Calculate first difference by group in R

Question

I was wondering if someone could help me calculate the first difference of a score by group. I know it should be a simple process but for some reason I\'m having trouble doi

Answer 1

This is one way using base R

df$diff <- unlist(by(df$score , list(df$group) , function(i) c(NA,diff(i))))

or

df$diff <- ave(df$score , df$group , FUN=function(i) c(NA,diff(i)))

or using data.table - this will be more efficient for larger data.frames

library(data.table)
dt <- data.table(df)
setkey(dt,group)
dt[,diff:=c(NA,diff(score)),by=group]