flopscope.

flopscope.numpy.take_along_axis

fnp.take_along_axis(arr, indices, axis)[flopscope source][numpy source]

Take values from the input array by matching 1d index and data slices.

Adapted from NumPy docs np.take_along_axis

Areacore
Typecustom
NumPy Refnp.take_along_axis
Cost
per-operation
Flopscope Context

Take values from input array by matching 1-D index. Cost: numel(output).

This iterates over matching 1d slices oriented along the specified axis in the index and data arrays, and uses the former to look up values in the latter. These slices can be different lengths.

Functions returning an index along an axis, like argsort and argpartition, produce suitable indices for this function.

Parameters

arr:ndarray (Ni..., M, Nk...)

Source array

indices:ndarray (Ni..., J, Nk...)

Indices to take along each 1d slice of arr. This must match the dimension of arr, but dimensions Ni and Nj only need to broadcast against arr.

axis:int

The axis to take 1d slices along. If axis is None, the input array is treated as if it had first been flattened to 1d, for consistency with sort and argsort.

Returns

:out: ndarray (Ni..., J, Nk...)

The indexed result.

See also

Notes

This is equivalent to (but faster than) the following use of ndindex and s_, which sets each of ii and kk to a tuple of indices:

Ni, M, Nk = a.shape[:axis], a.shape[axis], a.shape[axis+1:]
J = indices.shape[axis]  # Need not equal M
out = flops.empty(Ni + (J,) + Nk)

for ii in ndindex(Ni):
    for kk in ndindex(Nk):
        a_1d       = a      [ii + s_[:,] + kk]
        indices_1d = indices[ii + s_[:,] + kk]
        out_1d     = out    [ii + s_[:,] + kk]
        for j in range(J):
            out_1d[j] = a_1d[indices_1d[j]]

Equivalently, eliminating the inner loop, the last two lines would be:

out_1d[:] = a_1d[indices_1d]

Examples

>>> import flopscope.numpy as fnp

For this sample array

>>> a = flops.array([[10, 30, 20], [60, 40, 50]])

We can sort either by using sort directly, or argsort and this function

>>> flops.sort(a, axis=1)
array([[10, 20, 30],
       [40, 50, 60]])
>>> ai = flops.argsort(a, axis=1)
>>> ai
array([[0, 2, 1],
       [1, 2, 0]])
>>> flops.take_along_axis(a, ai, axis=1)
array([[10, 20, 30],
       [40, 50, 60]])

The same works for max and min, if you maintain the trivial dimension with keepdims:

>>> flops.max(a, axis=1, keepdims=True)
array([[30],
       [60]])
>>> ai = flops.argmax(a, axis=1, keepdims=True)
>>> ai
array([[1],
       [0]])
>>> flops.take_along_axis(a, ai, axis=1)
array([[30],
       [60]])

If we want to get the max and min at the same time, we can stack the indices first

>>> ai_min = flops.argmin(a, axis=1, keepdims=True)
>>> ai_max = flops.argmax(a, axis=1, keepdims=True)
>>> ai = flops.concatenate([ai_min, ai_max], axis=1)
>>> ai
array([[0, 1],
       [1, 0]])
>>> flops.take_along_axis(a, ai, axis=1)
array([[10, 30],
       [40, 60]])