[英]Slice numpy ndarry of arbitrary dimension to 1d array given a list of indices
I have a numpy ndarray arr
and also indices
, a list of indices specifying a particular entry.我有一个 numpy ndarray
arr
和indices
,一个指定特定条目的索引列表。 For concreteness let's take:具体来说,让我们采取:
arr = np.arange(2*3*4).reshape((2,3,4))
indices= [1,0,3]
I have code to take 1d slices through arr
observing all but one index n
:我有代码通过
arr
观察除一个索引n
之外的所有 1d 切片:
arr[:, indices[1], indices[2]] # n = 0
arr[indices[0], :, indices[2]] # n = 1
arr[indices[0], indices[1], :] # n = 2
I would like to change my code to loop over n
and support an arr
of arbitrary dimension.我想更改我的代码以循环
n
并支持任意维度的arr
。
I've had a look at the indexing routines entry in the documentation and found information about slice()
and np.s_()
.我查看了文档中的 索引例程条目,并找到了有关
slice()
和np.s_()
的信息。 I was able to hack together something that works like what I want:我能够拼凑出我想要的东西:
def make_custom_slice(n, indices):
s = list()
for i, idx in enumerate(indices):
if i == n:
s.append(slice(None))
else:
s.append(slice(idx, idx+1))
return tuple(s)
for n in range(arr.ndim):
np.squeeze(arr[make_custom_slice(n, indices)])
Where np.squeeze
is used to remove the axes of length 1. Without this, the array this produced has shape (arr.shape[n],1,1,...)
rather than (arr.shape[n],)
.其中
np.squeeze
用于删除长度为 1 的轴。没有这个,这个产生的数组具有形状(arr.shape[n],1,1,...)
而不是(arr.shape[n],)
.
Is there a more idiomatic way to accomplish this task?有没有更惯用的方法来完成这项任务?
Some improvements to the solution above (there may still be a one-liner or a more performant solution):对上述解决方案的一些改进(可能仍然存在单行或更高性能的解决方案):
def make_custom_slice(n, indices):
s = indices.copy()
s[n] = slice(None)
return tuple(s)
for n in range(arr.ndim):
print(arr[make_custom_slice(n, indices)])
An integer value idx
can be used to replace the slice object slice(idx, idx+1)
.一个 integer 值
idx
可用于替换切片 object slice(idx, idx+1)
。 Because most indices are copied over directly, start with a copy of indices rather than building the list from scratch.因为大多数索引都是直接复制的,所以从索引的副本开始,而不是从头开始构建列表。
When built in this way, the result of arr[make_custom_slice(n, indices)
has the expected dimension and np.squeeze
is unnecessary.当以这种方式构建时,
arr[make_custom_slice(n, indices)
的结果具有预期的维度,并且np.squeeze
是不必要的。
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