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For a numpy matrix in python

from numpy import matrix

A = matrix([[1,2],[3,4]])

How can I find the length of a row (or column) of this matrix? Equivalently, how can I know the number of rows or columns?

So far, the only solution I've found is:

len(A)

len(A[:,1])

len(A[1,:])

Which returns 2, 2, and 1, respectively. From this, I've gathered that len() will return the number of rows, so I can always use the transpose, len(A.T), for the number of columns. However, this feels unsatisfying and arbitrary, as when reading the line len(A), it isn't immediately obvious that this should return the number of rows. It actually works differently than len([1,2]) would for a 2D python array, as this would return 2.

So, is there a more intuitive way to find the size of a matrix, or is this the best I have?

by (106k points)

To find the length of a numpy matrix in Python you can use shape which is a property of both numpy ndarray's and matrices.

A.shape

The above code will return a tuple (m, n), where m is the number of rows, and n is the number of columns.

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