RE: Fast way to select those elements from a list that are in another

*To*: mathgroup at smc.vnet.net*Subject*: [mg87083] RE: Fast way to select those elements from a list that are in another*From*: "Andrew Moylan" <andrew.j.moylan at gmail.com>*Date*: Mon, 31 Mar 2008 02:05:04 -0500 (EST)*References*: <frt574$sj8$1@smc.vnet.net> <47EFACC1.90909@gmail.com>

Unfortunately, Pick[a, Thread[a == b]] doesn't perform the same task. In your example, s3 typically evaluates to Sequence[]. That's why s1===s2===s3 subsequently evaluates to true: s1===s2===s3 >> SameQ[s1,s2,Sequence[]] >> SameQ[s1,s2] >> True This brings up an interesting quirk: you can only use SameQ to verify that two things are identical if you can be sure that none of them is Sequence[]! -----Original Message----- From: Jean-Marc Gulliet [mailto:jeanmarc.gulliet at gmail.com] Sent: Monday, 31 March 2008 2:08 AM To: Andrew Moylan; mathgroup at smc.vnet.net Subject: [mg87083] Re: Fast way to select those elements from a list that are in another 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? Andrew, I have came up with the following, which is, I believe, the fastest among all solutions that have been posted (as of today): Pick[a, Thread[a == b]] For instance, {a, b} = Table[RandomInteger[10000, 1000], {2}]; s1 = Select[a, MemberQ[b, #] &]; // Timing // First s2 = With[{c = Intersection[a, b]}, Select[a, MemberQ[c, #] &]]; // Timing // First s3 = Pick[a, Thread[a == b]]; // Timing // First s1 === s2 === s3 (* 0.127442 0.01327 0.000111 True *) Regards, -- Jean-Marc