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

try this with tapply

df$diff<-as.vector(unlist(tapply(df$value,df$group,FUN=function(x){ return (c(NA,diff(x)))})))