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RE: st: Using foreach and two local macros


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Using foreach and two local macros
Date   Thu, 7 Aug 2008 17:39:20 +0100

Stefan's (non-)recommendation of -for- deserves some further explanation
and yet more  warnings. 

Note that this -for- in Stata is absolutely nothing to do with -for- in
Mata.

-for- was a prominent command in Stata, but has not been documented for
some time. 
That is, help is not even accessible via -help undocumented-. So -for-
is of no use to you unless you can remember the syntax or somehow have
kept on-line or printed documentation from Stata 7. 

It is true that -for- supported parallel lists and that nothing quite
equivalent is 
available via -foreach- or -forvalues-. That was, however, its only real
advantage in my view. In every other respect, -for- is a command that
can get you into real messes. Longer-standing members of this list may
remember a genre of anguished questions from people bitten by one or
more aspects of -for-. 

Echoes of that genre can be seen in 

http://www.stata.com/support/faqs/lang/macros.html
http://www.stata.com/support/faqs/lang/nestfor.html

Among other things, -for- entailed a considerable interpretative
overhead. So far from code being shorter, it was really much, much
longer than with -foreach- or -forvalues-, even for parallel lists. 

In terms of the original question, Jose evidently did not look at 

SJ-3-2  pr0009  . . . . . . . . . . . . . Speaking Stata:  Problems with
lists
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
J. Cox
        Q2/03   SJ 3(2):185--202                                 (no
commands)
        discusses ways of working through lists held in macros

SJ-2-2  pr0005  . . . . . .  Speaking Stata:  How to face lists with
fortitude
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
J. Cox
        Q2/02   SJ 2(2):202--222                                 (no
commands)
        demonstrates the usefulness of for, foreach, forvalues, and
        local macros for interactive (non programming) tasks

Under the Stata Journal's 3-year moving window, these papers are visible
to all via the Stata Journal website, regardless of whether you or your
institution subscribe. 

The more recent paper discusses parallel lists (and the demise of -for-)
in considerable detail. 

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

Gawrich Stefan

Looping over parallel lists is indeed a very powerful feature. 
To keep it flexible you'll often use a second extended function (word
count)
with the forval command.

local x "1 2"
local y "3 4"
local n : word count `x'
forvalues i = 1/`n' {
local x1 : word `i' of `x'
local y1 : word `i' of `y'
di `x1'+2*`x1'+`y1'
}

But in my view the syntax has two drawbacks:
1) It needs a lot of lines. So it bloats your do-files expecially when
you
use it with more than two locals. 
(I often use it for batch graph creation where variables, selections,
titles
and settings can make up to ten locals)
2) You have to create new local names inside the loop, which makes it
confusing and error prone 

So sometimes I find it convenient to use that good old (not longer
documented) "for" command again. 

for num 1 2 \ num 3 4 :  di X+2*X+Y  // Yes, you can do it in one line!

or 

local x "1 2"
local y "3 4"
for num `x' \ num `y' :  di X+2*X+Y

But foreach and forvalues are now state of the art, so I don't want to
advertise "for". 
But I think it could be very helpful to save the user from writing all
these
extended function lines and add this  
parallel lists functionality to "foreach". 


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