r cumsum differentiate reset at >0

Question

I am quite new to R and tried some formulas form the forum but somehow I could not modify them to meet my condition. I want to construct a cumsum which starts new when the variable in column E is >=0:

NR E Proability Cumsum    
1  -.2 .2 .2   
2  -.15 .2 .4     
3 -.1   .2 .6    
4 .1  .2 .2   
5 .2  .2 .4

The last column should be the result.

Does anyone have an idea?

Thank you very much :)

Answer 1

There's a very useful function called rleid in the data.table package which you can make use of for the grouping. I'll use it in combination with some base functions:

library(data.table)
df <- transform(df, cumsum2 = ave(Proability, rleid(E > 0), FUN = cumsum))

Or, if you want to fully use data.table functions, it would be

setDT(df)
df[, cumsum2 := cumsum(Proability), by = rleid(E > 0)]
setDF(df) # optional