filter rows in data.table with `by`

问题

I would like to filter group which following criteria. The DT brings unexpected results.

Input data

library(data.table)
library(dplyr)

dt <- data.table(
    logic = c(TRUE, TRUE, FALSE, TRUE, TRUE, TRUE),
    group = c("A" , "A",  "A"  , "B" , "B" , "B")
)

I would like to filter group, where logic field values are all TRUE.

Expected behavior (by dplyr)

As you can see dplyr works as expected, and brings back only values with group = B

dt %>% 
  group_by(group) %>% 
  filter(all(logic))
# Source: local data table [3 x 2]
# Groups: group

#   logic group
# 1  TRUE     B
# 2  TRUE     B
# 3  TRUE     B

Unexpected behavior by data.table

DT doesn't really filter rows, either bringing all table or nothing.

dt[all(logic), group, by = group]
# Empty data.table (0 rows) of 2 cols: group,group

dt[all(.SD$logic), group,by = group]
#    group group
# 1:     A     A
# 2:     B     B

回答1:

You could use [ as in

dt[, .SD[all(logic)], by = group]
#   group logic
#1:     B  TRUE
#2:     B  TRUE
#3:     B  TRUE

回答2:

We need to use if

dt[, if(all(logic)) .SD, by = group]
#    group logic
#1:     B  TRUE
#2:     B  TRUE
#3:     B  TRUE

来源：https://stackoverflow.com/questions/34393053/filter-rows-in-data-table-with-by