On Thu, Nov 21, 2024 at 2:00 PM Slavin, Jonathan via NumPy-Discussion <numpy-discussion@python.org> wrote:

Hi all,

I was trying to use meshgrid with three arrays and got some odd results. Here's a simple example:

xt = np.array([1,2,3,4])
yt = np.array([6,7,8])
zt = np.array([12,13])
xxx,yyy,zzz = np.meshgrid(xt,yt,zt)

So I would expect that xxx[0,0,:] = array([1,2,3,4])

instead I get xxx[0,0,:] = array([1,1]) and xxx[0,:,0] = array([1,2,3,4])

also yyy[:,0,0] = array([6,7,8]), whereas I would expect yyy[0,:,0] = array([6,7,8])

So what's going on? This seems like a bug to me.

Any suggestions for getting what I wanted -- i.e. xxx.shape = (2,3,4), with values as appropriate?

This is documented in the `Notes` section concerning the behavior with the default `indexing='xy'`, which is primarily for 2D arrays (and presumably following a convention from another language like MATLAB). In order to get the (2, 3, 4) arrays that you want, use `indexing='ij'`

  # Note the flipped order of inputs and outputs since you want the `zt` values across the first axis.

  zzz, yyy, xxx = np.meshgrid(zt, yt, xt, indexing='ij')

--

Robert Kern