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