Efficient ways to reshape huge data from long to wide format - similar to dcast

Question

This question pertains to creating \"wide\" tables similar to tables you could create using dcast from reshape2. I know this has been discussed many times before, but my que

Answer 1

For n=1e6 the following takes about 10 seconds with plain dcast and about 4 seconds with dcast.data.table:

library(reshape2)

dcast(z[, sum(price), by = list(c1, c2, c3)], c1 + c2 ~ c3)

# or with 1.8.11
dcast.data.table(z, c1 + c2 ~ c3, fun = sum)