问题描述
我有一个numpy的第二阵列。我的任务的简化版本,是从沿每个轴采取的向量。为了说明:
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]
但是我不一定知道尺寸x将具有的数目。因此,面临的挑战是如何将冒号:索引操作符在变量的位置。这样有什么语法的一个例子可能看起来像:
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]
或
y = indexer.index(x,ind)
对于某些模块索引。我可以写,但我喜欢这种感觉必须已经解决了,我不能谁愿意做这唯一的一个。在MATLAB中,例如,你可以使用的subsref()函数做到这一点。
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.
在是否蟒蛇/ numpy的/其他模块的任何结构存在?
Does any such construct exist in python / numpy / other module?
推荐答案
由于从 numpy的
提出的有关的可以使用的内置功能和元组串联创建变量指标。
As suggested from numpy
's documentation about indexing you can use the slice
built-in function and tuple concatenation to create variable indexes.
在事实上:
在标简直就是一个片的文字符号
文字
In fact the :
in the subscript is simply the literal notation for a slice
literal.
在特定的:
等同于片(无)
(其中,本身就是等于片(无,无,无)
这里的参数是启动
,停止
和步
)。
In particular :
is equivalent to slice(None)
(which, itself, is equivalent to slice(None, None, None)
where the arguments are start
, stop
and step
).
例如:
a[(0,) * N + (slice(None),)]
相当于:
a[0, 0, ..., 0, :] # with N zeros
的:对于片
符号只能直接下标内部使用。例如,这将失败:
The :
notation for slices can only be used directly inside a subscript. For example this fails:
In [10]: a[(0,0,:)]
File "<ipython-input-10-f41b33bd742f>", line 1
a[(0,0,:)]
^
SyntaxError: invalid syntax
要允许任意尺寸的阵列中提取切片你可以写一个简单的功能,如:
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)
和使用它作为:
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])
您可以概括 make_index
做任何事情。要记住的重要一点是,它应该在年底,返回包含无论是整数或片
个元组
。
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 slice
s.
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