Extract subarray between certain value in Python

arrays, list, numpy, python

Solution

You mentioned `numpy` in the tags. If you're using it, have a look at `np.split`.

For example:

import numpy as np

x = np.array([-1, 1, 2, 3, 4, 5, 7, -1, 4, 4, 4, 5, 6, 7, 7, 8, -1, 0, 2,
               3, 5, -1])
arrays = np.split(x, np.where(x == -1)[0])
arrays = [item[1:] for item in arrays if len(item) > 1]

This yields:

[array([1, 2, 3, 4, 5, 7]),
 array([4, 4, 4, 5, 6, 7, 7, 8]),
 array([0, 2, 3, 5])]

What's going on is that `where` will yield an array (actually a tuple of arrays, therefore the `where(blah)[0]`) of the indicies where the given expression is true. We can then pass these indicies to `split` to get a sequence of arrays.

However, the result will contain the `-1`'s and an empty array at the start, if the sequence starts with `-1`. Therefore, we need to filter these out.

If you're not already using `numpy`, though, your (or @DSM's) `itertools` solution is probably a better choice.

Problem

I have a list of values that are the result of merging many files. I need to pad some of the values. I know that each sub-section begins with the value -1. I am trying to basically extract a sub-array between -1's in the main array via iteration. For example supposed this is the main list: ``` -1 1 2 3 4 5 7 -1 4 4 4 5 6 7 7 8 -1 0 2 3 5 -1 ``` I would like to extract the values between the -1s: ``` list_a = 1 2 3 4 5 7 list_b = 4 4 4 5 6 7 7 8 list_c = 0 2 3 5 ... list_n = a1 a2 a3 ... aM ``` I have extracted the indices for each -1 by searching through the main list: ``` minus_ones = [i for i, j in izip(count(), q) if j == -1] ``` I also assembled them as pairs using a common recipe: ``` def pairwise(iterable): a, b = tee(iterable) next(b, None) return izip(a,b) for index in pairwise(minus_ones): print index ``` The next step I am trying to do is grab the values between the index pairs, for example: ``` list_b: (7 , 16) -> 4 4 4 5 6 7 7 8 ``` so I can then do some work to those values (I will add a fixed int. to each value in each sub-array).

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