CODEWITHSUNDEEP
pythonarraysnumpysum
noami shal
noami shal

Reputation: 79

Sum of negative elements in each column of a NumPy array

Do you know how to sum up all negative elements in each column of a NumPy array. For example 

>>> d
array([[ 1,  2,  3],
       [-1, -1,  9],
       [ 7, -6,  4]])

I need to get [-1,-7,0]. Is there a function to do so?

Upvotes: 3

Views: 7099

Answers (1)

Divakar
Divakar

Reputation: 221614

Few approaches could be suggested, as listed below -

((d<0)*d).sum(0)
np.where(d<0,d,0).sum(0)
np.einsum('ij,ij->j',d<0,d)
((d - np.abs(d)).sum(0))/2

Sample step-by-step run with explanation for all those approaches -

1) Input array :

In [3]: d
Out[3]: 
array([[ 1,  2,  3],
       [-1, -1,  9],
       [ 7, -6,  4]])

2) Mask of negative elements :

In [4]: d<0
Out[4]: 
array([[False, False, False],
       [ True,  True, False],
       [False,  True, False]], dtype=bool)

3) Get masked negative elements off input array using element-wise multiplication :

In [5]: (d<0)*d
Out[5]: 
array([[ 0,  0,  0],
       [-1, -1,  0],
       [ 0, -6,  0]])

4) Finally, sum along axis=0 to sum along each column :

In [6]: ((d<0)*d).sum(axis=0) # Or simply ((d<0)*d).sum(0)
Out[6]: array([-1, -7,  0])

Approach#2 : 3) Get step (3) results alternatively with np.where :

In [7]: np.where(d<0,d,0)
Out[7]: 
array([[ 0,  0,  0],
       [-1, -1,  0],
       [ 0, -6,  0]])

Approach#3 : 3,4) Perform elementwise multiplication between mask and array and get the summation, all in one step using np.einsum -

In [8]: np.einsum('ij,ij->j',d<0,d)
Out[8]: array([-1, -7,  0])

Approach#4 : Get the absolute values of input array and subtract from the array itself, giving us double of the negative elements and the positive values being canceled out :

In [9]: d - np.abs(d)
Out[9]: 
array([[  0,   0,   0],
       [ -2,  -2,   0],
       [  0, -12,   0]])

Sum along each column and divide by 2 for desired output :

In [10]: (d - np.abs(d)).sum(0)
Out[10]: array([ -2, -14,   0])

In [11]: ((d - np.abs(d)).sum(0))/2
Out[11]: array([-1, -7,  0])

Upvotes: 13

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