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For readability I like:

b <- c(0,a[-length(a)])
which(a != b & a == 0)

[1]  4 12 18

which(a != b & a == 1)

[1]  1  6 16 23


On 07 Aug 2014, at 17:23, William Dunlap <wdunlap at tibco.com> wrote:

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
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