Question

This is my first post and I’m still a Python and Scipy newcomer, so go easy on me! I’m trying to convert an Nx1 matrix into a python list. Say I have some 3×1 matrix

x = scipy.matrix([1,2,3]).transpose()

My aim is to create a list, y, from x so that

y = [1, 2, 3]

I’ve tried using the tolist() method, but it returns [[1], [2], [3]], which isn’t the result that I’m after. The best i can do is this

y = [xi for xi in x.flat]

but it’s a bit cumbersome, and I’m not sure if there’s an easier way to achieve the same result. Like I said, I’m still coming to grips with Python and Scipy…

Thanks

Answer 1

A question for your question

While Sven and Navi have answered your question on how to convert

x = scipy.matrix([1,2,3]).transpose()

into a list, I’ll ask a question before answering:

• Why are you using an Nx1 matrix instead of an array?

Using array instead of matrix

If you look at the Numpy for Matlab Users wiki/documentation page, section 3 discusses ‘array’ or ‘matrix’? Which should I use?. The short answer is that you should use array.

One of the advantages of using an array is that:

You can treat rank-1 arrays as either row or column vectors. dot(A,v) treats v as a column vector, while dot(v,A) treats v as a row vector. This can save you having to type a lot of transposes.

Also, as stated in the Numpy Reference Documentation, "Matrix objects are always two-dimensional." This is why x.tolist() returned a nested list of [[1], [2], [3]] for you.

Since you want an Nx1 object, I’d recommend using array as follows:

>>> import scipy
>>> x = scipy.array([1,2,3])
>>> x
array([1, 2, 3])
>>> y = x.tolist()     // That's it. A clean, succinct conversion to a list.
>>> y
[1, 2, 3]

If you really want to use matrix

If for some reason you truly need/want to use a matrix instead of an array, here’s what I would do:

>>> import scipy
>>> x = scipy.matrix([1,2,3]).transpose()
>>> x
matrix([[1],
        [2],
        [3]])
>>> y = x.T.tolist()[0]
>>> y
[1, 2, 3]

In words, the x.T.tolist()[0] will:

1. Transpose the x matrix using the .T attribute
2. Convert the transposed matrix into a nested list using .tolist()
3. Grab the first element of the nested listed using [0]