[Numpy-discussion] list to array is slow

[Skipper Seabold]

On Thu, Jul 22, 2010 at 5:09 PM, marco cammarata <marcocamma at gmail.com> wrote:
> Hi,
>
> any idea why the simple code below is so slow ?
>
> import numpy as n
> from time import time as t
>
> dims = (640,480)
> m = n.random.random( dims )
>
> l=[]
>
> for i in range(200):
>        l.append(m)
>
> t0=t()
> b=n.array(l)
> print t()-t0
>
>
> To convert the list into an array takes about 5 sec ...
>

There was a recent thread that covers similar ground.  "Stacking
arrays along new dimension in front"

http://www.mail-archive.com/numpy-discussion@scipy.org/msg26772.html

I think the fastest way was to do

b = n.concatenate(a_[None] for a_ in l)

but I could be mistaken, and there should probably be a more general
solution to this problem.

Skipper