[R] Conditionally incrementing a loop counter: Take 2

[John Fox]

Dear Mike,

You could use a repeat loop and manage the index yourself:

i <- 0
repeat{
	x <- runif(1)
	if (x < .1){
	   i <- i + 1
	   cat("x = ", x, "\n")
	   }
    if (i == 10) break
    }

But if your example problem reflects your actual application, why not just
generate uniform random numbers on the interval (0, .1)?

I hope this helps,
 John

--------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Mike Jones
> Sent: December-27-07 6:08 PM
> To: Peter Dalgaard
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] Conditionally incrementing a loop counter: Take 2
> 
> Since I didn't want the i to increment in the loop when the condition
> is not met, then in my example I wanted the loop to actually run 14
> times instead of the 10 since I wanted 4 of the iterations to be thrown
> away, or ignored.  I still haven't been able to figure this out.  Going
> the "while" route doesn't seem to work for me either.
> 
> 
> nums <- numeric(10)
> i <- 1
> garbage <- 0
> 
> while (i <= 10){
> 	x <- runif(1)
> 	cat("x = ",x,"\n")
> 	if (x < 0.1){
> 		nums[i] <- x
> 		i <- i + 1
> 	}
> 	else{
> 	        garbage <- garbage+1
> 	}
> cat("i = ",i,"garbage = ",garbage,"\n")
> }
> 
> -----Original Message-----
> From: Peter Dalgaard [mailto:p.dalgaard at biostat.ku.dk]
> Sent: Thursday, December 27, 2007 5:36 PM
> To: Mike Jones
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] Conditionally incrementing a loop counter: Take 2
> 
> 
> Mike Jones wrote:
> > My apologies for not including a working example.
> >
> > Here it is:
> >
> > for (i in 1:10){
> >    cat("initial i = ",i,"\n")
> >    x <- runif(1)
> >    if (x > 0.7){
> >       i <- i-1
> >    }
> >    cat("second i = ",i,"\n")
> > }
> >
> > When I ran this i got what follows, so there were four cases where I
> > wanted the i not to increment.
> >
> > initial i =  1
> > second i =  1
> > initial i =  2
> > second i =  1
> > initial i =  3
> > second i =  3
> > initial i =  4
> > second i =  3
> > initial i =  5
> > second i =  4
> > initial i =  6
> > second i =  6
> > initial i =  7
> > second i =  7
> > initial i =  8
> > second i =  7
> > initial i =  9
> > second i =  9
> > initial i =  10
> > second i =  10
> >
> >
> Is this the kind of effect you want?
> 
>  > x <- runif(10)
>  > cbind(x, 1:10, cumsum(x < .7))
>                 x
>  [1,] 0.384165631  1 1
>  [2,] 0.392715845  2 2
>  [3,] 0.895936431  3 2
>  [4,] 0.910242185  4 2
>  [5,] 0.689987301  5 3
>  [6,] 0.237071326  6 4
>  [7,] 0.225032680  7 5
>  [8,] 0.001856286  8 6
>  [9,] 0.392034868  9 7
> [10,] 0.655076045 10 8
> 
> If you insist on using a loop, you need to separate the loop control
> from the manipulation of i, as in (e.g.)
> 
> i <- 0
> for (j in 1:10){
>    i <- i + 1
>    cat("initial i = ",i,"\n")
>    x <- runif(1)
>    if (x > 0.7){
>       i <- i-1
>    }
>    cat("second i = ",i,"\n")
> }
> 
> 
> >>  -----Original Message-----
> >> From: 	Mike Jones
> >> Sent:	Thursday, December 27, 2007 4:35 PM
> >> To:	'r-help at lists.R-project.org'
> >> Subject:	Conditionally incrementing a loop counter
> >>
> >> Hi,
> >> I am trying a for loop from 1 to 10 by 1. However, if a condition
> >> does not get met, I want to "throw away" that iteration. So if my
> >> loop is for (i in 1:10) and i is say, 4 and the condition is not met
> >> then I don't want i to go up to 5.  Is there a way to do that? I
> >> can't seem to manually adjust i because from what I understand, R
> >> creates 10 long vector and uses that to "loops thru" and I'm not
> sure
> >> how to get at the index of that vector. Any suggestions? Thanks in
> >> advance.
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> Mike Jones
> >> Westat
> >> 1650 Research Blvd. RE401
> >> Rockville, MD 20850
> >> Ph: 240.314.2312
> >>
> >>
> >
> > 	[[alternative HTML version deleted]]
> >
> > ______________________________________________
> > 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.
> >
> 
> 
> --
>    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45)
> 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45)
> 35327907
> 
> ______________________________________________
> 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.