How can I add rows to an R data frame every other row?

Question

Brief: how can I add m rows to my m X n data frame, where each new row is inserted after each existing row? I will essentially copy the existing ro

Answer 1

Here's one possible approach if I understand what you're doing:

dat <- head(CO2, 10) # fake data set

L1 <- lapply(1:nrow(dat), function(i) {
    dat2x <-  dat[i, ]
    dat2x[4] <- dat[i, 4] - dat[i, 5]
    rbind(dat[i, ], dat2x)
})
do.call(rbind, L1)

EDIT: totally working off e4e5f4's excellent response:

new <- dat[rep(1:nrow(dat),1,each=2),]
new[c(F, T),4] <- dat[4] - dat[5]

Both are equivalent but I assume the alter is way faster.