iterative cumsum where sum determines the next position to be added

Question

I have a data.table as follows

set.seed(5)
x <- data.table(x=sample(1:20,15))

> x
     x
 1:  5
 2: 14
 3: 17
 4: 20
 5:  2
 6: 11
 7:  8
 8: 15
 9: 12
10         


        

Answer 1

We can use Reduce with accumulate = TRUE

accum <- Reduce(function(i, j) i + x$x[i], x$x, accumulate = TRUE)
c(1, accum[!is.na(accum)])
# [1]  1  5  7 15 28

or purrr::accumulate

library(purrr)

accum <- accumulate(x$x, ~ .x + x$x[.x])
c(1, accum[!is.na(accum)])
# [1]  1  5  7 15 28

Answer 2

Here's a function that takes how long you want your vector to be and produces a vector of that length:

recursiveadd<-function(x, n) {k<-x$x[1]
 for (i in 1:(n-1)) {
     k[i+1]<-sum(x$x[k[i]],k[i])
   }
 k
}

recursiveadd(x,4)
[1]  5  7 15 28

Answer 3

A base R solution:

i = 1
v = i
sum = 0
while (i <= nrow(x)) {
   v = c(v, i)
   sum = sum + x$x[i]
   i = sum
}