# [R] ask for help

William Dunlap wdunlap at tibco.com
Thu Aug 7 17:23:29 CEST 2014

```My solution may be a bit clearer if you define the function isFirstInRun
isFirstInRun <- function(x) {
if (length(x) == 0) {
logical(0)
} else {
c(TRUE, x[-1] != x[-length(x)])
}
}

Then that solution is equivalent to
which(isFirstInRun(a) & a==1)

If 'a' contains NA's then you have to decide how to deal with them.

(The call to 'which' is not needed if you are going to be using the
result as a subscript.)

You may also want isLastInRun
isLastInRun <- function(x) {
if (length(x) == 0) {
logical(0)
} else {
c(x[-1] != x[-length(x)], TRUE)
}
}
Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Thu, Aug 7, 2014 at 7:36 AM, William Dunlap <wdunlap at tibco.com> wrote:
>> a<-c(1,1,1,0,0,1,1,1,1,1,1,0,0,0,0,1,1,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1)
>> which( a==1 & c(TRUE, a[-length(a)]!=1) )
> [1]  1  6 16 23
>> which( a==0 & c(TRUE, a[-length(a)]!=0) )
> [1]  4 12 18
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
>
> On Wed, Aug 6, 2014 at 7:12 PM, Johnnycz <johnnycz at yeah.net> wrote:
>> Hello,everybody,
>>          I have a sequence,like a<-c(1,1,1,0,0,1,1,1,1,1,1,0,0,0,0,1,1,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1),how to get the position of each first 1 and 0, that's to say, how to get b<-c(1,6,16,23) for first 1 and d<-c(4,12,18) for first 0.
>>         Many thanks!
>> Johnny
>>         [[alternative HTML version deleted]]
>>
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```