Padding a numpy array with zeros, and using another array as an index for ones

Question

I'm trying to pad a numpy array, and I cannot seem to find the right approach from the documentation for numpy. I have an array:

a = array([2, 1, 3, 5, 7])

This represents the index for an array I wish to create. So at index value 2 or 1 or 3 etc I would like to have a one in the array, and everywhere else in the target array, to be padded with zeros. Sort of like an array mask. I would also like to specify the overall length of the target array, l. So my ideal function would like something like:

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

, where l=10 for the above example.

EDIT:

So I wrote this function:

def padwithones(a,l) :
    p = np.zeros(l)
    for i in a :
        p = np.insert(p,i,1)
    return p

Which gives:

Out[19]: 
array([ 0.,  1.,  0.,  1.,  1.,  1.,  0.,  1.,  0.,  0.,  0.,  0.,  0.,
        0.,  0.])

Which isn't correct!

have you tried anything? Maybe using np.zeros and then indexed assignment...?? — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jan 3, 2017 at 18:14
so use a generator maybe to determine when a one is to inserted into the np.zeros array? — Anthony W
– Anthony W, Commented Jan 3, 2017 at 18:18

sirfz · Accepted Answer · 2017-01-03 18:30:31Z

2

What you're looking for is basically a one-hot array:

def onehot(foo, l):
    a = np.zeros(l, dtype=np.int32)
    a[foo] = 1
    return a

Example:

In [126]: onehot([2, 1, 3, 5, 7], 10)
Out[126]: array([0,  1,  1,  1,  0,  1,  0,  1,  0,  0])

edited Jan 3, 2017 at 18:30

answered Jan 3, 2017 at 18:26

sirfz

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2 Comments

Anthony W Over a year ago

that's great. Thanks!

juanpa.arrivillaga Over a year ago

If you usedtype=bool it would take 1/4th the space.

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