r - 将 dplyr 转换为 data.table

Question

所以我正在尝试翻译一些 dplyr 代码。我尝试从将 dplyr 转换为 data.table 的包中获取帮助，但它仍然不起作用。错误来自 dplyr 的 row_number..

我需要 dplyr 代码中的所有步骤(尽管它们在 mtcars 中没有意义)

library(dplyr)
library(dtplyr) # from https://github.com/tidyverse/dtplyr
library(data.table)

mtcars %>% 
  distinct(mpg, .keep_all = TRUE) %>% 
  group_by(am) %>% 
  arrange(mpg, .by_group = TRUE) %>% 
  mutate(row_num = LETTERS[row_number()]) %>% 
  ungroup() 

# using dtplyr
dt <- lazy_dt(mtcars)

dt %>% 
  distinct(mpg, .keep_all = TRUE) %>% 
  group_by(am) %>% 
  arrange(mpg, .by_group = TRUE) %>% 
  mutate(row_num = LETTERS[row_number()]) %>% 
  ungroup() %>% 
  show_query()
#> unique(`_DT1`, by = "mpg")[order(am, mpg)][, `:=`(row_num = c("A", 
#> "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", 
#> "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z")[row_number()]), 
#>     keyby = .(am)]

# I then use the query from dtplyr 
DT <- as.data.table(mtcars)
unique(DT, by = "mpg")[order(am, mpg)][, `:=`(row_num = c("A", 
                                                              "B", "C", "D", "E", "F", "G", 
                                                              "H", "I", "J", "K", "L", "M", 
                                                              "N", "O", "P", "Q", "R", "S", 
                                                              "T", "U", "V", "W", "X", "Y", 
                                                              "Z")[row_number()]), keyby = .(am)]

#> row_number() should only be called in a data context

^{由reprex package于2019年7月14日创建(v0.3.0)}

Answer 1

我可以推荐 rowid 函数吗？它在“底层”执行分组步骤，您可能会发现它看起来更干净:

unique(DT, by='mpg')[order(am, mpg), row_num := LETTERS[rowid(am)]]

如果你喜欢链接，你也可以得到[]中的所有内容:

DT[ , .SD[1L], by = mpg
   ][order(am, mpg), row_num := LETTERS[rowid(am)]]