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MathGroup Archive 2010

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Extracting some elements, members of another list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112404] Extracting some elements, members of another list
  • From: Nacho <ncc1701zzz at gmail.com>
  • Date: Tue, 14 Sep 2010 05:14:25 -0400 (EDT)

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!


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