[R] modifynig some elements of a vector

[William Dunlap]

Try using tabulate() instead of table().  E.g.,
compare your original
  f0 <- function (x, u) {
      tu <- table(u)
      indices <- as.numeric(names(tu))
      x[indices] <- x[indices] + tu
      x
  }
to
  f1 <- function (x, u) {
      x + tabulate(u, nbins = length(x))
  }

I tried it for a 20-long x and 30-long u.  f0 and f1
gave identical results.  10^5 iterations of f0 took
62 seconds, f1 1.72 seconds.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com  

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Eric Elguero
> Sent: Thursday, February 10, 2011 3:50 AM
> To: r-help at r-project.org
> Subject: [R] modifynig some elements of a vector
> 
> He everybody,
> 
> I want to add 1 to some elements of  a vector:
> 
> x is a vector
> u is a vector of idices, that is, integers
> assumed to be within the range 1..length(x)
> and I want to add 1 to the elements of x
> each time their index appears in u
> 
> x[u]<-x[u]+1 works only when there are no
> duplicated values in u
> 
> I found this solution:
> 
> tu <- table(u)
> indices <- as.numeric(names(tu))
> x[indices] <- x[indices]+tu
> 
> but it looks ugly to me and I would
> prefer to avoid calling the function 'table'
> since this is to be done millions of times
> as part of a simulation program.
> 
> Eric Elguero
> Génétique & Adaptation des Plasmodium
> IRD
> Montpellier - FRance
> 
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