On 29/01/2010 2:03 PM, Saptarshi Guha wrote:
Dear R Users,
Using codetools I obtained the text representation of the parse tree for this
snippet
z=quote({x[1]<-2})
showTree(z)
> ("{" (<- ([ x 1) 2)) (A)
If I understand correctly, x[1]<-2 ought to be "[<-"(x,1,2), so shouldn't i see
("{" ( [<- x 1 2 ) )
If indeed the parse tree in (A) is correct, the operation ([ x 1)
returns the value of x[1], how then does
the <- operator know to change the 1st element of x ? Does the <-
implementation inspect the its 1st argument (e.g if it sees
[ call, it instead changes itself to implementation of "[<-"?
The displayed parse tree is correct. You could confirm it by looking at
z[[1]], z[[2]], z[[2]][[1]], etc.
The [ call is the first argument to <-, which is the destination of the
assignment. The conversion to [<- happens at evaluation time.
Duncan Murdoch
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