R: ddply() Possible to reuse generated columns?

Question

I have a script where I'm using ddply, as in the following example:

ddply(df, .(col),
function(x) data.frame(
col1=some_function(x$y),
col2=some_other_function(x$y)
)
)

Within ddply, is it possible to reuse col1 without calling the entire function again?

For example:

ddply(df, .(col),
function(x) data.frame(
col1=some_function(x$y),
col2=some_other_function(x$y)
col3=col1*col2
)
)

Answer 1

I don't think that's possible, but it shouldn't matter too much, because at that point it's not an aggregation function anymore. For example:

#use summarize() in ddply()
data.means <- ddply(data, .(groups), summarize, mean = mean(x), sd = sd(x), n = length(x))
data.means$se <- data.means$sd / sqrt(data.means$n)
data.means$Upper <- data.means$mean + (data.means$SE * 1.96)
data.means$Lower <- data.means$mean - (data.means$SE * 1.96)

So I didn't calculate the SEs directly, but it wasn't so bad calculating it outside of ddply(). If you really wanted to, you could also do

ddply(data, .(groups), summarize, se = sd(x) / sqrt(length(x)))

Or to put it in terms of your example

ddply(df, .(col), summarize,
      col1=some_function(y),
      col2=some_other_function(y)
      col3=some_function(y)*some_other_function(y)
    )

Answer 2

You've got a whole function to play with! Doesn't have to be a one-liner! This should work:

ddply(df, .(col), function(x) {
  tmp <- some_other_function(x$y)
  data.frame(
    col1=some_function(x$y),
    col2=tmp,
    col3=tmp
  )
})