It is surely an elegant way of doing things (although far from being easy to parse visually) but is it really faster than a loop?
After all, the indexing problem is the same and sapply simply does the same job as for in this case, plus "<<-" will _search_ through the environment on every single step. Where is the gain? Oleg -- Dr Oleg Sklyar | EBI-EMBL, Cambridge CB10 1SD, UK | +44-1223-494466 Byron Ellis wrote: > Actually, why not use a closure to store previous value(s)? > > In the simple case, which depends on x_i and y_{i-1} > > gen.iter = function(x) { > y = NA > function(i) { > y <<- if(is.na(y)) x[i] else y+x[i] > } > } > > y = sapply(1:10,gen.iter(x)) > > Obviously you can modify the function for the bookkeeping required to > manage whatever lag you need. I use this sometimes when I'm > implementing MCMC samplers of various kinds. > > > On 1/30/07, Herve Pages <[EMAIL PROTECTED]> wrote: >> Tom McCallum wrote: >>> Hi Everyone, >>> >>> I have a question about for loops. If you have something like: >>> >>> f <- function(x) { >>> y <- rep(NA,10); >>> for( i in 1:10 ) { >>> if ( i > 3 ) { >>> if ( is.na(y[i-3]) == FALSE ) { >>> # some calculation F which depends on one or >>> more of the previously >>> generated values in the series >>> y[i] = y[i-1]+x[i]; >>> } else { >>> y[i] <- x[i]; >>> } >>> } >>> } >>> y >>> } >>> >>> e.g. >>> >>>> f(c(1,2,3,4,5,6,7,8,9,10,11,12)); >>> [1] NA NA NA 4 5 6 13 21 30 40 >>> >>> is there a faster way to process this than with a 'for' loop? I have >>> looked at lapply as well but I have read that lapply is no faster than a >>> for loop and for my particular application it is easier to use a for loop. >>> Also I have seen 'rle' which I think may help me but am not sure as I have >>> only just come across it, any ideas? >> Hi Tom, >> >> In the general case, you need a loop in order to propagate calculations >> and their results across a vector. >> >> In _your_ particular case however, it seems that all you are doing is a >> cumulative sum on x (at least this is what's happening for i >= 6). >> So you could do: >> >> f2 <- function(x) >> { >> offset <- 3 >> start_propagate_at <- 6 >> y_length <- 10 >> init_range <- (offset+1):start_propagate_at >> y <- rep(NA, offset) >> y[init_range] <- x[init_range] >> y[start_propagate_at:y_length] <- cumsum(x[start_propagate_at:y_length]) >> y >> } >> >> and it will return the same thing as your function 'f' (at least when 'x' >> doesn't >> contain NAs) but it's not faster :-/ >> >> IMO, using sapply for propagating calculations across a vector is not >> appropriate >> because: >> >> (1) It requires special care. For example, this: >> >> > x <- 1:10 >> > sapply(2:length(x), function(i) {x[i] <- x[i-1]+x[i]}) >> >> doesn't work because the 'x' symbol on the left side of the <- in the >> anonymous function doesn't refer to the 'x' symbol defined in the >> global >> environment. So you need to use tricks like this: >> >> > sapply(2:length(x), >> function(i) {x[i] <- x[i-1]+x[i]; assign("x", x, >> envir=.GlobalEnv); x[i]}) >> >> (2) Because of this kind of tricks, then it is _very_ slow (about 10 times >> slower or more than a 'for' loop). >> >> Cheers, >> H. >> >> >>> Many thanks >>> >>> Tom >>> >>> >>> >> ______________________________________________ >> R-devel@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-devel >> > > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel