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Re: st: how can I use noof obs in Syntax


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: how can I use noof obs in Syntax
Date   Wed, 24 Oct 2012 09:53:21 +0100

Let's take that more slowly.

Rasool's intention is evidently that the number of observations in the
current dataset gives the default.

The immediate problem is that -syntax- won't evaluate expressions for you.

Maarten's solution subverts that by getting the part of Stata that
processes your input to substitute the current value of _N as an
integer before -syntax- sees "_N".

But that will wire into the definition of the program whatever that
currently is: 0, 74, or whatever. If Rasool or any other user changes
the size of the dataset while using the program, then there will be a
problem.

There are at least two easy work-arounds.

One is to have an impossible default that no-one would specify

noobs(integer -1)

Then change -1 to whatever _N is

if `noobs' == -1 local noobs = _N

That is, if the default value is still in place after -syntax-, the
option was never specified, so we use the value of _N. This should be
done when it is needed, and as before the effect is to define -noobs-
as a constant. If the dataset changes in size, -noobs- will not adjust
automatically.

Another is to use a -numlist-

noobs(numlist int max=1 >0)

Then

if "`noobs'" == "" local noobs = _N

That's a similar solution; some might regard it as more elegant or
more transparent.

Nick

On Wed, Oct 24, 2012 at 8:34 AM, Maarten Buis <maartenlbuis@gmail.com> wrote:
> On Wed, Oct 24, 2012 at 6:35 AM, rasool.bux wrote:
>> How I can use as an option in Syntax noobs(integer _N) is giving a syntax error.
>>
>> syntax varlist(min=2) [if] [in] [, REPS(integer 50) noobs(integer _N) seed(integer 1) ]
>
> noobs(integer `=_N')
>
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