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

You can use the base function ave() for this

df <- data.frame(group=rep(c(1,2),each=3),value=c(10,20,25,5,10,15))
df$diff <- ave(df$value, factor(df$group), FUN=function(x) c(NA,diff(x)))

which returns

  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