numpy.take¶

numpy.
take
(a, indices, axis=None, out=None, mode='raise')[source]¶ Take elements from an array along an axis.
When axis is not None, this function does the same thing as “fancy” indexing (indexing arrays using arrays); however, it can be easier to use if you need elements along a given axis. A call such as
np.take(arr, indices, axis=3)
is equivalent toarr[:,:,:,indices,...]
.Explained without fancy indexing, this is equivalent to the following use of
ndindex
, which sets each ofii
,jj
, andkk
to a tuple of indices:Ni, Nk = a.shape[:axis], a.shape[axis+1:] Nj = indices.shape for ii in ndindex(Ni): for jj in ndindex(Nj): for kk in ndindex(Nk): out[ii + jj + kk] = a[ii + (indices[jj],) + kk]
Parameters:  a : array_like (Ni…, M, Nk…)
The source array.
 indices : array_like (Nj…)
The indices of the values to extract.
New in version 1.8.0.
Also allow scalars for indices.
 axis : int, optional
The axis over which to select values. By default, the flattened input array is used.
 out : ndarray, optional (Ni…, Nj…, Nk…)
If provided, the result will be placed in this array. It should be of the appropriate shape and dtype. Note that out is always buffered if mode=’raise’; use other modes for better performance.
 mode : {‘raise’, ‘wrap’, ‘clip’}, optional
Specifies how outofbounds indices will behave.
 ‘raise’ – raise an error (default)
 ‘wrap’ – wrap around
 ‘clip’ – clip to the range
‘clip’ mode means that all indices that are too large are replaced by the index that addresses the last element along that axis. Note that this disables indexing with negative numbers.
Returns:  out : ndarray (Ni…, Nj…, Nk…)
The returned array has the same type as a.
See also
compress
 Take elements using a boolean mask
ndarray.take
 equivalent method
take_along_axis
 Take elements by matching the array and the index arrays
Notes
By eliminating the inner loop in the description above, and using
s_
to build simple slice objects,take
can be expressed in terms of applying fancy indexing to each 1d slice:Ni, Nk = a.shape[:axis], a.shape[axis+1:] for ii in ndindex(Ni): for kk in ndindex(Nj): out[ii + s_[...,] + kk] = a[ii + s_[:,] + kk][indices]
For this reason, it is equivalent to (but faster than) the following use of
apply_along_axis
:out = np.apply_along_axis(lambda a_1d: a_1d[indices], axis, a)
Examples
>>> a = [4, 3, 5, 7, 6, 8] >>> indices = [0, 1, 4] >>> np.take(a, indices) array([4, 3, 6])
In this example if a is an ndarray, “fancy” indexing can be used.
>>> a = np.array(a) >>> a[indices] array([4, 3, 6])
If
indices
is not one dimensional, the output also has these dimensions.>>> np.take(a, [[0, 1], [2, 3]]) array([[4, 3], [5, 7]])