On Tue, 2020-05-19 at 17:11 +0100, Eric Wieser wrote:

Hi Will,

To force an iteration to run along certain axes, I believe you should be using `op_axes`. Your diagnosis is correct that `external_loop` is trying to help you be more optimal, since it's purpose is exactly that: optimization.

Unfortunately, if you use `op_axes` you'll run into https://github.com/numpy/numpy/issues/9808.

Yeah, I do not think what you want is possible with nditer. `op_axes` allows you to ignore certain axis, but as Eric points out, that actually ignores that axis entirely. So it acts similar to slicing the whole axis away. I previously thought it could be nice to have a new `arr.iteraxis(axis=None)` command. As a base, it would act like a flat iter, otherwise iterate the axis listed. Maybe with an additional flag whether its allowed to optimize iteration order (although from the python side I expect we do not want to do that by default). The reason is also that I am not sure I like `arr.flat` and `for subarr in arr` too much, because the list-of-list like iteration seems only semi-natural for an N-D array. - Sebastian

Eric

On Tue, 19 May 2020 at 00:42, William Ayd <william.ayd@icloud.com> wrote:

I am trying to use the nditer to traverse each column of a 2D array, returning the column as a 1D array. Consulting the docs, I found this example which works perfectly fine:

In [*65*]: a = np.arange(6).reshape(2,3)

In [*66*]: *for* x *in* np.nditer(a, flags=['external_loop'], order='F'): ...: print(x, end=' ') ...:

[0 3] [1 4] [2 5]

When changing the shape of the input array to (1, 3) however, this doesn’t yield what I am hoping for any more (essentially [0], [1] [2]):

In [*68*]: *for* x *in* np.nditer(a, flags=['external_loop'], order='F'): ...: print(x, end=' ') ...:

[0 1 2]

I suspect this may have to do with the fact that the (1, 3) array is both C and F contiguous, and it is trying to return as large of a 1D F-contiguous array as it can. However, I didn’t see any way to really force it to go by columns. My best guess was the *itershape* argument though I couldn’t figure out how to get that to work and didn’t see much in the documentation.

Thanks in advance for the help!

- Will

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