问data.table R中的滞后

EN

panel_lag <- function(var, k) {
  if (k > 0) {
    # Bring past values forward k times
    return(c(rep(NA, k), head(var, -k)))
  } else {
    # Bring future values backward
    return(c(tail(var, k), rep(NA, -k)))
  }
}

x = data.table(a=1:10, 
               dte=sample(seq.Date(from=as.Date("2012-01-20"),
                                   to=as.Date("2012-01-30"), by=1),
                          10))
x[, L1_a:=panel_lag(a, 1)]  # This won't work correctly as `x` isn't keyed by date
setkey(x, dte)
x[, L1_a:=panel_lag(a, 1)]  # This will