Sum every nth points

Question

I have a vector and I need to sum every n numbers and return the results. This is the way I plan on doing it currently. Any better way to do this?

Answer 1

One way is to use rollapply from zoo:

rollapply(v, width=n, FUN=sum, by=n)
# [1]  55 155 255 355 455 555 655 755 855 955

And in case length(v) is not a multiple of n:

v <- 1:92

rollapply(v, width=n, FUN=sum, by=n, partial=T, align="left")
# [1]  55 155 255 355 455 555 655 755 855 183

Answer 2

v <- 1:100

n <- 10

cutpoints <- seq( 1 , length( v ) , by = n )

categories <- findInterval( 1:length( v ) , cutpoints )

tapply( v , categories , sum )

Answer 3

A little late to the party, but I don't see a rowsum() answer yet. rowsum() is proven more efficient than tapply() and I think it would also be very efficient relative to a few of the other responses as well.

rowsum(v, rep(seq_len(length(v)/n), each=n))[,1]
#  1   2   3   4   5   6   7   8   9  10 
# 55 155 255 355 455 555 655 755 855 955

Using @Josh O'Brien's grouping technique would likely improve efficiency even more.

rowsum(v, (seq_along(v)-1) %/% n)[,1]
#  0   1   2   3   4   5   6   7   8   9 
# 55 155 255 355 455 555 655 755 855 955 

Simply wrap in unname() to drop the group names.