CODEWITHSUNDEEP
pythonnumpy
Katerina
Katerina

Reputation: 2610

Assign value to numpy array with overlaid slices of indexes

I want to assign a value to some segments of an array. I've got indexes of the segments as tuples (start_idx, end_idx). Segments may overlay or be sub-segments of each other. 

a = np.zeros(12)
segments = np.array([(0, 3), (1, 2), (6, 8), (8, 10)])
a[segments] = 1

The results is:

a
>> array([1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 1, 0])

How can I mask all segments to get this output:

a
>> array([1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0])

Upvotes: 4

Views: 219

Answers (4)

MSeifert
MSeifert

Reputation: 152677

Just to mention the trivial solution: Use a for-loop over the segments and assign to the slices:

import numpy as np
a = np.zeros(12)
segments = np.array([(0, 3), (1, 2), (6, 8), (8, 10)])

for seg in segments.tolist():  # the "tolist" is just an optimization here, you *could* omit it.
    a[seg[0]: seg[1]+1] = 1    # or just "seq[1]" if you want to exclude the end point
print(a)
# array([ 1.,  1.,  1.,  1.,  0.,  0.,  1.,  1.,  1.,  1.,  1.,  0.])

Upvotes: 1

Divakar
Divakar

Reputation: 221584

Here's one vectorized approach with the idea being borrowed from this post -

def segment_arr(segments, L): # L being length of output array
    s = np.zeros(L,dtype=int)
    stop = segments[:,1]+1
    np.add.at(s,segments[:,0],1)
    np.add.at(s,stop[stop<len(s)],-1)
    return (s.cumsum()>0).astype(int)

Sample run -

In [298]: segments = np.array([(0, 3), (1, 2), (6, 8), (8, 10)])

In [299]: segment_arr(segments, L=12)
Out[299]: array([1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0])

Upvotes: 2

akuiper
akuiper

Reputation: 214987

One option is to simply loop through the segments, and convert ranges to actual index:

a = np.zeros(10)
segments = np.array([(0, 3), (1, 2), (6, 8), (8, 10)])

a[[i for s in segments for i in range(*s)]] = 1    
a
# array([ 1.,  1.,  1.,  0.,  0.,  0.,  1.,  1.,  1.,  1.])

Upvotes: 1

d_void
d_void

Reputation: 123

Try this:

a = np.zeros(10)
segments = np.array([(0, 3), (1, 2), (6, 8), (8, 10)])
a[range(3)+range(1,2)+range(6,8)+range(8,10)] = 1
print (a)

Upvotes: 1

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