[R] Count matches of a sequence in a vector?

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Jeff Brown
> Sent: Wednesday, April 21, 2010 8:08 AM
> To: r-help at r-project.org
> Subject: Re: [R] Count matches of a sequence in a vector?
> 
> 
> This sort of calculation can't be vectorized; you'll have to 
> iterate through
> the sequence, e.g. with a "for" loop.  I don't know if a 
> routine has already
> been written.

It can be partially vectorized:
 f2 <- function (v, p) {
    retval <- TRUE
    i <- seq_len(length(v) - length(p) + 1L) - 1L
    for (j in seq_along(p)) {
        retval <- retval & v[i + j] == p[j]
    }
    retval
 }
E.g., for the following data
 set.seed(1)
 v <- sample(1:10, size=1e6, replace=TRUE)
 p <- 2:4
compare using zoo::rollapply (which loops over the long v)
 f1 <- function(v, p)rollapply(zoo(v), length(p), function(x)all(x==p))
and f2 (which loops over the short p).  I get

 > library(zoo)
 > system.time(r1 <- f1(v,p))
    user  system elapsed 
   13.17    0.06   13.25 
 > system.time(r2 <- f2(v,p))
    user  system elapsed 
    0.12    0.00    0.12 
 > identical(which(r1), which(r2))
 [1] TRUE

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> -- 
> View this message in context: 
> http://n4.nabble.com/Count-matches-of-a-sequence-in-a-vector-t
p2019018p2019108.html
> Sent from the R help mailing list archive at Nabble.com.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>