Need help on calling Head from C

Hi,
PROTECT(dfm=lang3(install("data.frame"),df,ScalarLogical(FALSE)));
SET_TAG(CDDR(dfm), install("stringsAsFactors")) ;
SEXP res = PROTECT(eval(dfm,R_GlobalEnv));
PROTECT(head=lang3(install("head"),res,ScalarInteger(1)));
head  = PROTECT(eval(head,R_GlobalEnv));

I tried the above following , it seemed to be not working . Can you please
help.

Can you elaborate? The above code works AFAICT ...
The code is actually not safe.  Both "install" and "SalarLogical/Integer"
potentially allocate memory, so at least one needs to be protected before
callling lang3.  (Passing one such argument would be OK, since lang3
protects its arguments, but it doesn't get a chance to do that while they
are still being evaluated.)
How true but who can blame him? The Writing R Extensions manual
contains the same mistake:

   SEXP mkans(double x)
   {
     SEXP ans;
     ans = PROTECT(allocVector(REALSXP, 1));
     REAL(ans)[0] = x;
     UNPROTECT(1);
     return ans;
   }

   double feval(double x, SEXP f, SEXP rho)
   {
     defineVar(install("x"), mkans(x), rho);
     return REAL(eval(f, rho))[0];
   }

Reported here more than 6 years ago and never fixed:

   https://stat.ethz.ch/pipermail/r-devel/2008-January/048040.html

Cheers,
H.
Now, I'm not sure this is the source of the actual problem, since both
"data.frame" and "head" presumably already exist, so the install won't
actually allocate memory.  But this is not a safe coding method in
general.

    Radford Neal

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Need help on calling Head from C

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