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RE: st: How do i substitute the return local of program in the if expression of the egen command?


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: How do i substitute the return local of program in the if expression of the egen command?
Date   Tue, 30 Mar 2010 13:36:35 +0100

Several questions are bundled up together here at different levels. Let me try to separate them. Let's put aside questions about ` ' (right) and ' ' (wrong) on one side as understood. 

1. The underlying problems
==========================

Antonis wants the row total of variables if and only if all variables are zero or positive. Otherwise put: if the row minimum is zero or positive. Given the second formulation, no program is needed:

egen rowmin = rowmin(<varlist>)
egen rowtotal = rowtotal(<varlist>) if rowmin >= 0 

Antonis' other code looks like more -egen- calls once he's done this. Correct me if I'm wrong. 

2. Why a call to r() didn't work initially
==========================================

Maarten explained this. Antonis wants the incarnation of his r() result as a local macro and must use ` ' to get its contents. 

3. Why the second call to r() still does not work
=================================================

This is more subtle. The first call to -egen- internally calls -count-. A side-effect of that is that existing r() results are zapped. Thus by the time of his second -egen- call the r() result Antonis wants no longer exists. He is then calling -egen- with an empty -if- condition, which is illegal. 

4. What could be done instead
=============================

If you wanted to program this, and didn't like the solution in #1 for some reason, it is better to join the two programs into one based on something like 

gen rowtotal = 0 
foreach v of local varlist { 
	replace rowtotal = rowtotal + cond(`v' >= 0, `v', .)  
} 

Adding one missing whenever a variable is negative should be sufficient to get the result desired. 

Antonis had a helper program that given (say) 

local varlist var1 var2 var3 var4 var5 

produces 

var1 >= 0 & var2 >= 0 & var3 >= 0  & var4 >= 0 & var5 >= 0 

There are tricks to do that more directly. 

local cond : subinstr local varlist " " ">= 0 & ", all 
local cond `cond' >= 0 

That is, replace each internal space by ">= 0 &" and then append a final ">= 0". 

Nick 
n.j.cox@durham.ac.uk 

Martin Weiss

set trace on

then, and see where the thing goes awry, i.e. post the lines surrounding the
error...

A Loumiotis

Sorry.... I mistyped the wrong left single quotation mark in my email
reply, but in my code I used the correct left quotation mark "`" and I
still get for the !(`r(andpve)') if expression

invalid syntax
r(198);

Martin Weiss

> Seems the left single quote is not correct. Replace "'" with "`", w/o the
> quotation marks, of course...

A Loumiotis

> Thanks for your help!
>
> using `r(andpve)' resolves the first problem...
>
> but for !('r(andpve)') i get the following
>
> invalid syntax
> r(198);

Maarten buis 

A Loumiotis wrote:

>>> I have written a program (with the name andpve) that
>>> generates an expression of the form:
>>>
>>> var1>=0 & var2>=0 & ... & varN>=0
>>>
>>> and returns this value as a local with name r(andpve).
>>>
>>> I then want to use r(andpve) in another progam (rowctotal)
>>> where U substitute it in the if expression of the egen command.
>>>
>>> egen `left'_a = rowtotal(`right') if r(andpve), missing
>>> egen `left'_b = rowmin(`right') if !(r(andpve))
>>
>> egen `left'_a = rowtotal(`right') if `r(andpve)', missing
>> egen `left'_b = rowmin(`right') if !(`r(andpve)')

>>> But the egen command does not seem understand the r(andpve)
>>> as an if
>>> expression or the !(r(andpve)) as an if expression.
>>>
>>> What did I do wrong?  I'm new to Stata.  The code is
>>> below.
>
>>> program rowctotal, rclass
>>>     version 11
>>> gettoken left right: 0
>>> capture drop `left'
>>> andpve `right'
>>>     di r(andpve)
>>>     egen `left'_a = rowtotal(`right') if r(andpve),
>>> missing
>>>     egen `left'_b = rowmin(`right') if !(r(andpve))
>>>     egen `left' = rowtotal(`left'_*), missing
>>>     *drop `left'_*
>>> end
>>>
>>> program andpve, rclass
>>>     local k = 1
>>>     while "``k''" != "" {
>>>     local ++k
>>>     }
>>> local --k
>>> local i = 1
>>> local andpve ""
>>> while `i' <= `k'-1 {
>>>     local andpve "`andpve'``i''>=0 & "
>>>     local ++i
>>> }
>>> local andpve "`andpve'``i''>=0"
>>> return local andpve `andpve'
>>> end
>>>

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