Numpy, adding a row to a matrix

sapsi saptarshi.guha at gmail.com
Tue Jun 10 03:06:12 CEST 2008


Hello,
Thank you. Yes, I will post to the numpy mailing list in future.
Regards
Saptarshi


On Jun 9, 4:04 pm, Robert Kern <robert.k... at gmail.com> wrote:
> sapsi wrote:
> > Hello,
> > I have a numpy array (2 rows 3 colums)
>
> > import numpy
> > a=numpy.array(  [ [1,2,3] , [3,3,1] ])
>
> > I wish to add a row, this is how i do it
>
> > s=a.shape
> > numpy.resize(a,s[0]+1,s[1])
> > a[s[0]]=new row vector.
>
> > Q: Is this a costly operation?
>
> It can be if you have large arrays.
>
> > What happens if i have to it several
> > (and unknown) number of times?
> > is there a simpler way to add a row?
>
> numpy.vstack([a, newrow])
>
> Generally speaking, you shouldn't resize numpy arrays. If you need to construct
> an array by appending, build up a list instead and use vstack() (or hstack() or
> dstack() or column_stack() or concatenate() depending on the geometry).
>
> We also have a numpy mailing list, which you should direct future numpy
> questions to:
>
>    http://www.scipy.org/Mailing_Lists
>
> --
> Robert Kern
>
> "I have come to believe that the whole world is an enigma, a harmless enigma
>   that is made terrible by our own mad attempt to interpret it as though it had
>   an underlying truth."
>    -- Umberto Eco




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