Creating a variable number of list in Python

Question

I want to read data from a .txt file and enter each column as a variable (say x_1, x_2, etc) Normally I use

x_1, x_2, x_3 = numpy.loadtxt('filename.txt', delimiter=',' , unpack = True)

Now the problem is each data file is going to have different number of columns. So I was looking for a way to create multiple variable names x_i

I tried using a dictionary with each key as an empty list in the following way:

features = dict(('x_%d' %i,[]) for i in range (1,n))   # n is specified by user

The problem is I want to convert these lists(x_1, x_2, etc into a matrix and carry out some matrix operations later. And I just cannot refer these keys as variables as in:

x_1.T                        # Transpose  OR
x_2 - Y                      # both x_2 and Y are matrices

And I don't want to use something like features['x_3'] every time. Any suggestions?

update: Okay found out a way: create a list of matrices and then refer to them as x[o], x[1], etc. Even though x is a list, x[2] is a matrix.

Any better/smarter solution?

You can utilize the locals() method. It'd be okay for smaller scripts and applications but it gets unruly fast IMO. — Henrik Andersson
– Henrik Andersson, Commented Aug 2, 2013 at 7:57
What's wrong with just using a 2 dimensional list? so you'd do x = numpy.loadtxt(...) and then x[0].T, x[1] - Y? — DanielB
– DanielB, Commented Aug 2, 2013 at 7:59

bennihepp · Accepted Answer · 2013-08-02 08:00:54Z

1

First load the file into a big array (matrix):

x = numpy.loadtxt('filename.txt', delimiter=',', unpack=True)

Now you can access the column vectors with

x1 = x[:,0]
x2 = x[:,1]

and use numpy functions on these variables

y = x1.dot(x2)

answered Aug 2, 2013 at 8:00

bennihepp

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Comments

Lauritz V. Thaulow · Accepted Answer · 2013-08-02 08:02:44Z

0

Why don't you use a list called x instead?

x[1].T
x[2] - Y

It's only one extra keystroke compared to x_1, x_2, etc. If you don't want to use x[0], just put a None there.

Dynamically generating local or global varibles is a bad idea.

answered Aug 2, 2013 at 8:02

Lauritz V. Thaulow

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1 Comment

Ally Over a year ago

@ Lauritz: Yes exactly this is what I did as mentioned in my update, thanks anyways for the fast reply

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