http://stackoverflow.com/a/30193788/1270695

dcastInDataTable.R

dt <- data.table(
  Row = c(rep(1, 3), rep(2, 3), 3),
  Col = factor(c(1, 2, 8, 1, 2, 9, 2), 1:9),
  Value = c(31L, 56L, 13L, 83L,51L, 16L, 53L)
)
str(dt)
# Classes ‘data.table’ and 'data.frame':	7 obs. of  3 variables:
#  $ Row  : num  1 1 1 2 2 2 3
#  $ Col  : Factor w/ 9 levels "1","2","3","4",..: 1 2 8 1 2 9 2
#  $ Value: int  31 56 13 83 51 16 53
#  - attr(*, ".internal.selfref")=<externalptr> 
dcast.data.table(dt, Row ~ Col, value.var = "Value", drop = FALSE)
#    Row  1  2  8  9
# 1:   1 31 56 13 NA
# 2:   2 83 51 NA 16
# 3:   3 NA 53 NA NA
dcast.data.table(dt, Row ~ Col, value.var = "Value", drop = FALSE, fun.aggregate = I)
# Error in dcast.data.table(dt, Row ~ Col, value.var = "Value", drop = FALSE,  : 
#   Aggregating function provided to argument 'fun.aggregate' should always return a length 1 vector, but returns 0-length value for fun.aggregate(integer(0)). This value will have to be used to fill missing combinations, if any, and therefore can not be of length 0. Either override by setting the 'fill' argument explicitly or modify your function to handle this case appropriately.
dcast.data.table(dt, Row ~ Col, value.var = "Value", drop = FALSE, fun.aggregate = I, fill = NA_integer_)
#    Row  1  2  8  9
# 1:   1 31 56 13 NA
# 2:   2 83 51 NA 16
# 3:   3 NA 53 NA NA