Python numpy array replacing

Question

I have an array which looks like this for example:

array([[ 1,  1,  2,  0,  4],
       [ 5,  6,  7,  8,  9],
       [10,  0,  0, 13, 14],
       [15, 16, 17, 18, 19],
       [20, 21, 22,  0, 24],
       [25, 26, 27, 28, 29],
       [30, 31, 32, 33, 34],
       [35, 36, 37, 38, 39],
       [40, 41, 42, 43, 44],
       [45, 46, 47, 48, 49]])

I have another two arrays which are like:

array([[   0,    0,    0,    0],
       [   0,    0,    0,    0],
       [   0, 2891,    0,    0],
       [   0,    0,    0,    0],
       [   0,    0,    0, 2891]])

and

array([[  0,   0,   0, 643],
       [  0,   0,   0,   0],
       [  0,   0, 643,   0],
       [  0,   0,   0,   0],
       [  0,   0,   0,   0]])

What I want is to pick value 2891 from the 2nd array to the first array in the corresponding position and also 643 from the third array to the first array in the corresponding position so that the final array should look like this:

array([[   1,    1,    2,  643,    4],
       [   5,    6,    7,    8,    9],
       [  10, 2891,  643,   13,   14],
       [  15,   16,   17,   18,   19],
       [  20,   21,   22, 2891,   24],
       [  25,   26,   27,   28,   29],
       [  30,   31,   32,   33,   34],
       [  35,   36,   37,   38,   39],
       [  40,   41,   42,   43,   44],
       [  45,   46,   47,   48,   49]])

So far I have tried this command:

np.place(a,a<1, np.amax(b))

where a referred to the first array and b referred to the 2nd array. What it does it just replace all the 0 value with 2891 value. Can someone help?

Answer 1

You can find the indices where y and z are nonzero using the nonzero method:

In [9]: y.nonzero()
Out[9]: (array([2, 4]), array([1, 3]))

In [10]: z.nonzero()
Out[10]: (array([0, 2]), array([3, 2]))

You can select the associated values through fancing indexing:

In [11]: y[y.nonzero()]
Out[11]: array([2891, 2891])

and you can assign these values to locations in x with

In [13]: x[y.nonzero()] = y[y.nonzero()]

---

import numpy as np
x = np.array([[ 1,  1,  2,  0,  4],
              [ 5,  6,  7,  8,  9],
              [10,  0,  0, 13, 14],
              [15, 16, 17, 18, 19],
              [20, 21, 22,  0, 24],
              [25, 26, 27, 28, 29],
              [30, 31, 32, 33, 34],
              [35, 36, 37, 38, 39],
              [40, 41, 42, 43, 44],
              [45, 46, 47, 48, 49]])

y = np.array([[   0,    0,    0,    0],
              [   0,    0,    0,    0],
              [   0, 2891,    0,    0],
              [   0,    0,    0,    0],
              [   0,    0,    0, 2891]])

z = np.array([[  0,   0,   0, 643],
              [  0,   0,   0,   0],
              [  0,   0, 643,   0],
              [  0,   0,   0,   0],
              [  0,   0,   0,   0]])

x[y.nonzero()] = y[y.nonzero()]
x[z.nonzero()] = z[z.nonzero()]
print(x)

yields

[[   1    1    2  643    4]
 [   5    6    7    8    9]
 [  10 2891  643   13   14]
 [  15   16   17   18   19]
 [  20   21   22 2891   24]
 [  25   26   27   28   29]
 [  30   31   32   33   34]
 [  35   36   37   38   39]
 [  40   41   42   43   44]
 [  45   46   47   48   49]]