len() of a numpy array in python

python numpy multidimensional-array variable-length
Monica Heddneck picture Monica Heddneck · Mar 29, 2017 · Viewed 113.1k times · Source

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?

Answer

hpaulj picture hpaulj · Mar 29, 2017

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).

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