Here is a simple tidyverse solution...
library(tidyverse) #or just dplyr and tidyrset.seed(100)x <- round(rnorm(20, sd = 0.02), 3)df <- tibble(x = x) %>% mutate(seqno = cumsum(c(1, diff(sign(x)) != 0))) %>% #identify sequence ids group_by(seqno) %>% #group by sequences mutate(n_of_sequence = row_number(), #count row numbers for each group sum = cumsum(x)) %>% #cumulative sum for each group ungroup() %>% select(-seqno) #remove sequence iddf# A tibble: 20 x 3 x n_of_sequence sum<dbl> <int> <dbl> 1 -0.01 1 -0.01 2 0.003 1 0.003 3 -0.002 1 -0.002 4 0.018 1 0.018 5 0.002 2 0.0200 6 0.006 3 0.026 7 -0.012 1 -0.012 8 0.014 1 0.014 9 -0.017 1 -0.017 10 -0.007 2 -0.024 11 0.002 1 0.002 12 0.002 2 0.004 13 -0.004 1 -0.004 14 0.015 1 0.015 15 0.002 2 0.017 16 -0.001 1 -0.001 17 -0.008 2 -0.009 18 0.01 1 0.01 19 -0.018 1 -0.018 20 0.046 1 0.046