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Re: st: funny behaviour of _N


From   Philip Ryan <philip.ryan@adelaide.edu.au>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: funny behaviour of _N
Date   Thu, 28 Aug 2008 16:55:19 +0930

Ashim

Funny perhaps, but logical to Stata.

In code fragment 2. below you are using the -in- qualifier. -help in- explicitly states that -in- is followed by a range and gives allowable syntax for that range.
_N is not allowable syntax. Essentially it is not a number; it is what Stata calls a system variable. But its content as a number can be exposed by evaluation "on the fly". Try:

replace a=b in `=_N'

In code fragment 1. below you have forced the evaluation of the *expression* _N by using the "=". The number represented by _N now resides in local macro num. When num is subsequently expanded it presents to -in- as a syntactically correct range.

Probably about now, or shortly thereafter, you should read the User's Guide and at least selected parts of the Data Management volume of the documentation. It will repay the effort.

Phil



At 03:53 PM 28/08/2008, you wrote:

Hello everyone,

Please compare the following 2 code snippets : -

1.

local num=_N
replace a=b in `num'

2.

replace a=b in _N

Surprisingly only the 1st one works ! Could some one tell me why ?

Thank you,
Ashim
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