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Re: A list based table join?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112964] Re: A list based table join?
  • From: Patrick Scheibe <pscheibe at trm.uni-leipzig.de>
  • Date: Fri, 8 Oct 2010 04:49:09 -0400 (EDT)

Hi,

you *could*:

- gather all elements of x by the words, so that you have elements with
the same word collected
- transform this into a list of rules where each rule looks like
"word"->{num1,...}
- use this to replace the words in y

selectItems[x_, y_] := 
 Flatten[y /. ((Transpose /@ 
       Gather[x, First@#1 === First@#2 &]) /. {{word_String ..}, 
       vals : {__Integer}} :> Rule[word, vals])]

x = {{"hello", 27}, {"bye", 30}, {"hello", 54}, {"seeya", 100}, {"hi",
    9}};
y = {"hello", "hi"};
selectItems[x,y]

Cheers
Patrick

On Thu, 2010-10-07 at 03:39 -0400, Leo Ditolaghi wrote:
> Hi all,
> 
> (noob Mathematica question:)
> 
> If I have two lists, how do I determine the indices at which the 1st list
> elements are matched by the elements of the second, and then use these
> matching indices to extract corresponding columns in the 1st list (sort of a
> simulated database join)? I.e.,
> 
> x = {{"hello", 27},
> {"bye", 30},
> {"hello", 54},
> {"seeya", 100},
> {"hi", 9}}
> 
> y = {"hello", "hi"}
> 
> where x is the source table, and y is the select criteria (so to speak.)
> 
> I wish to produce z, where y defines which elements of x to extract:
> 
> z = {27,54,9}
> 
> I know this should be easy and elegant, but I am going in circles
> with list functions.
> 
> Thanks in advance,
> 
> Leo
> 
> 



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