Converting a data frame to a matrix with plyr daply

Question

I\'m trying to use the daply function in the plyr package but I cannot get it to output properly. Even though the variable that makes up the matrix is

Answer 1

The identity function isn't what you want here; from the help page, "All plyr functions use the same split-apply-combine strategy: they split the input into simpler pieces, apply .fun to each piece, and then combine the pieces into a single data structure." The simpler pieces in this case are subsets of the original data frame with unique Vehicle/Month combinations; the identity function just returns that subset, and these subsets are then used to fill the resulting matrix.

That is, each element of the matrix you got is a data frame (which is a type of list) with the rows with that Month/Vehicle combination.

> try1 <- daply(DF, .(Vehicle, Month), identity)
> try1[1,1]
[[1]]
   Month Vehicle Samples
1 Oct-10   31057     256

You instead want to use a function that just gets the Samples portion of that data frame, like this:

daply(DF, .(Vehicle, Month), function(x) x$Samples)

which results in

       Month
Vehicle Oct-10 Nov-10 Dec-10
  31057    256    267    159
  31059    316    293    268
  31060    348    250    206

A few alternate ways of doing this are with cast from the reshape package (which returns a data frame)

cast(DF, Vehicle~Month, value="Samples")

the revised version in reshape2; the first returns a data frame, the second a matrix

dcast(DF, Vehicle~Month, value_var="Samples")
acast(DF, Vehicle~Month, value_var="Samples")

with xtabs from the stats package

xtabs(Samples ~ Vehicle + Month, DF)

or by hand, which isn't hard at all using matrix indexing; almost all the code is just setting up the matrix.

with(DF, {
  out <- matrix(nrow=nlevels(Vehicle), ncol=nlevels(Month),
                dimnames=list(Vehicle=levels(Vehicle), Month=levels(Month)))
  out[cbind(Vehicle, Month)] <- Samples
  out
})

The reshape function in the stats package can also be used to do this, but the syntax is difficult and I haven't used it once since learning cast and melt from the reshape package.