R help - Mar 2009 - FW: flow control

Hi all,

I need a little help with flow control in R.  What I'd like to do is to
advance a for loop by changing its counter.  However, what seems obvious
to me does not yield the proper results.  An example of my problem is


for (i in seq(1, some_number, some_increment)){
	<some stuff>
	if (some_condition == T) i <- i + 2;  #want to advance the loop
by 2
}

Whenever the counter goes to the next step, the next item in the
original sequence seq(1, some_number, some_increment) replaces the
counter value.  Is there a way for me to do this?

Best,

Ken

Hi Ken,

The help page for ?"for" says that:

The index seq in a for loop is evaluated at the start of the loop; changing
it subsequently does not affect the loop. The variable var has the same type
as seq, and is read-only: assigning to it does not alter seq.

So you cannot do what you want to do with a for loop.  But you could do what
you want with a while loop:

i <- 0
while(i < 20) {
	i <- i + 1
	cat(i, "\n")
	if(i %% 5 == 0) i <- i + 2
}

-Christos
> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Lo, Ken
> Sent: Wednesday, March 04, 2009 3:53 PM
> To: r-help at r-project.org
> Subject: [R] FW: flow control
> 
> Hi all,
> 
> I need a little help with flow control in R.  What I'd like 
> to do is to advance a for loop by changing its counter.  
> However, what seems obvious to me does not yield the proper 
> results.  An example of my problem is
> 
> 
> for (i in seq(1, some_number, some_increment)){
> 	<some stuff>
> 	if (some_condition == T) i <- i + 2;  #want to advance 
> the loop by 2 }
> 
> Whenever the counter goes to the next step, the next item in 
> the original sequence seq(1, some_number, some_increment) 
> replaces the counter value.  Is there a way for me to do this?
> 
> Best,
> 
> Ken
> 
> 	
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
>

The help page for ?"for" says that:

   The index seq in a for loop is evaluated at the start of the loop;
changing
   it subsequently does not affect the loop. The variable var has the
same type
   as seq, and is read-only: assigning to it does not alter seq.

The help file is not right when seq is a list() or other recursive
type.  In that case var has the type of seq[[i]] where i is the current
iteration count.  (I think this is true in general, since [ and [[ act
the same for nonrecursive types when the indices are such that a scalar
would be returned.  However that explanation is unnecessarily
complicated
in the nonrecursive case.)

Also, the variable var is not really read-only.  You can alter it but
it gets reset to the next value in seq at the start of each iteration.
You cannot affect the meaning of 'next' to force it to, e.g, omit or
repeat iterations.

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com