On Thu, Feb 10, 2011 at 12:50:03PM +0100, Eric Elguero wrote: > 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.
If we start with a zero vector x, then the operation, which you ask for, is equivalent to computing the frequency table of u. So, i think, using function table() may be adequate. However, let me suggest a slightly different code n <- 8 x <- rep(0, times=8) u <- c(1, 2, 5, 2, 5, 1, 2, 8, 2, 1) x <- x + unclass(table(factor(u, levels=1:n))) x 1 2 3 4 5 6 7 8 3 4 0 0 2 0 0 1 Another solution may be used, if the vectors u all have the same length (not only the same range of values). Then it is possible to compute first the frequency table for each component of u separately in different rows of a matrix x m <- 10 x <- matrix(0, nrow=m, ncol=n) x[cbind(1:m, u)] <- x[cbind(1:m, u)] + 1 This may be repeated for different vectors u and when all repetitions are finished, then the final result may be obtained as colSums(x) [1] 3 4 0 0 2 0 0 1 Hope this helps. Petr Savicky. ______________________________________________ R-help@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.
