dplyr lag function returns NAs

问题

Does anybody have an explanation for such result using dplyr package?

I have a data.frame df

    library(dplyr)
    df = data_frame(
      'id' = c(1,2,2,2,2,3,3,3,3),
      'start' = c(881, 1611, 1611, 1642, 1764, 0, 0, 28, 59),
      'end' = c(1089, 1819, 1819, 1850, 1972, 208,  208,236, 267))

That looks like

    # Source: local data frame [9 x 3]
    #
    # id start   end
    # (dbl) (dbl) (dbl)
    # 1     1   881  1089
    # 2     2  1611  1819
    # 3     2  1611  1819
    # 4     2  1642  1850
    # 5     2  1764  1972
    # 6     3     0   208
    # 7     3     0   208
    # 8     3    28   236
    # 9     3    59   267

After grouping by id and applying a lag in end column, I was expecting to have one missing for each id.

    df %>% 
      group_by(id) %>%
      mutate(end.prev = lag(end))

But I have

    # Source: local data frame [9 x 4]
    # Groups: id [3]
    # 
    # id start   end end.prev
    # (dbl) (dbl) (dbl)    (dbl)
    # 1     1   881  1089       NA
    # 2     2  1611  1819       NA
    # 3     2  1611  1819     1819
    # 4     2  1642  1850     1819
    # 5     2  1764  1972     1850
    # 6     3     0   208       NA
    # 7     3     0   208       NA  <- I don't understant this NA
    # 8     3    28   236       NA  <- Neither this one
    # 9     3    59   267       NA  <- nor this other

I am using the last version available in cran dplyr 0.4.3 (my R version is 3.2.5)

来源：https://stackoverflow.com/questions/37706633/dplyr-lag-function-returns-nas