R help - Nov 2007 - running sum of a vector

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

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
> 
> ______________________________________________
> R-help at 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. 
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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
>
> ______________________________________________
> R-help at 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.