R data.table get maximum value per row for multiple columns

Question

I've got a data.table in R which looks like that one:

dat <- structure(list(de = c(1470L, 8511L, 3527L, 2846L, 2652L, 831L
), fr = c(14L, 81L, 36L, 16L, 30L, 6L), it = c(9L, 514L, 73L, 
37L, 91L, 2L), ro = c(1L, 14L, 11L, 1L, 9L, 0L)), .Names = c("de", 
"fr", "it", "ro"), class = c("data.table", "data.frame"), row.names = c(NA, 
-6L))

I now wanna create a new data.table (having exactly the same columns) but holding only the maximum value per row. The values in the other columns should simply be NA.

The data.table could have any number of columns (the data.table above is just an example).

The desired output table would look like this:

     de fr it ro
1: 1470 NA NA NA
2: 8511 NA NA NA
3: 3527 NA NA NA
4: 2846 NA NA NA
5: 2652 NA NA NA
6:  831 NA NA NA

Answer 1

There are several issues with what the OP is attempting here: (1) this really looks like a case where data should be kept in a matrix rather than a data.frame or data.table; (2) there's no reason to want this sort of output that I can think of; and (3) doing any standard operations with the output will be a hassle.

With that said...

dat2 = dat
is.na(dat2)[-( 1:nrow(dat) + (max.col(dat)-1)*nrow(dat) )] <- TRUE
# or, as @PierreLafortune suggested
is.na(dat2)[col(dat) != max.col(dat)] <- TRUE

# or using the data.table package
dat2 = dat[rep(NA_integer_, nrow(dat)), ]
mc   = max.col(dat)
for (i in seq_along(mc)) set(dat2, i = i, j = mc[i], v = dat[i, mc[i]])

Answer 2

It's not clear to me whether you mean that you want to use the data.table package, or if you are satisfied with making a data.frame using only base functions. It is certainly possible to do the latter.

Here is one solution, which uses only max() and which.max() and relies on the fact that an empty data.frame will fill in all of the remaining cells with NA to achieve a rectangular structure.

maxdat <- data.frame()
  for (col in names(dat)) {
  maxdat[which.max(dat[,col]), col] <- max(dat[,col])
}