filling numpy array by index

Question

I have a function which gives me the index for a given value. Eg,

def F(value):
   index = do_something(value)
   return index

I want to use this index to fill a huge numpy array by 1s. Lets call array features

l = [1,4,2,3,7,5,3,6,.....]

NOTE: features.shape[0] = len(l)

for i in range(features.shape[0]):
    idx = F(l[i])
    features[i, idx] = 1

Is there a pythonic way to perform this (as the loop takes a lot of time if the array is huge)?

Blaz Bratanic · Accepted Answer · 2014-04-02 13:56:37Z

2

If you can vectorize F(value) you could write something like

indices = np.arange(features.shape[0])
feature_indices = F(l)

features.flat[indices, feature_indices] = 1

answered Apr 2, 2014 at 13:43

Blaz Bratanic

2,27914 silver badges19 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

F(value) is not vectorized

behzad.nouri · Accepted Answer · 2014-04-02 13:51:00Z

1

try this:

i = np.arange(features.shape[0]) # rows
j = np.vectorize(F)(np.array(l)) # columns
features[i,j] = 1

answered Apr 2, 2014 at 13:51

behzad.nouri

78.5k18 gold badges130 silver badges127 bronze badges