问题
If I use len(np.array([[2,3,1,0], [2,3,1,0], [3,2,1,1]])), I get back 3.
Why is there no argument for len() about which axis I want the length of in multidimensional arrays? This is alarming. Is there an alternative?
回答1:
What is the len of the equivalent nested list?
len([[2,3,1,0], [2,3,1,0], [3,2,1,1]])
With the more general concept of shape, numpy developers choose to implement __len__ as the first dimension. Python maps len(obj) onto obj.__len__.
X.shape returns a tuple, which does have a len - which is the number of dimensions, X.ndim. X.shape[i] selects the ith dimension (a straight forward application of tuple indexing).
回答2:
Easy. Use .shape.
>>> nparray.shape
(5, 6) #Returns a tuple of array dimensions.
回答3:
You can transpose the array if you want to get the length of the other dimension.
len(np.array([[2,3,1,0], [2,3,1,0], [3,2,1,1]]).T)
来源：https://stackoverflow.com/questions/43081809/len-of-a-numpy-array-in-python