[Numpy-discussion] how to add columns

Benjamin Root ben.root@ou....
Wed Jul 28 10:39:20 CDT 2010

On Wed, Jul 28, 2010 at 7:43 AM, wheres pythonmonks <
wherespythonmonks@gmail.com> wrote:

> I have a rec array and I want to add an additional column.
> I've seen at least two solutions to this problem:
> mlab.rec_append_fields (matplotlib)
> And append_field from
> http://mail.scipy.org/pipermail/numpy-discussion/2007-September/029357.html
> In [19]: def append_field(rec, name, arr, dtype=None):
>    arr = np.asarray(arr)
>    if dtype is None:
>        dtype = arr.dtype
>    newdtype = np.dtype(rec.dtype.descr + [(name, dtype)])
>    newrec = np.empty(rec.shape, dtype=newdtype)
>    for field in rec.dtype.fields:
>        newrec[field] = rec[field]
>    newrec[name] = arr
>    return newrec
> Is there a best solution?  I don't like the matplotlib solution b/c of
> the "dll-hell" anti-pattern.  But the pure numpy solution looks like
> it has too many copies. (or are recarrays pointers-of-pointers as
> opposed to contiguous memory?)
> Help!
> W


You can import numpy.lib.recfunctions module and use the .append_fields()
function.  I am pretty sure it is still making a copy, but it is a
numpy-native function rather than rolling your own and/or using mlab.  Note,
that it will try to turn it into a masked record array by default.  You can
turn that off by using usemask=False.

I hope that helps,

Ben Root
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