Bart Kastermans kasterma at kasterma.net
Thu Aug 7 17:28:33 CEST 2014

```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
>>>        [[alternative HTML version deleted]]
>>>
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