R Dynamically build “list” in data.table (or ddply)

Question

My aggregation needs vary among columns / data.frames. I would like to pass the \"list\" argument to the data.table dynamically.

As a minimal example:

Answer 1

Another method (supporting the use of paste or paste0 to build the expression):

expr <- parse(text=mylist)
DT[, eval( expr ), by=type]
#-------
    type lengtha      maxb     meanc
1: hello       3 0.8265407 0.5244094
2:   bye       3 0.4955301 0.6289475
3:    ok       3 0.9527455 0.5600915