st: Re: Merge in Mata

[Gordon Hughes <G.A.Hughes@ed.ac.uk>]

If I understand the original posting correctly, what you are trying 
to carry out is a true match merge [using the terminology of the 
Stata manual entry for -merge-] with general values of the vector and 
matrix.  Assuming that is correct, then Mata does not provide any 
direct way of doing this.  Then, the question is how general you want to be.

A sketch of some special purpose code for vector v and matrix M would 
be as follows:

1.  Use -sort()- to row-sort both M and a matrix V consisting of v 
augmented by a column of original row numbers.

2.  Loop through V & M to match rows identified by the element in the 
first column of each, thus creating V_M.

3.  Decide how to treat non-matched rows in V & M and cases with 
multiple matches.

4.  Finally, re-sort V_M on the row indices of v in column 2 and drop 
this column.

None of this is difficult - provided that you are clear in dealing 
with non-matches and other special cases - and it can be easily 
implemented as a Mata function.

An alternative would be to write v & M to temporary Stata datasets 
and use -merge- to join them.  This will certainly be more 
expensive.  It may or may not be simpler from your perspective, 
depending upon how you are using Mata - e.g. you would not want to do 
this when programming a stand-alone Mata function or procedure.

Gordon Hughes
g.a.hughes@ed.ac.uk

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