Efficiently inserting default missing rows in a data.table

Question

Suppose I\'ve got the following data.table :

dt <- data.table(id=c(1,1,1,1,1,1,2,2,2,2),
           wday=c(\"mon\",\"tue\",\"wed\",\"thu\",\"         


        

Answer 1

One other possilibity with match and ddply :

FUN <- function(x) {
y <- x$val[match(c("mon", "tue", "wed", "thu", "fri", "sat", "sun"), x$wday, nomatch=NA)]
y[is.na(y)] <- 0
y <- data.frame(wday=c("mon", "tue", "wed", "thu", "fri", "sat", "sun"), val=y)
y
}
ddply(dt, .(id), FUN)