R apply function to data based on index column value

Question

Example:

require(data.table)
example = matrix(c(rnorm(15, 5, 1), rep(1:3, each=5)), ncol = 2, nrow = 15)
example = data.table(example)
setnames(example, old=c("V1","V2"), new=c("target", "index"))
example


threshold = 100

accumulating_cost = function(x,y) { x-cumsum(y) }
whats_left = accumulating_cost(threshold, example$target)
whats_left

I want whats_left to consist of the difference between threshold and the cumulative sum of values in example$target for which example$index = 1, and 2, and 3. So I used the following for loop:

rm(whats_left)

whats_left = vector("list")
for(i in 1:max(example$index)) {
  whats_left[[i]] = accumulating_cost(threshold, example$target[example$index==i])
}

whats_left = unlist(whats_left)
whats_left

plot(whats_left~c(1:15))

I know for loops aren't the devil in R, but I'm habituating myself to use vectorization when possible (including getting away from apply, being a for loop wrapper). I'm pretty sure it's possible here, but I can't figure out how to do it. Any help would be much appreciated.

If you already using data.table you might as well use it most basic capabilities, more specifically, the by argument. Try example[, accumulating_cost(threshold, target), by = index] — David Arenburg
– David Arenburg, Commented Mar 11, 2015 at 21:17
That's it! These basic capabilities are exactly what I'm struggling to learn. Thanks! Care to make this the answer? — Nigel Stackhouse
– Nigel Stackhouse, Commented Mar 11, 2015 at 21:19
@NigelStackhouse, check the new HTML vignettes. Keep track of this post for the vignettes that are planned. — Arun
– Arun, Commented Mar 11, 2015 at 21:24

David Arenburg · Accepted Answer · 2015-03-11 21:21:33Z

3

All you trying to do is accumulate the cost by index. Thus, you might want to use the by argument as in

example[, accumulating_cost(threshold, target), by = index]

answered Mar 11, 2015 at 21:21

David Arenburg

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