How to create an array of matrices in python?

Question

How to create an array of matrices in python?

In MATLAB, I do something like this:

for i = 1:n
 a{i} = f(i)
end

where f(i) is a function that returns a random matrix of fixed size.

In python, I am working with numpy but I do not understand how to do this.

import numpy as np
a = np.array([])
for i in range(0, n):
 # a.insert(i, f(i)) and does not work
 # a[i] = f(i) and does not work

• Will Vousden: Do you know the shape of the array returned by f in advance?
• Kira: Yes I know the dimensions of the matrix returned by f.

Answer 1

1. If you want a rank-3 Numpy array, and you know the shape of f(i) in advance, you can pre-allocate the array:

   a = np.zeros((n,) + shape)
   for i in range(n):
    a[i] = f(i)
   

2. If you just want a list (rather than a Numpy array) of matrices, use a list comprehension:

   a = [f(i) for i in range(n)]
   

3. Another way to get a Numpy array is to convert from the list comprehension above:

   a = np.array([f(i) for i in range(n)])
   

   However, this will be less efficient than #1 because the results of f(i) are first buffered in a dynamically-sized list before the Numpy array is built.

Answer 2

The best equivalent of a matlab cell array here is a list in python:

a = []
for i in range(n):
 a.append(f(i))

• Daniel: left-over from the question.