Calculate difference between values in consecutive rows by group

Question

This is a my df (data.frame):

group value
1     10
1     20
1     25
2     5
2     10
2     15 

I need to calculate difference

Answer 1

The package data.table can do this fairly quickly, using the shift function.

require(data.table)
df <- data.table(group = rep(c(1, 2), each = 3), value = c(10,20,25,5,10,15))
#setDT(df) #if df is already a data frame

df[ , diff := value - shift(value), by = group]    
#   group value diff
#1:     1    10   NA
#2:     1    20   10
#3:     1    25    5
#4:     2     5   NA
#5:     2    10    5
#6:     2    15    5
setDF(df) #if you want to convert back to old data.frame syntax

---

Or using the lag function in dplyr

df %>%
    group_by(group) %>%
    mutate(Diff = value - lag(value))
#   group value  Diff
#     
# 1     1    10    NA
# 2     1    20    10
# 3     1    25     5
# 4     2     5    NA
# 5     2    10     5
# 6     2    15     5

---

For alternatives pre-data.table::shift and pre-dplyr::lag, see edits.