looping and searching in numpy array

[Albert-Jan Roskam]

> Date: Thu, 10 Mar 2016 08:48:48 -0800
> Subject: Re: looping and searching in numpy array
> From: hemla21 at gmail.com
> To: python-list at python.org
> 
> On Thursday, March 10, 2016 at 2:02:57 PM UTC+1, Peter Otten wrote:
> > Heli wrote:
> > 
> > > Dear all,
> > > 
> > > I need to loop over a numpy array and then do the following search. The
> > > following is taking almost 60(s) for an array (npArray1 and npArray2 in
> > > the example below) with around 300K values.
> > > 
> > > 
> > > for id in np.nditer(npArray1):
> > >                   
> > >        newId=(np.where(npArray2==id))[0][0]
> > > 
> > > 
> > > Is there anyway I can make the above faster? I need to run the script
> > > above on much bigger arrays (50M). Please note that my two numpy arrays in
> > > the lines above, npArray1 and npArray2  are not necessarily the same size,
> > > but they are both 1d.
> > 
> > You mean you are looking for the index of the first occurence in npArray2 
> > for every value of npArray1?
> > 
> > I don't know how to do this in numpy (I'm not an expert), but even basic 
> > Python might be acceptable:
> > 
> > lookup = {}
> > for i, v in enumerate(npArray2):
> >     if v not in lookup:
> >         lookup[v] = i
> > 
> > for v in npArray1:
> >     print(lookup.get(v, "<not found>"))
> > 
> > That way you iterate once (in Python) instead of 2*len(npArray1) times (in 
> > C) over npArray2.
> 
> Dear Peter, 
> 
> Thanks for your reply. This really helped. It reduces the script time from 61(s) to 2(s). 
> 
> I am still very interested in knowing the correct numpy way to do this, but till then your fix works great. 


Hi, I suppose you have seen this already (in particular the first link): http://numpy-discussion.10968.n7.nabble.com/Implementing-a-quot-find-first-quot-style-function-td33085.htmlI don't thonk it's part of numpy yet.
Albert-Jan