Re: Re: Fast way to select those elements from a list

*To*: mathgroup at smc.vnet.net*Subject*: [mg86818] Re: [mg86806] Re: Fast way to select those elements from a list*From*: danl at wolfram.com*Date*: Sat, 22 Mar 2008 00:52:39 -0500 (EST)*References*: <frt574$sj8$1@smc.vnet.net> <47E23A75.7030909@gmail.com>

> To clarify: the list I want to construct is those elements from a that are > members of b: > Select[a, MemberQ[b, #] &] > > This is (I think!) the same as > > With[ > {c = Intersection[a, b]}, > Select[a, MemberQ[c, #] &] > ] > > but the latter is faster. > > > On 20/03/2008, Szabolcs Horv=E1t <szhorvat at gmail.com> wrote: >> >> Andrew Moylan wrote: >> > Two lists of integers: >> > >> > {a, b} = Table[RandomInteger[10000, 1000], {2}]; >> > >> > Which elements from a are in b? >> > >> > Select[a, MemberQ[b, #] &]; // Timing >> >>> {0.351, Null} >> > >> > It takes about 0.351 seconds (on my slow laptop). Specialised >> > functions for related tasks, such as Intersection[a, b], are much >> > faster: >> > >> > Intersection[a, b]; // Timing >> >>> {0., Null} >> > >> > Using Intersection, here's a somewhat faster way to select those >> > elements from a that are in b: >> > >> > With[ >> > {c = Intersection[a, b]}, >> > Select[a, MemberQ[c, #] &] >> > ]; // Timing >> >>> {0.09, Null} >> > >> > Is there a better, faster way to do this? >> > >> [...] This seems fairly fast. In[23]:= {a, b} = Table[RandomInteger[10000, 1000], {2}]; Timing[s1 = Select[a, MemberQ[b, #] &];] Out[24]= {0.621, Null} In[25]:= Timing[s2 = Module[{mq}, Map[(mq[#] = True) &, b]; Reap[Map[If[mq[#] === True, Sow[#]] &, a]]][[2, 1]];] Out[25]= {0.02, Null} In[26]:= s2 === s1 Out[26]= True Daniel Lichtblau Wolfram Research