R: default value using other argument modified when other argument modified -

when defining default argument y r function being equal other argument x. ex: function(x,y=x). found if change other argument (x) before using default argument (y), changes value of y.

i understand has "delayed copying until modified" desired behavior?

hereafter full example:

oh_my <- function(x, y=x){     x <- rev(x)     y <- rev(y) # value of y here rev of initial x!     print(y) } oh_my(1:5) 

[1] 1 2 3 4 5

an easy solution be:

ok <- function(x, y=null){     if(is.null(y)){         y<-x     }     x <- rev(x)     y <- rev(y)     print(y) } ok(1:5) 

[1] 5 4 3 2 1

but fact default obvious in first option (including in automatically generated files).

an other solution be:

pfiouu <- function(x, y=x){     y <- rev(y) # value of y here rev of initial x!     x <- rev(x)     print(y) } 

pfiouu(1:5)

[1] 5 4 3 2 1

but seems awkward me pfiouu , oh_my give different results 2 exchanged lines not mention explicitly each others variables , yet yield different results!

am missing practice keep default obvious , avoid kind of fall-trap?

as suggested mrflick, adding force(y) @ start of function allows force evaluation of y , prevents issues that. being desirable behavior in such high level programming language r remains open though.

no_surprise<- function(x, y=x){     force(y)     x <- rev(x)     y <- rev(y)      print(y) } no_surprise(1:5) 

[1] 5 4 3 2 1

as expected.

wiki