[Numpy-discussion] extract elements of an array that are contained in another array?

[Ning Sean]

Hi, I want to extract elements of an array (say, a) that are contained in
another array (say, b). That is, if a=array([1,1,2,3,3,4]), b=array([1,4]),
then I want array([1,1,4]).

I did the following but the speed is very slow (maybe because a is very
long):

c=array([])
for x in b:
   c=append(c,a[a==x])

any way to speed it up?

Thanks!
-Ning
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Re: [Numpy-discussion] extract elements of an array that are contained in another array?

[Ning Sean]

Thanks! Tried it and it is about twice as fast as my approach.

-Ning

On Wed, Jun 3, 2009 at 7:45 PM, josef.p...@gmail.com wrote:

 On Wed, Jun 3, 2009 at 8:29 PM, Ning Sean nings...@gmail.com wrote:
  Hi, I want to extract elements of an array (say, a) that are contained in
  another array (say, b). That is, if a=array([1,1,2,3,3,4]),
 b=array([1,4]),
  then I want array([1,1,4]).
 
  I did the following but the speed is very slow (maybe because a is very
  long):
 
  c=array([])
  for x in b:
 c=append(c,a[a==x])
 
  any way to speed it up?
 
  Thanks!
  -Ning
 


 It's waiting in Trac for inclusion in numpy
 http://projects.scipy.org/numpy/ticket/1036
 The current version only handles arrays with unique elements.

 You can copy the ticket attachment, the version there is very fast.

 Josef
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