Cumulative sums over run lengths. Can this loop be vectorized?

Question

I have a data frame on which I calculate a run length encoding for a specific column. The values of the column, dir, are either -1, 0, or 1.

dir.r

Answer 1

Add a 'group' column to the data frame. Something like:

df=data.frame(z=rnorm(100)) # dummy data
df$dir = sign(df$z) # dummy +/- 1
rl = rle(df$dir)
df$group = rep(1:length(rl$lengths),times=rl$lengths)

then use tapply to sum within groups:

tapply(df$z,df$group,sum)