Jean,

Ista is right, but:

In your function you are asking as.Date to convert the whole data.frame df
rather than just your daterep column.

out<-ddply(d2, .(daterep), function(df)
as.Date(strptime(df$daterep,format='%Y%m%d')))
>str(out)
'data.frame':    30 obs. of  2 variables:
 $ daterep: num  20100801 20100802 20100803 20100804 20100805 ...
 $ V1     : Date, format: "2010-08-01" "2010-08-02" "2010-08-03"
"2010-08-04" ...


On Tue, Aug 23, 2011 at 3:16 PM, jjap <jean.plamondon@fpinnovations.ca>wrote:

> 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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