动态Python数组切片 [英] Dynamic Python Array Slicing

问题描述

我面临的情况是我有一个非常大的numpy.ndarray(实际上是hdf5数据集)，我需要快速找到一个子集，因为它们的整个数组无法保存在内存中.但是，我也不想遍历这样的数组(即使声明内置的numpy迭代器也会抛出MemoryError)，因为我的脚本实际上需要几天才能运行.

I am facing a situation where I have a VERY large numpy.ndarray (really, it's an hdf5 dataset) that I need to find a subset of quickly because they entire array cannot be held in memory. However, I also do not want to iterate through such an array (even declaring the built-in numpy iterator throws a MemoryError) because my script would take literally days to run.

因此，我面临着遍历数组某些维度的情况，以便可以对完整数组的精简子集执行数组操作.为此，我需要能够动态切出数组的一个子集.动态切片意味着构造一个元组并将其传递.

As such, I'm faced with the situation of iterating through some dimensions of the array so that I can perform array-operations on pared down subsets of the full array. To do that, I need to be able to dynamically slice out a subset of the array. Dynamic slicing means constructing a tuple and passing it.

例如，代替

my_array[0,0,0]

我可能会使用

my_array[(0,0,0,)]

这是问题所在:如果我想沿着数组的特定维度/轴手动切出所有值，我可以做类似的事情

Here's the problem: if I want to slice out all values along a particular dimension/axis of the array manually, I could do something like

my_array[0,:,0]
> array([1, 4, 7])

但是，如果我使用元组，这是行不通的:

However, I this does not work if I use a tuple:

my_array[(0,:,0,)]

我会得到一个SyntaxError.

当我必须动态构造切片以将某些内容放在数组的括号中时，该怎么办?

How can I do this when I have to construct the slice dynamically to put something in the brackets of the array?

推荐答案

您可以使用python的

You could slice automaticaly using python's slice:

>>> a = np.random.rand(3, 4, 5)
>>> a[0, :, 0]
array([ 0.48054702,  0.88728858,  0.83225113,  0.12491976])
>>> a[(0, slice(None), 0)]
array([ 0.48054702,  0.88728858,  0.83225113,  0.12491976])

slice方法读取为slice(*start*, stop[, step]).如果仅传递一个参数，则将其解释为slice(0, stop).

The slice method reads as slice(*start*, stop[, step]). If only one argument is passed, then it is interpreted as slice(0, stop).

在上面的示例中，:被翻译为slice(0, end)，等效于slice(None).

In the example above : is translated to slice(0, end) which is equivalent to slice(None).

其他切片示例:

:5 -> slice(5)
1:5 -> slice(1, 5)
1: -> slice(1, None)
1::2 -> slice(1, None, 2)

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