I've found a temporary workaround that may be useful to nail down the problem (if problem there is), If I quote() each additional argument to be passed to fun., everything works fine: df <- data.frame(a=1:10 , b=1:10) foo1 <- function(a, b, cc=0, d=0){ a + b + cc + d } mdply(df, foo1, cc=1, d=2) # obscure (to me) failure mdply(df, foo1, quote(cc=1), quote(d=2)) Best wishes, baptiste ________________________________________ From: Auguie, Baptiste Sent: 05 October 2008 14:02 To: r-help at r-project.org Cc: h.wickham at gmail.com Subject: plyr package: passing further arguments fail Dear list and Hadley, The new plyr package seems to provide a clean and consistent way to apply a function on several arguments. However, I don't understand why the following example does not work like the standard mapply, library(plyr) df <- data.frame(a=1:10 , b=1:10) foo1 <- function(a, b, cc=0, d=0){ a + b + cc + d } mdply(df, foo1, cc=1) # fine mdply(df, foo1, d=1) # fails mdply(df, foo1, cc=1, d=2) # fails mapply(foo1, a=df$a, b=df$b, MoreArgs=list(cc=1)) mapply(foo1, a=df$a, b=df$b, MoreArgs=list(d=1)) mapply(foo1, a=df$a, b=df$b, MoreArgs=list(cc=1, d=2)) Best regards, baptiste