Update by an index and a mask?

Question

I have a big 2-dimensional array which I access by indexes. I want to update only the values of the indexed array which are not zero.

arrayx = np.random.random((10,10))

Let's say I have indexes (this is just example, the actual indexes are generated by separate process):

idxs = np.array([[4],
        [5],
        [6],
        [7],
        [8]]), np.array([[5, 9]])

Given these indexes, this should work, but it doesn't.

arrayx[idxs]

array([[0.7 , 0.1 ],
       [0.79, 0.51],
       [0.  , 0.8 ],
       [0.82, 0.32],
       [0.82, 0.89]], dtype=float16)

// note from editor: '<>' is equivalent to '!='
// but I agree that '>' 0 is more correct
// mask = mapx[idxs] <> 0 // original
mask = arrayx[idxs] > 0 // better

array([[ True,  True],
       [ True,  True],
       [False,  True],
       [ True,  True],
       [ True,  True]])

arrayx[idxs][mask] += 1

However, this does not update the array. How can I solve this?

so your map's 2D and indexes have a weird shape can you please update your question with proper data? I mean you have something understandable for map but not for idxs — ted, Dec 05 '19 at 04:59
also, you should probably avoid using `map` as a variable name as it would overwrite python's buil-in map function — ted, Dec 05 '19 at 04:59

Divakar · Accepted Answer · 2019-12-05T05:43:20.620

1

A simple one with np.where with a mask as the first input to choose and assign -

mapx[idxs] = np.where(mask,mapx[idxs]+1,mapx[idxs])

Custom update values

The second argument (here mapx[idxs]+1) could be edited to any complex update that you might be doing for the masked places corresponding to True ones in mask. So, let's say you were doing an update for the masked places with :

mapx[idxs] += x * (A - mapx[idxs])

Then, replace the second arg to mapx[idxs] + x * (A - mapx[idxs]).

Another way would be to extract integer indices off True ones in mask and then create new idxs that is selective based on the mask, like so -

r,c = np.nonzero(mask)
idxs_new = (idxs[0][:,0][r], idxs[1][0][c])
mapx[idxs_new] += 1

The last step could be edited similarly for a custom update. Just use idxs_new in place of idxs to update.

edited Dec 05 '19 at 05:43

answered Dec 05 '19 at 05:19

Divakar

218,885
19
262
358

i simplified the update is not simple += 1, but a complex formula ! – sten Dec 05 '19 at 05:22
@sten Could that complex formula be put as something involving `mapx[idxs]`? If so, then just replace that complex formula with the second arg to `np.where`? – Divakar Dec 05 '19 at 05:23
yep.. mapx[idxs] += x * (A - mapx[idxs]) – sten Dec 05 '19 at 05:26
@sten So, just replace the second arg to `np.where` to that : `mapx[idxs] + x * (A - mapx[idxs])`. – Divakar Dec 05 '19 at 05:27
oo i see never used .where() like this... checked the docs :) – sten Dec 05 '19 at 05:30

Update by an index and a mask?

1 Answers1