[Tutor] a code question, but don't know question's name

Eike Welk eike.welk at gmx.net
Sun Oct 7 23:20:52 CEST 2007

On Sunday 07 October 2007 21:29, Happy Deer wrote:
> Thank all for the discussion.
> Maybe I can separate my question into two.
> First, I have experience in Matlab, where I can use "eval". I
> wonder whether someone knows about it.

I suspect you are using Numpy, you should subscribe to the Numpy 
mailing-list. There are people that can compare Numpy to Matlab. 
Probably you should also subscribe to the Scipy mailing list(s).


> Second, if I just want to return data[:,1], ...data[:,-1]
> separately without knowing ahead how many columns data has. What
> should I do?

eike at bookxie:~> ipython --pylab
<much text deleted>

In [1]:a = array([[1,2,3], [4,5,6]])

In [2]:a
array([[1, 2, 3],
       [4, 5, 6]])

In [3]:a[:,0]
Out[3]:array([1, 4])

In [4]:a.shape
Out[4]:(2, 3)

In [6]:for i in range(a.shape[1]):
   .6.:  print a[:,i]
[1 4]
[2 5]
[3 6]

If you want a[:,i] be/stay a column vector use the matrix class. The 
mat constructor function even has Matlab like syntax.

In [8]:import numpy as np

In [9]:a = np.mat("1,2,3; 4,5,6")

In [11]:a
matrix([[1, 2, 3],
        [4, 5, 6]])

In [12]:a[:,0]

I shortened the Ipython session a little, you don't want to see my 
syntax errors I guess.


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