[R] running sum of a vector

[Chuck Cleland]

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?

?cumsum

X <- runif(20)

sumr.1(X)
 [1] 0.6359909 0.9435293 1.2167988 1.6229179
 [5] 2.2816672 3.2687057 4.1973724 4.4421475
 [9] 4.5601287 4.7500524 5.0639924 5.5831643
[13] 6.5071247 6.9861566 7.0352500 7.6723079
[17] 7.8560394 7.9281423 8.4757938 8.9985340

sumr.2(X)
 [1] 0.6359909 0.9435293 1.2167988 1.6229179
 [5] 2.2816672 3.2687057 4.1973724 4.4421475
 [9] 4.5601287 4.7500524 5.0639924 5.5831643
[13] 6.5071247 6.9861566 7.0352500 7.6723079
[17] 7.8560394 7.9281423 8.4757938 8.9985340

cumsum(X)
 [1] 0.6359909 0.9435293 1.2167988 1.6229179
 [5] 2.2816672 3.2687057 4.1973724 4.4421475
 [9] 4.5601287 4.7500524 5.0639924 5.5831643
[13] 6.5071247 6.9861566 7.0352500 7.6723079
[17] 7.8560394 7.9281423 8.4757938 8.9985340

all(cumsum(X) == sumr.1(X))
[1] TRUE

> Cheers,
> Alexy
> 
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-- 
Chuck Cleland, Ph.D.
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