R subtract value for the same ID (from the first ID that shows)

Question

I have a similar question to the question found here: R, subtract value from previous row, group by (slight modification; see below):

In R, lets say I have this da

Answer 1

We can use functions from dplyr. dt is the original data. dt2 is the final output.

library(dplyr)

dt2 <- dt %>%
  group_by(id) %>%
  mutate(diff = value - first(value))

dt2
Source: local data frame [10 x 4]
Groups: id [3]

      id       date value  diff
           
1   2380 10/30/2012 21.01  0.00
2   2380 10/31/2012 22.04  1.03
3   2380  11/1/2012 22.65  1.64
4   2380  11/2/2012 23.11  2.10
5  20100 10/30/2012 35.21  0.00
6  20100 10/31/2012 37.07  1.86
7  20100  11/1/2012 38.17  2.96
8  20100  11/2/2012 38.97  3.76
9  20103 10/30/2012 57.98  0.00
10 20103 10/31/2012 60.83  2.85