[R] running sum of a vector
Benilton Carvalho
bcarvalh at jhsph.edu
Thu Nov 8 00:08:56 CET 2007
x <- 1:10
cumsum(x)
b
On Nov 7, 2007, at 5:59 PM, Alexy Khrabrov wrote:
> I need a vector with sums of vectors up to each position in the
> original. The imperative version is simple:
>
> # running sum: the traditional imperative way
> sumr.1 <- function(x) {
> s <- c()
> ss <- 0
> for (i in 1:length(x)) {
> ss <- ss + x[i]
> s[i] <- ss
> }
> s
> }
>
> Yet I want a functional way, which is shorter:
>
> # running sum: functional way, but inefficient one!
> sumr.2 <- function(x) {
> sapply(1:length(x), function(i) sum(x[1:i]))
> }
>
> -- the problem with the latter is, we need to create indices to run
> over them, and the sum is recomputed anew for each position, while
> the imperative version iterates without recomputing. Is there a
> better functional solution?
>
> Cheers,
> Alexy
>
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