How to calculate consecutive sum in R?

Question

If for example I have a vector of numbers.

c(1:9)

Now, if I want to compute sum of three consecutive number in the following way:

(1+2+3,4+5+6,7+8+9)

how should I approach?

Probably better to use `colSums(matrix(1:9,nrow=3))` instead of `rowSums(matrix(1:9,nrow=3, byrow=TRUE))`. — shadow, Feb 11 '14 at 15:20
I like games :) `sapply(0:2, function(x) sum(c(1:9)[(3*x+1):(3*x+3)]))` gives: `6 15 24` — rawr, Feb 11 '14 at 15:21
None of these have conditional statements. @BenBolker you needed `colSums` instead — rawr, Feb 11 '14 at 15:28
Here are a cpuple more: `tapply(1:9, (seq_along(1:9)-1) %/% 3 , sum)` and `library(zoo); rollapply(1:9, 3, sum, by = 3)` — G. Grothendieck, Feb 11 '14 at 15:31
@BenBolker no, that was a response to OP who subsequently deleted his comment, ha. — rawr, Feb 11 '14 at 16:30
someone could put this up on http://codegolf.stackexchange.com/ ... — Ben Bolker, Feb 11 '14 at 16:41
My 2c: 3*((1:9)[1:9%%3==0]-1) or 3*(seq(1,9,by=3)+1)- just cause 3 is a lovely number — user20650, Feb 12 '14 at 00:36

Carl Witthoft · Answer 1 · 2014-02-11T18:03:51.360

I learned to use this one correctly:

foo<-1:9
aggregate(foo,by=list(rep(1:3,each=3) ),FUN=sum)

For a longer input, do this:

function (x,len) aggregate(x,by=list(rep(1:(length(x)/len),each=len) ),FUN=sum)

And because I'm still waiting for what-if.xkcd.com to update, here's the timings: (and I did check that all calculate the right thing)

x<-1:99
len<-3
 microbenchmark(shadow(x,len),rawr(x,len),groth(x,len),carl(x,len),times=10)
Unit: microseconds
           expr      min       lq    median       uq      max
 shadow(x, len)   62.006   70.130   75.0480   97.926  180.884
   rawr(x, len)  541.797  564.033  571.9445  585.842  639.295
  groth(x, len)  514.002  525.548  547.5705  551.633  568.310
   carl(x, len) 1045.536 1099.844 1120.5840 1166.553 1346.582
 neval
    10
    10
    10
    10

So the matrix -- colSums is fastest. I'll leave it to someone else to figure out the memory requirements of each approach.

How to calculate consecutive sum in R?

1 Answers1