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Date: Sun, 21 Jun 2015 03:20:24 +0100
From: MRAB <python@mrabarnett.plus.com>
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Subject: Re: How to construct matrix from vectors?
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On 2015-06-21 02:57, Nasser M. Abbasi wrote:
> I just started to learn some python today for first time,
> so be easy on me.
>
> I am having some trouble figuring how do the problem shown in this link
>
> http://12000.org/my_notes/mma_matlab_control/KERNEL/KEse44.htm
>
> Given 4 column vectors, v1,v2,v3,v4, each is 3 rows.
>
> I want to use these to construct matrix mat, which is
>
>         [[v1,v2],
>          [v3,v4]]
>
> So the resulting matrix is as shown in the link.  i.e.
> it will be 6 rows and 2 columns.
>
> This is what I tried:
>
> import numpy as np
> v1=np.array([1,2,3]);
> v2=np.array([4,5,6]);
> v3=np.array([7,8,9]);
> v4=np.array([10,11,12]);
>
> And now I get stuck, I tried
>
> m=np.array([[v1,v2],[v3,v4]])  #no good
>
> Also
>
> m=np.array([v1,v2,v3,v4])
> m.shape
>      Out[153]: (4L, 3L)
> m.T
>
> array([[ 1,  4,  7, 10],
>          [ 2,  5,  8, 11],
>          [ 3,  6,  9, 12]])
>
> Not what I want.
>
> I need to get the shape as in the above link, 6 rows by 2 columns,
> where each column vector is stacked as shown.  I also tried
>
> v1=np.array([1,2,3]); v1.shape=3,1
> v2=np.array([4,5,6]); v2.shape=3,1
> v3=np.array([7,8,9]); v3.shape=3,1
> v4=np.array([10,11,12]); v4.shape=3,1
> mat=np.array([[v1,v2],[v3,v4]])
>
> What is the correct way to do this in Python?
>
Here's one way, one step at a time:

>>> r1 = np.concatenate([v1, v2])
>>> r1
array([1, 2, 3, 4, 5, 6])
>>> r2 = np.concatenate([v3, v4])
>>> r2
array([ 7,  8,  9, 10, 11, 12])
>>> m = np.array([r1, r2])
>>> m
array([[ 1,  2,  3,  4,  5,  6],
        [ 7,  8,  9, 10, 11, 12]])
>>> m.transpose()
array([[ 1,  7],
        [ 2,  8],
        [ 3,  9],
        [ 4, 10],
        [ 5, 11],
        [ 6, 12]])
>>>