code_01.R

# Data
dat = data.frame(
    id = c(1, 1, 1, 2, 2, 2),
    date = c("2020-01-01", "2020-03-01", "2020-04-30", "2020-02-02", "2020-02-02", "2020-04-05"),
    num = c(10, 20, 30, 10, 20, 30),
    interactions = c(2, 6, 7, 3, 6, 8)
)
dat

##   id       date num interactions
## 1  1 2020-01-01  10            2
## 2  1 2020-03-01  20            6
## 3  1 2020-04-30  30            7
## 4  2 2020-02-02  10            3
## 5  2 2020-02-02  20            6
## 6  2 2020-04-05  30            8

###########################

# Original observations
grid = dat[, c("id", "date", "num")]

# Aggregate
dat = aggregate(dat["interactions"], dat[c("id", "date")], sum)

# Order
dat = dat[order(dat$id, dat$date), ]

# Split by 'id'
dat = split(dat, dat$id)

# Cumulative sum excluding self
f = function(x) {x$interactions = cumsum(x$interactions) - x$interactions; x}
dat = lapply(dat, f)

# Combine
dat = do.call(rbind, dat)

# Join with original observations
dat = merge(grid, dat, by = c("id", "date"), all.x = TRUE)

# Final result
dat

##   id       date num interactions
## 1  1 2020-01-01  10            0
## 2  1 2020-03-01  20            2
## 3  1 2020-04-30  30            8
## 4  2 2020-02-02  10            0
## 5  2 2020-02-02  20            0
## 6  2 2020-04-05  30            9