numpy update array indices

Question

I have following indices

indices = array([[ 0,  0],
   [ 1,  1],
   [ 5,  3],
   [ 7,  9]])

I want to update a 2d numpy array with these indices as one

img = np.zeros((12,12), np.uint8)

I tried the following

img[indices] = 1
indices_x = indices[:, 0]
indices_y = indices[:, 1]
img[np.ix_(indices_x,indices_y)] = 1

Like this img[indices[:, 0], indices[:, 1]] = 1

cs95
– cs95

2018-07-09 00:15:52 +00:00
Commented Jul 9, 2018 at 0:15 — cs95
– cs95, Commented Jul 9, 2018 at 0:15
Thanks coldspeed. Cannot upvote because of low reputation

Joe
– Joe

2018-07-09 00:24:09 +00:00
Commented Jul 9, 2018 at 0:24 — Joe
– Joe, Commented Jul 9, 2018 at 0:24

Oliver Evans · Accepted Answer · 2018-07-09 00:25:11Z

3

Try specifying the x and y indices separately.

In [0]: img[indices[:,0], indices[:,1]] = 3

In [1]: img
Out[1]: 
array([[3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]], dtype=uint8)

Alternatively, you can give a tuple or list:

In [2]: new_inds = tuple(zip(*indices))

In [3]: new_inds
Out[3]: ((0, 1, 5, 7), (0, 1, 3, 9))

In [4]: img[new_inds] = 4

In [5]: img
Out[5]: 
array([[4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]], dtype=uint8)

edited Jul 9, 2018 at 0:25

answered Jul 9, 2018 at 0:21

Oliver Evans

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2 Comments

user3483203 Over a year ago

One of these things is not like the other.

Joe Over a year ago

Thanks Oliver. Cannot upvote because of low reputation

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