subtract value from previous row by group

Question

In R, let's say I have this data frame:

Data id      date        value 2380    10/30/12    21.01 2380    10/31/12    22.04 2380    11/1/12     22.65 2380    11/2/12     23.11 20100   10/30/12    35.21 20100   10/31/12    37.07 20100   11/1/12     38.17 20100   11/2/12     38.97 20103   10/30/12    57.98 20103   10/31/12    60.83  

And I want to subtract the previous value from the current value, by group ID date, to create this:

id      date        value   diff 2380    10/30/12    21.01   0 2380    10/31/12    22.04   1.03 2380    11/1/12     22.65   0.61 2380    11/2/12     23.11   0.46 20100   10/30/12    35.21   0 20100   10/31/12    37.07   1.86 20100   11/1/12     38.17   1.1 20100   11/2/12     38.97   0.8 20103   10/30/12    57.98   0 20103   10/31/12    60.83   2.85 

Answer 1

With dplyr:

library(dplyr)  data %>%     group_by(id) %>%     arrange(date) %>%     mutate(diff = value - lag(value, default = first(value))) 

For clarity you can arrange by date and grouping column (as per comment by lawyer)

data %>%     group_by(id) %>%     arrange(date, .by_group = TRUE) %>%     mutate(diff = value - lag(value, default = first(value))) 

or lag with order_by:

data %>%     group_by(id) %>%     mutate(diff = value - lag(value, default = first(value), order_by = date)) 

With data.table:

library(data.table)  dt <- as.data.table(data) setkey(dt, id, date) dt[, diff := value - shift(value, fill = first(value)), by = id]