[Numpy-discussion] Getting 3-D array values from a 2-D array of indexes

[Robert Kern]

Jordan Dawe wrote:
> I have a 3D array A of shape (nz, ny, nx) and a 2D array B of shape (ny, 
> nx) that contains integer indexes.  I want to generate a 2D array C of 
> shape (ny, nx) composed of the values of A at the z-indexes specified in 
> B.  Is there an easy way to do this?

An alternative to Pau's suggestion is to use the .choose() method, which is
tailor-made for this use-case.


In [9]: nz, ny, nx = (3, 4, 5)

In [10]: B = array([[0, 1, 2, 1, 0], [1, 2, 1, 0, 1], [2, 1, 0, 1, 2], [1, 0, 1,
2, 1]])

In [11]: B
Out[11]:
array([[0, 1, 2, 1, 0],
       [1, 2, 1, 0, 1],
       [2, 1, 0, 1, 2],
       [1, 0, 1, 2, 1]])

In [12]: A = arange(nz*ny*nx).reshape((nz,ny,nx))

In [13]: A
Out[13]:
array([[[ 0,  1,  2,  3,  4],
        [ 5,  6,  7,  8,  9],
        [10, 11, 12, 13, 14],
        [15, 16, 17, 18, 19]],

       [[20, 21, 22, 23, 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],
        [50, 51, 52, 53, 54],
        [55, 56, 57, 58, 59]]])

In [14]: B.choose(A)
Out[14]:
array([[ 0, 21, 42, 23,  4],
       [25, 46, 27,  8, 29],
       [50, 31, 12, 33, 54],
       [35, 16, 37, 58, 39]])


-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco