R help - Apr 2004 - Index of a Loop Variable?

On Tue, 6 Apr 2004, Jim Java wrote:
> Hi Everyone:--
>
> Is it possible, within a for loop not explicitly using whole-number
> indexing, to find out the index value of the loop variable within the
> vector or list that's being looped through? For example, in --
>
> x <- seq(5, 50, by=5)
> index.in.x <- 1
> for (i in x) {
>   cat(paste(" index of i-value ", i, " within x: ",
index.in.x, sep=""), fill=T)
>   index.in.x <- index.in.x + 1
> }
>
>  -- is it in general possible to get values of "index.in.x"
without
> making it a count variable, as above?
No.

Sometimes it is possible to avoid the need to do this by replacing the
loop with sapply().

Otherwise you probably should use  for(i in seq(length=length(x)) )
and get the elements of x as x[i].

	-thomas

Hi Jim,

Use the build "seq(along=vec))" as in:

x <- seq(5, 50, by=5)
for (i in seq(along=x))
cat(paste(" index of i-value ", i, " with x-value: ", x[i],
sep=""), fill=T)

Eric


At 00:55 7/04/2004, Jim Java wrote:
>Hi Everyone:--
>
>Is it possible, within a for loop not explicitly using whole-number
>indexing, to find out the index value of the loop variable within the
>vector or list that's being looped through? For example, in --
>
>x <- seq(5, 50, by=5)
>index.in.x <- 1
>for (i in x) {
>   cat(paste(" index of i-value ", i, " within x: ",
index.in.x, sep=""),
> fill=T)
>   index.in.x <- index.in.x + 1
>}
>
>  -- is it in general possible to get values of "index.in.x"
without
>making it a count variable, as above?
>
>Thank you.
>
>  -- Jim Java
>
>______________________________________________
>R-help at stat.math.ethz.ch mailing list
>https://www.stat.math.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
Eric Lecoutre
UCL /  Institut de Statistique
Voie du Roman Pays, 20
1348 Louvain-la-Neuve
Belgium

tel: (+32)(0)10473050
lecoutre at stat.ucl.ac.be
http://www.stat.ucl.ac.be/ISpersonnel/lecoutre

If the statistics are boring, then you've got the wrong numbers. -Edward 
Tufte

Hi Everyone:--

Is it possible, within a for loop not explicitly using whole-number
indexing, to find out the index value of the loop variable within the
vector or list that's being looped through? For example, in --

x <- seq(5, 50, by=5)
index.in.x <- 1
for (i in x) {
  cat(paste(" index of i-value ", i, " within x: ",
index.in.x, sep=""), fill=T)
  index.in.x <- index.in.x + 1
}

 -- is it in general possible to get values of "index.in.x" without
making it a count variable, as above?

Thank you.

 -- Jim Java

On Tue, 06-Apr-2004 at 03:55PM -0700, Jim Java wrote:

|> Hi Everyone:--
|> 
|> Is it possible, within a for loop not explicitly using whole-number
|> indexing, to find out the index value of the loop variable within the

Depending on what you do with the index in the loop, it's simple
enough to use the names of a list instead of the indexes.  For
example, you can use the part of the list identified by its name
without needing to know what its index is.

for(i in names(xx)){
  do.something(xx[[i]], ...)
  etc
}

But if it's the index you need, the answer would be No.

HTH


-- 
Patrick Connolly
HortResearch
Mt Albert
Auckland
New Zealand 
Ph: +64-9 815 4200 x 7188
~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~
I have the world`s largest collection of seashells. I keep it on all
the beaches of the world ... Perhaps you`ve seen it.  ---Steven Wright 
~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~

>
You could define the utility function iter:

iter <- function(x) mapply( x, seq(along=x),
                      FUN=function(x,i)list(x=x,i=i), SIMPLIFY=F )

and use it like this to loop over seq(5,50,5) and 1:10 simultaneously:

  > for(idx in iter(seq(5,50,5))) with(idx, cat(x,i,"\n"))
  5 1 
  10 2 
  15 3 
  20 4 
  25 5 
  30 6 
  35 7 
  40 8 
  45 9 
  50 10 



Jim Java writes--

 
 Hi Everyone:--
 
 Is it possible, within a for loop not explicitly using whole-number
 indexing, to find out the index value of the loop variable within the
 vector or list that's being looped through? For example, in --
 
 x <- seq(5, 50, by=5)
 index.in.x <- 1
 for (i in x) {
   cat(paste(" index of i-value ", i, " within x: ",
index.in.x, sep=""),
fill=T)
   index.in.x <- index.in.x + 1
 }
 
  -- is it in general possible to get values of "index.in.x" without
 making it a count variable, as above?
 
 Thank you.
 
  -- Jim Java