> -----Original Message-----
> From: r-help-boun...@r-project.org
> [mailto:r-help-boun...@r-project.org] On Behalf Of boshao zhang
> Sent: Monday, January 11, 2010 1:20 PM
> To: r-help@r-project.org
> Subject: [R] help needed to find zero areas in a vector
>
> Dear Helpers:
>
> I spend more than half a day to solve this problem in R:
>
> Let x be a vector of a string of 0s and 1s, such as
> x<-c(0,0,1,1,0,0,0,0,1,1,0,0,0,0). It can be a very long vector.
> How to sub vectors of 0s? In the above example, I would like
> get the vectors (0,0), (0,0,0,0), (0,0,0,0).
> I can use which(x==0) to get the index of the 0 elements, but
> I don't know how to get the starting indices and end indices
> of the subvectors. Maybe we should use which(x==1), but I
> don't know how.
You can use the following functions, which return TRUE
for elements which are last (or first) in a run of equal
values and FALSE for other elements:
lastInRun <- function(x)c(x[-1]!=x[-length(x)], TRUE)
firstInRun <- function(x)c(TRUE, x[-1]!=x[-length(x)])
They are fast for long vectors but don't handle NA's.
E.g.,
> x<-c(0,0,1,1,0,0,0,0,1,1,0,0,0,0)
> data.frame(x, first=firstInRun(x), last=lastInRun(x))
x first last
1 0 TRUE FALSE
2 0 FALSE TRUE
3 1 TRUE FALSE
4 1 FALSE TRUE
5 0 TRUE FALSE
...
14 0 FALSE TRUE
If you are only interested only in runs of 0's use
firstInRun(x) & x==0
to flag elements of x that start a run of zeros.
For many things the logical vector output will suffice
but you can use which(logical) to convert it to the
equivalent integer indices.
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
>
> Thanks.
>
> Bob
>
>
>
> [[alternative HTML version deleted]]
>
>
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