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 us 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?
22 Answers
shape is a property of both numpy ndarray's and matrices.
A.shape will return a tuple (m, n), where m is the number of rows, and n is the number of columns.
In fact, the numpy matrix object is built on top of the ndarray object, one of numpy's two fundamental objects (along with a universal function object), so it inherits from ndarray
matrix.size according to the numpy docs returns the Number of elements in the array. Hope that helps.
