将Numpy数组索引存储在变量中 [英] Store Numpy array index in variable

问题描述

我想将索引切片作为参数传递给函数:

I want to pass an index slice as an argument to a function:

def myfunction(some_object_from_which_an_array_will_be_made, my_index=[1:5:2,::3]):
    my_array = whatever(some_object_from_which_an_array_will_be_made)
    return my_array[my_index]

显然这是行不通的，并且显然在这种特殊情况下，可能还有其他方法可以做到这一点，但是假设我真的想要这样做，我该如何使用变量来切片一个numpy的数组?

Obviously this will not work, and obviously in this particular case there might be other ways to do this, but supposing I really want to do stuff this way, how can I use a variable to slice a numpy array?

推荐答案

np.lib.index_tricks具有许多可以简化索引编制的函数(和类). np.s_是这样的功能之一.它实际上是具有__get_item__方法的类的实例，因此它使用所需的[]表示法.

np.lib.index_tricks has a number of functions (and classes) that can streamline indexing. np.s_ is one such function. It is actually an instance of a class that has a __get_item__ method, so it uses the [] notation that you want.

使用说明:

In [249]: np.s_[1:5:2,::3]
Out[249]: (slice(1, 5, 2), slice(None, None, 3))

In [250]: np.arange(2*10*4).reshape(2,10,4)[_]
Out[250]: 
array([[[40, 41, 42, 43],
        [52, 53, 54, 55],
        [64, 65, 66, 67],
        [76, 77, 78, 79]]])

In [251]: np.arange(2*10*4).reshape(2,10,4)[1:5:2,::3]
Out[251]: 
array([[[40, 41, 42, 43],
        [52, 53, 54, 55],
        [64, 65, 66, 67],
        [76, 77, 78, 79]]])

注意，它构造的切片元组与ajcr相同. _是IPython用于最后结果的临时变量.

Notice that it constructs the same tuple of slices that ajcr did. _ is the temporary variable that IPython uses for the last result.

要将此类元组传递给函数，请尝试:

To pass such a tuple to a function, try:

def myfunction(some_object_from_which_an_array_will_be_made, my_index=np.s_[:,:]):
    my_array = whatever(some_object_from_which_an_array_will_be_made)
    return my_array[my_index]
I = np.s_[1:5:2,::3]
myfunction(obj, my_index=I)

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