numpy.take_along_axis#
- numpy.take_along_axis(arr, indices, axis)[source]#
Take values from the input array by matching 1d index and data slices.
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
andargpartition
, produce suitable indices for this function.New in version 1.15.0.
- Parameters
- arrndarray (Ni…, M, Nk…)
Source array
- indicesndarray (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.
- axisint
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
andargsort
.
- Returns
- out: ndarray (Ni…, J, Nk…)
The indexed result.
See also
take
Take along an axis, using the same indices for every 1d slice
put_along_axis
Put values into the destination array by matching 1d index and data slices
Notes
This is equivalent to (but faster than) the following use of
ndindex
ands_
, which sets each ofii
andkk
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 = np.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
For this sample array
>>> a = np.array([[10, 30, 20], [60, 40, 50]])
We can sort either by using sort directly, or argsort and this function
>>> np.sort(a, axis=1) array([[10, 20, 30], [40, 50, 60]]) >>> ai = np.argsort(a, axis=1); ai array([[0, 2, 1], [1, 2, 0]]) >>> np.take_along_axis(a, ai, axis=1) array([[10, 20, 30], [40, 50, 60]])
The same works for max and min, if you expand the dimensions:
>>> np.expand_dims(np.max(a, axis=1), axis=1) array([[30], [60]]) >>> ai = np.expand_dims(np.argmax(a, axis=1), axis=1) >>> ai array([[1], [0]]) >>> np.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 = np.expand_dims(np.argmin(a, axis=1), axis=1) >>> ai_max = np.expand_dims(np.argmax(a, axis=1), axis=1) >>> ai = np.concatenate([ai_min, ai_max], axis=1) >>> ai array([[0, 1], [1, 0]]) >>> np.take_along_axis(a, ai, axis=1) array([[10, 30], [40, 60]])