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Re: Extracting some elements, members of another list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112418] Re: Extracting some elements, members of another list
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Wed, 15 Sep 2010 04:37:08 -0400 (EDT)

Nacho wrote:
> Hi!
> 
> I would like your advice about this problem:
> 
> I have a list of elements, consisting in phone numbers with some data:
> 
> list1= { { phone1, data1}, {phone2, data2}, {phone3, data3} .... }
> 
> And a list of phones, a subset of the phone numbers in list1
> 
> list2= {phone2, phone3, phone7... }
> 
> I'd like to extract the elements from list1 whose phone numbers are
> present in list 2, that is:
> 
> result= { { phone2, data2}, {phone3, data3, {phone7, data7} .... }
> 
> I've used this with small lists and it works fine:
> 
> result = Select[list1, MemberQ[list2, #[[1]]] &];
> 
> The problem is that now I would like to use it with big lists. list1
> is over 1.000.000 elements long and list2 is about 500.000 elements
> long.  Ordering is not a problem, I could resort the lists.
> 
> Any hint to extract this list faster? It seems to take a lot of time
> (estimation is about 5 hours and I have to do it repeatedly)
> 
> 
> Thanks!

Could do as follows.

overlapsNth[l1_,l2_,n_:1] := Module[
   {h, res},
   Do[h[l1[[j,n]]] = True, {j,Length[list1]}];
   res = Reap[Do[If[TrueQ[h[l2[[j,n]]]], Sow[list2[[j]]]],
     {j,Length[l2]}]][[2,1]];
   Clear[h];
   res
   ]

I've not thoroughly checked for correctness so you might want to do 
that. As for speed, here is an example.

n = 6;
list1 = RandomInteger[10^9, {10^n,2}];
list2 = RandomInteger[10^9, {10^n,2}];

In[24]:= Timing[intersect = overlapsNth[list1,list2,1];]
Out[24]= {3.86441, Null}

In[25]:= Length[intersect]
Out[25]= 1033

Daniel Lichtblau
Wolfram Research



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