Re: Extracting some elements, members of another list

*To*: mathgroup at smc.vnet.net*Subject*: [mg112469] Re: Extracting some elements, members of another list*From*: Ray Koopman <koopman at sfu.ca>*Date*: Thu, 16 Sep 2010 06:01:44 -0400 (EDT)*References*: <i6q0l9$l5o$1@smc.vnet.net>

On Sep 15, 1:38 am, Leonid Shifrin <lsh... at gmail.com> wrote: > Hi, > > The following function: > > Clear[memberPositions]; > memberPositions[x_List, y_List] := > Module[{tag}, > Pick[Range[Length[x]], > Replace[x, Dispatch[Thread[Rule[Intersection[x, y], tag]]], {1}], > tag]] > > Is a fast way to find positions in list <x> which are also in <y>, for > arbitrary lists <x> and <y>. Using it, it worked in about 5 seconds on my > machine (M7.0 32bit WinXP AMD Phenom II - 2800 Ghz). Here is how you do it: > > Model your lists: > > In[16]:= list1 = Transpose[ > {RandomInteger[{1000000,9999999},1000000], > Table[ToExpression["data"<>ToString[i]],{i,1,1000000}]}]; > > In[17]:= list2 = RandomSample[list1[[All,1]],500000]; > > Get the result: > > In[18]:= list1[[memberPositions[list1[[All,1]],list2]]]//Short//Timing > Out[18]= {5.203,{{9506759,data1},{3854419,data2},{1093656,data3}, > <<526944>>,{6991722,data999997},{9278309,data1000000}}} This is very quick the first time thru, but each successive call slows by an increasing amount. Here are 5 sequential sample times for the same two input lists: {4.84, 5.34, 6.39, 7.21, 8.95}. What's causing the slowdown, and how can it be fixed?