.BY experimenting

datatable_grouping_experiments.R

library(data.table)

dta <- data.table(gr = LETTERS[1:3], 
                  a = 1,  
                  b = 1)

add <- function(x, y, group) if(group == "B") x + y else x - y

dta[,.BY, by = .(gr)]

#    gr BY
# 1:  A  A
# 2:  B  B
# 3:  C  C

dta[,str(.BY), by = .(gr)]
# List of 1
# $ gr: chr "A"
# List of 1
# $ gr: chr "B"
# List of 1
# $ gr: chr "C"
# Empty data.table (0 rows) of 1 col: gr

dta[,str(.BY$gr), by = .(gr)]
# chr "A"
# chr "B"
# chr "C"
# Empty data.table (0 rows) of 1 col: gr


dta[, newcol := add(a, b, .BY), by = .(gr)]

dta
#    gr a b newcol
# 1:  A 1 1      0
# 2:  B 1 1      2
# 3:  C 1 1      0


## Another Example: 

set.seed(1)
DT <- data.table(x = rep(LETTERS[1:3],4),
                 y = rep(letters[1:2],6),
                 z = rnorm(12,10))

DT[, .(x_GroupValue = .BY$x,
       y_GroupValue = .BY$y,
       RowCount = .N,
       z_Average = mean(z)), by = .(x,y)]

#    x y x_GroupValue y_GroupValue RowCount z_Average
# 1: A a            A            a        2  9.930488
# 2: B b            B            b        2 10.460984
# 3: C a            C            a        2  9.870076
# 4: A b            A            b        2 10.644946
# 5: B a            B            a        2 10.920644
# 6: C b            C            b        2  9.784687