Matthew Ciaramitaro - 3 years ago 259

Python Question

Let's say I have a

`numpy.ndarray`

`shape (2,3,2)`

`arr = np.array([[[1,3], [2,5], [1,2]],[[3,3], [6,5], [5,2]]])`

I want to reshape it in such a way that:

`arr.shape == (2,3)`

arr == [[(1,3), (2,5), (1,2)],[(3,3), (6,5), (5,2)]]

and

each value of

`arr`

`tuple`

The reason I want to do this is that I want to take the minimum along axis 0 of the 3dimensional array, but I want to preserve the value that the min of the rows in paired with.

`arr = np.array(`

[[[1, 4],

[2, 1],

[5, 2]],

[[3, 3],

[6, 5],

[1, 7]]])

print(np.min(arr, axis=0))

>>> [[1,3],

[2,1],

[1,2]]

>>>Should be

[[1,4],

[2,1],

[1,7]]

If the array contained tuples, it would be 2 dimensional, and the comparison operator for minimize would still function correctly,

so I would get the correct result. But I haven't found any way to do this besides iterating over the arrays, which is inefficient and obvious in implementation.

Is it possible to perform this conversion efficiently in numpy?

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Answer Source

First, let's find out which pairs to take:

```
first_eq = arr[0,:,0] == arr[1,:,0]
which_compare = np.where(first_eq, 1, 0)[0]
winner = arr[:,:,which_compare].argmin(axis=0)
```

Here, `first_eq`

is True where the first elements match, so we would need to compare the second elements. It's `[False, False, False]`

in your example. `which_compare`

then is `[0, 0, 0]`

(because the first element of each pair is what we will compare). Finally, `winner`

tells us which of the two pairs to choose along the second axis. It is `[0, 0, 1]`

.

The last step is to extract the winners:

```
arr[winner, np.arange(arr.shape[1])]
```

That is, take the winner (0 or 1) at each point along the second axis.

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