Re: Correlating two lists

• To: mathgroup at smc.vnet.net
• Subject: [mg100200] Re: Correlating two lists
• From: Ray Koopman <koopman at sfu.ca>
• Date: Thu, 28 May 2009 04:26:49 -0400 (EDT)
• References: <gv2jr1\$9af\$1@smc.vnet.net> <gvhm8r\$82s\$1@smc.vnet.net>

```Function[xi,{xi,Sequence@@Select[y,R[xi,#]&,1]}] /@ x

On May 26, 2:17 pm, anguz... at ing.uchile.cl wrote:
> Hi all.
> I have the following problem:
> Two lists: "x" and "y";  and a boolean relation R[a,b] with a in x and
> b in y, and for each "a" there is at most one "b" in "y" such that
> R[xi,yj]is true.
> R is also "expensive" to calculate. I would like to group each xi in
> "x" with its partner in "y" (if there is one) and create a merged list:
> {{x1,y1},{x2},{x3},{x4,y4},...}
> such that R[xi,yi]  is true
> I'm looking for the adequate Mathematica function to do this as
> efficiently as possible. by the way, I'm using now this
> straightforward-naive approach:
>
> merged={};
> Do[Do[If[R[xi,yj],merged=Append[merged,{xi,yj}]],{yj,y}];If[MemberQ[merge[[All,1]],xi],merged=Append[merged,{xi}]],{xi,x}]
>
> but I'm sure there must be a better one
> (without the Do's would be beautiful).
> Any thoughts will be greatly appreciated.
> Thanks in advance, and cheers for the 100.000
>
> Atte. Andres Guzman
>
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```

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