CODEWITHSUNDEEP
pythonarraysnumpy
T.Poe
T.Poe

Reputation: 2079

Effective way to find and delete certain elements from a numpy array

I have a numpy array with some positive numbers and some -1 elements. I want to find these elements with -1 values, delete them and store their indeces.

One way of doing it is iterating through the array and cheking if the value is -1. Is this the only way? If not, what about its effectivness? Isn't there a more effective python tool then?

Upvotes: 0

Views: 474

Answers (3)

cs95
cs95

Reputation: 402493

Use a combination of np.flatnonzero and simple boolean indexing.

x = array([ 0,  0, -1,  0,  0, -1,  0, -2,  0,  0])
m = x != -1  # generate a mask

idx = np.flatnonzero(~m)
x = x[m]

idx
array([2, 5])

x
array([ 0,  0,  0,  0,  0, -2,  0,  0])

Upvotes: 0

RomanPerekhrest
RomanPerekhrest

Reputation: 92854

With numpy.argwhere() and numpy.delete() routines:

import numpy as np

arr = np.array([1, 2, 3, -1, 4, -1, 5, 10, -1, 14])
indices = np.argwhere(arr == -1).flatten()
new_arr = np.delete(arr, indices)

print(new_arr)            # [ 1  2  3  4  5 10 14]
print(indices.tolist())   # [3, 5, 8]

https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.argwhere.html https://docs.scipy.org/doc/numpy/reference/generated/numpy.delete.html

Upvotes: 2

Nimish Bansal
Nimish Bansal

Reputation: 1759

import numpy as np
yourarray=np.array([4,5,6,7,-1,2,3,-1,9,-1]) #say
rangenumpyarray=np.arange(len(yourarray)) # to create a column adjacent to your array of range
arra=np.hstack((rangenumpyarray.reshape(-1,1),yourarray.reshape(-1,1))) # combining both arrays as two columns
arra[arra[:,1]==-1][:,0]  # learn boolean indexing

Upvotes: 0

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