[Rd] Why does typeof() modify an object's "named" field?

Josh O'Brien joshmobrien at gmail.com
Fri Mar 22 20:43:05 CET 2013


Doing typeof() on an object appears to reset the "named" field in its
sxpinfo header to 2, which can change the way that subsequent
subassignment operations are carried out:

X <- 1:5e7
# @4eeb0008 13 INTSXP g0c7 [NAM(1)] (len=50000000, tl=0) 1,2,3,4,5,...
system.time(X[1] <- 9L)
#    user  system elapsed
#       0       0       0


# @4eeb0008 13 INTSXP g1c7 [MARK,NAM(2)] (len=50000000, tl=0) 9,2,3,4,5,...
system.time(X[2] <- 9L)
#    user  system elapsed
#    0.16    0.08    0.23

Some other functions that query the nature of an object (e.g. class(),
length(), attributes()) do not modify the object's "named" field. Is
there a reason that typeof() should?

(Possibly of interest is this somewhat related thread on Stack
Overflow: http://stackoverflow.com/questions/15559387/operator-in-rstudio-and-r/15559956#15559956

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