# Re: [R] Conditionally incrementing a loop counter: Take 2

From: John Fox <jfox_at_mcmaster.ca>
Date: Thu, 27 Dec 2007 18:36:14 -0500

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
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
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> 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
> > http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
>
>
> --
> O__ ---- Peter Dalgaard ุster 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
>
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