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Efficient way to merge lists--Programming help

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:

  test = ReplacePart[test,test2[[i]],
     test//Position[#,{test2[[i]][[1]],test2[[i]][[2]],_}]& //Flatten]];

Thanks for any help.

Joel W. Cannon                 |   (318)869-5160          Dept. of
Physics               |   (318)869-5026  FAX    Centenary College of
Louisiana |       P. O. Box 41188		       |	     
Shreveport, LA 71134-1188      |      

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