[R] ddply - how to transform df column "in place"

[jjap]

Dear R-users,

I am trying to get the plyr syntax right, without much success.

Given:
d<- data.frame(cbind(x=1,y=seq(20100801,20100830,1)))
names(d)<-c("first", "daterep")
d2<-d

# I can convert the daterep column in place the classic way:
d$daterep<-as.Date(strptime(d$daterep, format="%Y%m%d"))

# How to do it the plyr way?
ddply(d2, c("daterep"), function(df){as.Date(df, format="%Y%m%d")})
# returns: Error in as.Date.default(df, format = "%Y%m%d") : 
#   do not know how to convert 'df' to class "Date"

Thanks for any hints,

---jean

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