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RE: st: What is EGEN_Varname and EGEN_SVarname ?

From   "William Buchanan" <[email protected]>
To   <[email protected]>
Subject   RE: st: What is EGEN_Varname and EGEN_SVarname ?
Date   Mon, 30 Jul 2012 14:09:54 -0700

Hi Pradipto,

I'll try to answer what I can, but it may not be much and others on the list
would likely have much more refined/precise answers.  

1. The lines of code:

  if _by() { 
                local byopt "by(`_byvars')"
                local cma ","

might be what you are looking for, but you should also read the help file
for the -byable- option in the programming manual.  This will better explain

2. Not sure about all of your question here, but you should consider the
context in which this line occurs 

if `"`args'"' == "_all" | `"`args'"' == "*" {
                version 7.0, missing
                unab args : _all
                local args : subinstr local args "`_sortindex'"  "", all
                version 6.0, missing

Which seems like it is parsing different ways that you could include the
entire varlist in your dataset.  And _all is used in more than just -drop-.
For example, -log query _all- or -program drop _all- are other uses of

3. The global macros need to be defined so they can be passed to the other
functions (which is why Nick explicitly mentioned that they are there
because local macros would not work in this context).

Have you read the programming manual for Stata yet?  If not I would highly
suggest going through that and possibly looking into other resources (Kit
Baum's book is pretty awesome).  Otherwise, you could probably just as
easily create a work around using tempvars that you could combine into a
single variable that is stored at the end of the routine with a loop.  


-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Pradipto Banerjee
Sent: Monday, July 30, 2012 1:47 PM
To: [email protected]
Subject: RE: st: What is EGEN_Varname and EGEN_SVarname ?


I'm primarily trying to understand Stata syntax within the -egen- code to
write my "own" function rather than modify the -egen- code. Perhaps you can
help me. For instance,

1. How does the egen function handle the "by" part:

. by var1: egen ...
. bysort var1 (var2): egen ...

i.e., where in the egen code, does it figure out that it needs to sort
before doing a by operation etc (i.e. where does it figure out that it was
supplied with -by- instead of -bysort- etc). Similarly, in another part of
the function, it's written

. capture noisily _g`fcn' `type' `dummy' = (`args') `if' `in' /*
                */ `cma' `byopt' `options'

Why is the "by" part done this way?, i.e. why isn't this given as

. capture noisily by _byvars: _g`fcn' `type' `dummy' = (`args') `if' `in' ,

2. In the part of the code where it is written:

(a) . local args : subinstr local args "`_sortindex'"  "", all word

Why are we changing all instances of _sortindex as blanks? Isn't the
sortindex typically like __000000 (or something similar), and it isn't
normally provided when a person is calling the egen function?

(b) . if `"`args'"' =="_all"

Usually, I had seen _all been used as drop _all. What does the above part of
the code mean?

3. (a) Why do we need to define EGEN_SVarname and EGEN_Varname to `name' and
_sortindex before calling the _g`fcn', AND (b) why do we need to define them
again to Null?


-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of William Buchanan
Sent: Monday, July 30, 2012 1:00 PM
To: [email protected]
Subject: RE: st: What is EGEN_Varname and EGEN_SVarname ?


Another important thing to consider, with regards to Nick's advice, is that
he is basically trying to prevent you from making any changes to your
installed copy of Stata that might "break" it.  If you're changing the
global macros in the egen files then you run the risk of making a change
that prevents egen from working correctly in the future.  If you wanted to
see what those variables are, you could do something like:

viewsource egen.ado (copy the material in the window into the do file editor
or another text editor of your choice)

save your clone as myegen.ado to your personal directory (if you're unsure
where this is you can use the -sysdir- command)

pick the function that you were trying to modify and do the same thing
(e.g., copy the source into a new file myconcat.ado for example).

Now if you make a call to -myegen- make sure that you are using the function
that you've duplicated and you can change the global macros in your
duplicate files.

The point, however, is to avoid editing any of the files that are native to
the Stata installation unless you're willing to risk making potentially
harmful changes.  Additionally, Nick also provided you with an example
earlier illustrating how you could make the egen subroutine that you were
interested in byable, so you could just start working with that code and
modifying it as necessary.


-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Nick Cox
Sent: Monday, July 30, 2012 9:41 AM
To: [email protected]
Subject: Re: st: What is EGEN_Varname and EGEN_SVarname ?

I am mixing explanations and advice on different levels. Feel free to take
the explanations when correct and ignore the advice if it doesn't appeal.
Otherwise I think Maarten's reply to you captures my attitude well. It is a
defining characteristic of a discussion list that people aren't obliged to
offer exactly the kind of support that posters of questions might prefer.


On Mon, Jul 30, 2012 at 8:48 AM, Pradipto Banerjee
<[email protected]> wrote:
> Nick
> I don't understand your line of thought ... you actually want me NOT 
> to
advance my understanding of programming in Stata? Why are some folks allowed
to understand advanced programming and others not?
> Sorry to say this, but one huge negative of Stata is that there is so 
> much
undocumented stuff - and for efficient programming we have to repeatedly
rely on email lists like this.
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of Nick Cox
> Sent: Saturday, July 28, 2012 9:33 AM
> To: [email protected]
> Subject: Re: st: What is EGEN_Varname and EGEN_SVarname ?
> These are names of globals defined in -egen- to communicate between 
> programs, which manifestly can't be done through locals, apart from 
> using non-documented features. If you're following my earlier advice 
> you will leave them in peace.
> Nick
> On 27 Jul 2012, at 16:03, Pradipto Banerjee 
> <[email protected]
>  > wrote:
>> I noticed some of the codes, e.g. _ggroup2 use EGEN_Varname and 
>> EGEN_SVarname and recently one of the codes shared by Nick (called
>> ereplace) use these. What is EGEN_Varname and EGEN_SVarname ?
> *
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