Re: Extracting some elements, members of another list
- To: mathgroup at smc.vnet.net
- Subject: [mg112435] Re: Extracting some elements, members of another list
- From: Adriano Pascoletti <adriano.pascoletti at uniud.it>
- Date: Wed, 15 Sep 2010 04:40:35 -0400 (EDT)
Dispatch should be a good candidate: In[1]:= list1 = {{phone1, data1}, {phone2, data2}, {phone3, data3}, {phone4, data4}, {phone5, data5}, {phone6, data6}, {phone7, data7}, {phone8, data8}}; rules = Dispatch[Apply[#1 -> {##1} & , list1, {1}]] Out[2]= Dispatch[{phone1 -> {phone1, data1}, phone2 -> {phone2, data2}, phone3 -> {phone3, data3}, phone4 -> {phone4, data4}, phone5 -> {phone5, data5}, phone6 -> {phone6, data6}, phone7 -> {phone7, data7}, phone8 -> {phone8, data8}}] In[5]:= list2 = {phone2, phone3, phone7}; list2 /. rules Out[6]= {{phone2, data2}, {phone3, data3}, {phone7, data7}} Adriano Pascoletti 2010/9/14 Nacho <ncc1701zzz at gmail.com> > 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! > >