Re: st: Using scalars when defining matrices

["Michael S. Hanson" <mshanson@mac.com>]

On Dec 7, 2006, at 7:48 PM, Rachel wrote:

> I'm having trouble with the syntax of matrix define (or input) when
> using scalars.  When I try the following:
>
> scalar b3=2
> matrix input Matrixnew=(1,`b3',`b3',`b3'\ `b3',1,0,0\ `b3',0,1,0\ 
> `b3',0,0,1)
	[snip]

> What am I doing wrong?
	You are confusing scalars with local macros.  Consider the following:

scalar b3 = 2
local b3 = 4
matrix Ms = (1,b3,b3,b3 \ b3,1,0,0 \ b3,0,1,0 \ b3,0,0,1)
matrix Ml = (1,`b3',`b3',`b3' \ `b3',1,0,0 \ `b3',0,1,0 \ `b3',0,0,1)
mat list Ms
mat list Ml

	Notice that b3 -- the scalar -- and b3 -- the local macro -- are two 
*different* objects with two *different* values.  That may be confusing 
to you, but Stata understands the difference.  And it's important it 
does, otherwise programs (do and ado files) could accidentally clobber 
a variable, scalar, matrix, etc. that might exist in the user's active 
memory prior to a program being executed if it just so happened that 
they had the same name.

	As long as you are consistent -- that is, use a scalar and do not 
quote it, or use a local macro and quote it appropriately -- you likely 
will not get confused either.  I find that using scalars is more 
convenient when working interactively in Stata, but for any kind of 
programming (i.e. do or ado files), local macros are well worth 
learning and using.  The manuals are the best place to learn this topic 
-- far more complete than the on-line help.

                                        -- Mike

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