Concise/terse filtering R data.table rows by condition within groups (inside a magrittr)

data.table_filtering_by_groups.R

library(data.table)
library(magrittr)

## Pattern:
# DT[DT[,row_filtering_expression, by=grouping_column][[2]]] 
## or generally
# DT[DT[,row_filtering_expression, by=.(grouping_col1,grouping_col2,etc.)][[number_of_grouping_cols+1]]]

## Example:
data.table(a=c(1,1,1,2,2),
           b=c(1,0,3,-1,5),
           v=1:5) %>%
  print() %>%
  #    a  b v
  # 1: 1  1 1
  # 2: 1  0 2
  # 3: 1  3 3
  # 4: 2 -1 4
  # 5: 2  5 5
  .[.[,b==min(b), by=a][[2]]] %>%
  print()
  #    a  b v
  # 1: 1  0 2
  # 2: 2 -1 4

## A helper function:
filterDTrowsWithinGroups <-
  function(DT, row_filtering_expression, by, dot_is_list=TRUE)
    eval(bquote({
      if (dot_is_list)
        . <- list # data.table's alias
      DT[DT[, .(substitute(row_filtering_expression))
            , by = .(substitute(by))]
         [[if (is.list(.(substitute(by))))
             length(.(substitute(by))) + 1
           else
             2]]]
    }))

## Usage example (the same as before):
data.table(a=c(1,1,1,2,2),
           b=c(1,0,3,-1,5),
           v=1:5) %>%
  filterDTrowsWithinGroups(b==min(b), by=a) %>%
  print()
  ## Returns again:
  #    a  b v
  # 1: 1  0 2
  # 2: 2 -1 4