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rcumsum

Reversing cumulative sum to determine actuals in R

I have a df where each row is the cumulative sum of the row above it. Is there a way to derive the original values from this df?

   X1 X2
1  1  5
2  3  9
3  6 12
4 10 14
5 15 15

Desired output:

   X1 X2
1  1  5
2  2  4
3  3  3
4  4  2
5  5  1

Thanks

Solution

  • Just use diff. Assuming your dataset is called "mydf" and you want to do this for all columns, try:

    mydf[] <- lapply(mydf, function(x) diff(c(0, x)))
mydf
#   X1 X2
# 1  1  5
# 2  2  4
# 3  3  3
# 4  4  2
# 5  5  1

    Since diff returns a vector of length one less than the input, you need to pad the input with a 0 (thus also retaining the original value in that column).

    As @DavidArenburg mentions, you could also easily adapt this to "data.table" code too, like this:

    library(data.table)
as.data.table(mydf)[, lapply(.SD, function(x) diff(c(0, x)))]