[Numpy-discussion] saving the lower half of matrix

Kumar Appaiah a.kumar@alumni.iitm.ac...
Mon Jun 17 06:14:56 CDT 2013


On Mon, Jun 17, 2013 at 01:07:34PM +0200, Bala subramanian wrote:
>    Friends,
>    I have to save only the lower half of a symmetric matrix to a file. I used
>    numpy.tril to extract the lower half. However when i use 'numpy.savetxt',
>    numpy saves the whole matrix (with upper half values replaced as zeros)
>    rather than only the lower half. Any better idea to achieve this.
>    as an example
>    >>>import numpy as np
>    >>> d=np.arange(1,26,1)
>    >>> d=d.reshape(5,5)
>    >>> np.tril(d)
>    array([[ 1,� 0,� 0,� 0,� 0],
>    ������ [ 6,� 7,� 0,� 0,� 0],
>    ������ [11, 12, 13,� 0,� 0],
>    ������ [16, 17, 18, 19,� 0],
>    ������ [21, 22, 23, 24, 25]])
> 
>    >>> np.savetxt( 'test', np.tril(d))
>    output test also contains the zeros, what i want is only the lower half
>    like below.
>    1
>    6��� 7
>    11 12 13
>    �-���� -���� -��� -� etc

How about saving numpy.concatenate([x[i,:i+1] for i in range(x.shape[0])])
instead? If you remove the concatenate, you'll get the individual
arrays that have the data you need as well.

Kumar
-- 
Kumar Appaiah


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