Gabriel Gabriel - 8 days ago 5
Python Question

Extract elements from numpy array, that are not in list of indexes

I want to do something similar to what was asked here NumPy array, change the values that are NOT in a list of indices, but not quite the same.

Consider a

numpy
array:

> a = np.array([0.2, 5.6, 88, 12, 1.3, 6, 8.9])


I know I can access its elements via a list of indexes, like:

> indxs = [1, 2, 5]
> a[indxs]
array([ 5.6, 88. , 6. ])


But I also need to access those elements which are not in the
indxs
list. Naively, this is:

> a[not in indxs]
> array([0.2, 12, 1.3, 8.9])


What is the proper way to do this?

Answer
In [170]: a = np.array([0.2, 5.6, 88, 12, 1.3, 6, 8.9])
In [171]: idx=[1,2,5]
In [172]: a[idx]
Out[172]: array([  5.6,  88. ,   6. ])
In [173]: np.delete(a,idx)
Out[173]: array([  0.2,  12. ,   1.3,   8.9])

delete is more general than you really need, using different strategies depending on the inputs. I think in this case it uses the boolean mask approach (timings should be similar).

In [175]: mask=np.ones_like(a, bool)
In [176]: mask
Out[176]: array([ True,  True,  True,  True,  True,  True,  True], dtype=bool)
In [177]: mask[idx]=False
In [178]: mask
Out[178]: array([ True, False, False,  True,  True, False,  True], dtype=bool)
In [179]: a[mask]
Out[179]: array([  0.2,  12. ,   1.3,   8.9])