将总和应用于R中的向量块？

Question

I have a vector v containing the number of elements I need to add up from another vector w . How can I do this in R?

Example

v = c(4,0,3,3,0,0)
w = rexp(10,1)

So, I need a new vector containing c(w[1]+w[2]+w[3]+w[4],0,w[5]+w[6]+w[7],w[8]+w[9]+w[10],0,0) . How can I construct such vector? I thought of using apply but I cannot see how to do it. A for loop could also work, but there is a problem with the zeroes.

Answer 1

We can use rep on the sequence of 'v' to create a grouping variable, get the sum of 'w' with tapply using the grouping variable and append the 0's based on the position of '0' in 'v'

r1 <- unname(tapply(w, rep(seq_along(v), v), FUN = sum))
i1 <- which(v==0)-1
for(i in seq_along(i1)) r1 <- append(r1, 0, after = i1[i])
r1
#[1] 2.951259 0.000000 2.982644 4.233564 0.000000 0.000000