Ian Watson
> My problem is this: how can -for each- be made to do
> something like:
>
> for var v27-v32 \ num 1/6 : recode X (99=3), gen(item_Y)
>
> I should add to my earlier posting, that I realise one can have a
> -foreach- loop nested inside another -for each- loop, but I hope that
> is not the answer to my query. If so, it means several lines of code
> instead of just one.
> And, if one fully exploits -for- (with a third placeholder Z), then it
> would mean even more lines of code in -for each- to replicate this
> behaviour.
Ian's specific example is akin to a problem posed by Dev Vencappa
a few days ago and answered by Chris Wallace, Tom Steichen and
Ernest Berkhout.
-foreach- [sic] can be used in this problem in several different
ways. As it happens, all are, I think, longer-winded than the -for-
statement. However, the problem is not a _nested_ problem at
all, and so does not call for nested loops. It is a problem
of going through two lists in parallel.
Code first, and then comment.
0.
local i = 0
foreach v of var v27-v32 {
local i = `i' + 1
recode `v' 99=3, gen(item_`i')
}
1. You can cut a line from that:
local i = 0
foreach v of var v27-v32 {
recode `v' 99=3, gen(item_`++i')
}
2.
foreach i of num 1/6 {
local j = `i' + 21
recode v`j' 99=3, gen(item_`i')
}
3.
foreach i of num 27/32 {
recode v`i' 99=3, gen(item_`=`i'-21')
}
Instead of cycling through a numlist with -foreach-,
-forval- offers yet other ways of doing it. Here's one.
4.
forval i = 27/32 {
recode v`i' 99=3, gen(item_`=`i'-21')
}
That's enough variations on a theme. In essence, the
recipe is to step through one list with -foreach- or -forval-,
and to step through another parallel list using some
operation on a local macro.
What seems to underlie Ian's question is some incredulity
that -foreach- (introduced in Stata 7) can be so long-winded
when -for- (with a much longer history, but now undocumented
as of Stata 8) offers -- for this kind of problem -- a compact
solution.
He is right to be puzzled. -for- could be used very concisely
for problems like this, and was designed with precisely that
aim in mind. In fact, I'd call this -for-'s biggest single advantage.
-foreach- and -forvalues- in contrast offer only the means of
stepping through one list at a time. They score much much better
for nested problems, and in most other ways.
The merits and demerits of -for- featured frequently on this
list up to about two years ago. Some of those threads are echoed
directly or indirectly in a discussion in
"Speaking Stata: Problems with lists" Stata Journal
3(2): 185--202 (2003).
As it also happens, a single call to -recode- can be used
for the problem, but I take that to be incidental.
Nick
[email protected]
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