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MathGroup Archive 1998

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Re: Efficient way to merge lists--Programming help

  • To: mathgroup at smc.vnet.net
  • Subject: [mg12725] Re: Efficient way to merge lists--Programming help
  • From: "Allan Hayes" <hay at haystack.demon.cc.uk>
  • Date: Thu, 4 Jun 1998 02:52:11 -0400
  • References: <6l2r6l$e3p@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Joel Cannon wrote in message <6l2r6l$e3p at smc.vnet.net>...
>Cam someone suggest better ways to accomplish the following?
>
>I have two lists containing triplets--test is a rectangular array of
>triplets {x,y,z} (e.g. Dimensions {4,40,3}) and test2 is an array of
>triplets (e.g. Dimensions {50,3}). test2 is to replace equivalent
>elements in test (when the x and y values are equal--the test2 z values
>have been calculated more accurately).
>
>The task can be understood to be:
>
>1. Given a triplet {x_i,y_i,z_i} from test2, find the position in test
>which has the same {x_i,y_i,_}.
>
>2. Replace the element in test by the triplet from test2.
>
>
>I resorted to a loop after I decided that it would take me too much time
>to figure out a better, more elegant solution. Here is what I did:
>
>For[i=1,i<=Length[test2],i++,
>  test = ReplacePart[test,test2[[i]],
>     test//Position[#,{test2[[i]][[1]],test2[[i]][[2]],_}]& //Flatten]];
>


I suspect that the following may be slower, but its different.

test={{1,2,3},{2,3,4},{3,4,5},{4,5,6}}; test2={{2,3,-4},{4,5,-6}};

test/.(test2/.{x_?NumericQ,y_,z_}-> ({x,y,_}->{x,y,z}))

{{1,2,3},{2,3,-4},{3,4,5},{4,5,-6}}

------------------------------------------------------------- 
Allan Hayes
Training and Consulting
Leicester UK
http://www.haystack.demon.co.uk
hay at haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44(0)116 271 8642



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