re: st: Use of matrix values in generate statements

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

I am sorry for being a little pernickety about the implications of 
this discussion, but I would like to be clear about what is or is not 
possible in this respect.

The original observation was that the code:

	matrix input stdvalues (3700 6200...\3800 6350...\...
	gen stded = stdvalues[year-1992,filestat]

does not work.  Dan found that

	gen int i = flpdyr-1992
 	gen int j = filestat
	gen stded = stdvals[i,j]

does work and Section 14.9 of my User Guide (Stata 10) says that

	gen stded = stdvals[2,3]

should also work.

So one inference is that -generate- can access matrix sub-elements 
when either the sub-elements are either (a) constants, or (b) 
variables, but not if they are defined as (c) expressions.  This 
would be odd given the way most of Stata works.

An alternative inference is that the problem lies in the variable 
type - i.e. if variables or variable expressions are used to specify 
sub-elements they must be of type int (or byte) but not float - and 
Dan's second piece of code is required to force the result of the 
expression to be of type int.

Can anyone tells us what is the correct inference?

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

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