cumsum() from last entry and moving backward

user_1867328 0
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I want to calculate cumulative sum of a vector of numbers from the last value and moving backward. the standard cumsum() function in R generally start from the first value and move forward as in simple example -

cumsum(1:5)
## 1  3  6 10 15

But I want a result like -

15, 14, 12, 9, 5

Is there any R function to directly calculate that?

cumsum r
3 Answers

the solution you are looking for is:

rev(cumsum(rev(x)))
user_78471160

You can do:

Reduce(sum, 1:5, right = TRUE, accumulate = TRUE)

[1] 15 14 12  9  5
user_59645570

Another base R option

> v <- 1:5

> sum(v) - cumsum(c(0, head(v, -1)))
[1] 15 14 12  9  5
user_121587570
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