create a column for baseline values in timeseries data. are there better ways?

repeatcolumn.R

library(plyr)

testdata = data.frame(
  ID = rep(c("A", "B", "C"), each = 6),
  time = rep(seq(0, 5, 1), 3),
  output = rnorm(3 * 6)
  )

testdata = merge(testdata,
  ddply(
    subset(testdata, time == 0), .(ID), summarize,
    BaselineValue = output
    ),
  by = "ID"
  )