Retain and lag function in R as SAS

Question

I am looking for a function in R similar to lag1, lag2 and retain functions in SAS which I can use with data.tables.

I know th

Answer 1

For retain, try this :

retain<-function(x,event,outside=NA)
{
  indices <- c(1,which(event==TRUE), nrow(df)+1)
  values <- c(outside,x[event==TRUE])
  y<- rep(values, diff(indices)) 
}

With data : I want to retain down the value when w==b

df <- data.frame(w = c("a","b","c","a","b","c"), x = 1:6, y = c(1,1,2,2,2,3), stringsAsFactors = FALSE)
df$z<-retain(df$x-df$y,df$w=="b")
df

And here's the contrary obtain, that does not exist in SAS:

obtain<-function(x,event,outside=NA)
{
  indices <- c(0,which(event==TRUE), nrow(df))
  values <- c(x[event==TRUE],outside)
  y<- rep(values, diff(indices)) 
}

Here's an example. I want to obtain the value in advance where w==b

df$z2<-obtain(df$x-df$y,df$w=="b")
df

Thanks to Julien for helping.