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library(data.table)
library(dtplyr)
library(dplyr, warn.conflicts = FALSE)

12.1 dtplyr 语法

DTPLYR 使用 DPLYR 的语法s实现 data.table 的速度；编写 DPLYR（和 TidyR）代码，DTPLYR 将其转换为等效的 Data.Table。

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mtcars2 <- lazy_dt(mtcars)
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dtplyr <- mtcars2 %>% 
  filter(wt < 5) %>% 
  mutate(l100k = 235.21 / mpg) %>% # liters / 100 km
  group_by(cyl) %>% 
  summarise(l100k = mean(l100k))
dtplyr
#> Source: local data table [3 x 2]
#> Call:   `_DT1`[wt < 5][, `:=`(l100k = 235.21/mpg)][, .(l100k = mean(l100k)), 
#>     keyby = .(cyl)]
#> 
#>     cyl l100k
#>   <dbl> <dbl>
#> 1     4  9.05
#> 2     6 12.0 
#> 3     8 14.9 
#> 
#> # Use as.data.table()/as.data.frame()/as_tibble() to access results

dtplyr %>% show_query()
#> `_DT1`[wt < 5][, `:=`(l100k = 235.21/mpg)][, .(l100k = mean(l100k)), 
#>     keyby = .(cyl)]

dtplyr %>% as_tibble()
cyl l100k
4 9.048898
6 11.970180
8 14.871885
Show the code
dt <- data.table::as.data.table(mtcars)
dt[wt<5][, `:=`(l100k = 235.21/mpg)][, .(l100k = mean(l100k)), keyby = .(cyl)]
cyl l100k
4 9.048898
6 11.970180
8 14.871885

12.2 data.table 语法

DT[i, j, by]

##   R:                 i                 j        by
## SQL:  where | order by   select | update  group by

https://cran.r-project.org/web/packages/data.table/vignettes/datatable-intro.html