# numpy - 2D matrix/array - initialization like in Matlab...

someone newsboost at gmail.com
Tue Oct 16 00:42:34 CEST 2012

```On 10/15/2012 11:26 PM, MRAB wrote:
> On 2012-10-15 22:09, someone wrote:
>>
>> See this:
>>
>> ==========================================================
>> In [5]: Dx = numpy.matrix('1 0 0; 0 0.5 -0.5; 0 -0.5 1.5')
>>
>> In [6]: Dx
>> Out[6]:
>> matrix([[ 1. ,  0. ,  0. ],
>>           [ 0. ,  0.5, -0.5],
>>           [ 0. , -0.5,  1.5]])
>> ==========================================================
>>
>>
>>
>> Ok... So now test = 33 and instead of the value 1.5 I want to use the
>> value of "test" and put it directly into the matrix (or array):
>>
>> ==========================================================
>> In [7]: test=33
>>
>> In [8]: Dx = numpy.matrix('1 0 0; 0 0.5 -0.5; 0 -0.5 test')
>> ---------------------------------------------------------------------------
>>
>> NameError                                 Traceback (most recent call
>> last)
>> /home/user/something/<ipython-input-8-5a43575649e1> in <module>()
>> ----> 1 Dx = numpy.matrix('1 0 0; 0 0.5 -0.5; 0 -0.5 test')
>>
>> /usr/lib/python2.7/dist-packages/numpy/matrixlib/defmatrix.pyc in
>> __new__(subtype, data, dtype, copy)
>>       252
>>       253         if isinstance(data, str):
>> --> 254             data = _convert_from_string(data)
>>       255
>>       256         # now convert data to an array
>> ...... etc...
>> ==========================================================
>>
>>
>>
>> So obviously it doesn't understand that I want this:
>>
>> ==========================================================
>> In [21]: Dx[2,2]=test
>>
>> In [22]: Dx
>> Out[22]:
>> matrix([[  1. ,   0. ,   0. ],
>>           [  0. ,  33. ,  -0.5],
>>           [  0. ,  -0.5,  33. ]])
>> ==========================================================
>>
>> Without having to manually change all the individual places using my
>> variables (test is actually many variables, not just one but I think you
>> should understand the problem now).
>>
>>
>> How to initialize my array directly using variables ?
>>
>> It could also be that I wanted:
>>
>> test11 = 1
>> test12 = 1.5
>> test13 = 2
>> test21 = 0
>> test22 = 5
>>
>> Dx = numpy.matrix('test11 test12 test13; test21 test22 -0.5; 0 -0.5 1.5')
>>
>> Etc... for many variables...
>>
>> Appreciate ANY help, thank you very much!
>>
> What it prints should give you a hint:
>
>  >>> Dx = numpy.matrix([[test11, test12, test13], [test21, test22,
> -0.5], [0, -0.5, 1.5]])
>  >>> Dx
> matrix([[ 1. ,  1.5,  2. ],
>          [ 0. ,  5. , -0.5],
>          [ 0. , -0.5,  1.5]])

Uh, great - thank you very much!

As you maybe see, I'm only a python newbie so I'm not so good at
understanding the error messages and reading the source code yet.

Thank you very much for the solution to the problem! It's highly
appreciated. Thanks.

```