How to use the dimension of a python matrix in a loop

Question

I am working with a matrix, lets call it X, in python.

I know how to get the dimension of the matrix using X.shape but I am interested specially on using the number of rows of the matrix in a for loop, and I dont know how to get this value in a datatype suitable for a loop.

For example, imagine tihs simple situation:

a = np.matrix([[1,2,3],[4,5,6]])
for i in 1:(number of rows of a)
 print i

How can I get automatically that "number of rows of a"?

• miradulo: Possible duplicate of Numpy array dimensions

Answer 1

X.shape[0] == number of rows in X

Answer 2

A superficial search on numpy will lead you to shape. It returns a tuple of array dimensions.

In your case, the first dimension (axe) concerns the columns. You can access it as you access a tuple's element:

import numpy as np
a = np.matrix([[1,2,3],[4,5,6]])
# a. shape[1]: columns
for i in range(0,a.shape[1]):
 print 'column '+format(i)
# a. shape[0]: rows 
for i in range(0, a.shape[0]):
 print 'row '+format(i)

This will print:

column 0
column 1
column 2
row 0
row 1