Passing variables to plyr

plyr_demo.R

my.reduce <- function(df.in, f1, f2){
  x <- ddply(df.in, .(f1, f2), summarize,
             value=sum(value)
             )
  x
}

df <- data.frame(
  state=rep(c("MA", "NY", "CA"), each=10),
  year=rep((2001:2010), 3),
  other=sample(c("A", "B", "C", "D"), 30, replace=TRUE),
  value = runif(30)
  )

# state year other     value
# 1    MA 2001     C 0.6021182
# 2    MA 2002     B 0.5948861
# 3    MA 2003     A 0.7402534
# 4    MA 2004     A 0.8149549
# 5    MA 2005     A 0.2495546
# 6    MA 2006     B 0.4370473

# I would like these two lines to return the same result.
ddply(df, .(state, other), summarize, value=sum(value))
my.reduce(df, "state", "other")

SOLUTION (thanks Hadley):

ddply(df.in, c(f1, f2), summarize,
         value=sum(value)
         )