python 冒号运算符_的Python：指数使用冒号运算符中的任意维数组

I have a numpy nd array. A simplified version of my task is to take a vector from along each axis. To illustrate:

import numpy

x = numpy.array(range(24)).reshape((2,3,4))

x0 = x[0,0,:]

x1 = x[0,:,0]

x2 = x[:,0,0]

However I do not necessarily know the number of dimensions x will have. So the challenge is how to place the colon : indexing operator in a variable location. An example of what such syntax could look like:

n = x.ndim

ind = list(np.zeros(n))

dim = 0

ind[dim] = ':'

y = x[ind]

or

y = indexer.index(x,ind)

for some module indexer. I could write it, but I feel like this must already be solved, I can't be the only one who wants to do this. In MATLAB, for example, you can do this with the subsref() function.

Does any such construct exist in python / numpy / other module?

解决方案

As suggested from numpy's documentation about indexing you can use the slice built-in function and tuple concatenation to create variable indexes.

In fact the : in the subscript is simply the literal notation for a slice literal.

In particular : is equivalent to slice(None) (which, itself, is equivalent to slice(None, None, None) where the arguments are start, stop and step).

For example:

a[(0,) * N + (slice(None),)]

is equivalent to:

a[0, 0, ..., 0, :] # with N zeros

The : notation for slices can only be used directly inside a subscript. For example this fails:

In [10]: a[(0,0,:)]

File "", line 1

a[(0,0,:)]

^

SyntaxError: invalid syntax

To allow extracting a slice from an array of arbitrary dimensions you can write a simple function such as:

def make_index(num_dimension, slice_pos):

zeros = [0] * num_dimension

zeros[slice_pos] = slice(None)

return tuple(zeros)

And use it as in:

In [3]: a = np.array(range(24)).reshape((2, 3, 4))

In [4]: a[make_index(3, 2)]

Out[4]: array([0, 1, 2, 3])

In [5]: a[make_index(3, 1)]

Out[5]: array([0, 4, 8])

In [6]: a[make_index(3, 0)]

Out[6]: array([ 0, 12])

You can generalize make_index to do any kind of things. The important thing to remember is that it should, in the end, return a tuple containing either integers or slices.