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

Mike Jones MikeJones at westat.com
Fri Dec 28 00:08:26 CET 2007

```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]]
>
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>

--
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

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