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Re: Extracting some elements, members of another list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112436] Re: Extracting some elements, members of another list
  • From: "Carl K. Woll" <carlw at wolfram.com>
  • Date: Wed, 15 Sep 2010 04:40:46 -0400 (EDT)

On 9/14/2010 4:14 AM, 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!
>
>    

One possibility is to use an auxiliary function. I'll call it hashed.

Clear[hashed]
SetAttributes[hashed, Listable]
With[{lhs = hashed[list1[[All, 1]]]}, lhs = list1];

Then, use:

hashed[list2]

to get your result.

Carl Woll
Wolfram Research


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