Mailman 3 Create a method to index N-dim tensors using 1D index #23992 - NumPy-Discussion

June 19, 2023

      ### Proposed new feature or change:

I work with geospatial data that requires tensors with many dimensions. One challenge I used to have was when I had to implement a `__getitem__` to access those tensors using a 1D index. One use case is machine learning, when one needs to feed models sequentially with sub-tensors of the original one. A simple example is a matrix of shape 2x2 that has the positions `(0, 0)`, `(0, 1)`, `(1, 0)`, `(1, 1)`, and I want to access it via indices `[0, 1, 2, 3]`. So, when I say `__getitem__(2)`, I want the position `(1, 0)`.

The internals of numpy probably has such information because arrays are usually stored contiguously in memory, but I couldn't find a way to access this information.

I came up with this function that calculates tensor indices given the 1D index and the shape of the tensor:

``` python
from typing import Tuple
import numpy as np

def f(i: int, s: Tuple[int]) -> Tuple[int]:
    return (i,) if len(s) == 1 else f(i // s[-1], s[:-1]) + (i % s[-1],)
```

How to use it:

``` python
tensor = np.arange(12).reshape((3, 4))
row_id, col_id = f(i=6, s=(3, 4))
```

Would numpy like to include this method in its code? If yes, I can check how to submit a PR. If not, I would appreciate an indication of a more suitable library (`scipy`, maybe `xarray`).

Daniel

Create a method to index N-dim tensors using 1D index #23992

Daniel Salles Civitarese

Robert Kern

Sebastian Berg

Robert Kern

Sebastian Berg

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