Hi,
I would like to report a small bug in one of the code examples given in
the 'Writing R extensions' manual. Section 5.9.2 ("Calling
.External")
defines the function "showArgs" in order to demonstrate how to access
in
C the pair-listed arguments passed by .External. The function aims at
printing echoing all passed arguments to the screen but only prints
every second argument.
The loop to go through the pair list starts as follows
args = CDR(args); /* skip 'name' */
for(i = 0; args != R_NilValue; i++, args = CDR(args)) {
args = CDR(args); /* <--- This line is wrong. */
As you can see, "args = CDR(args)" is called twice, resulting in every
other argument skipped. The marked line should be deleted.
Furthermore, the function requires all arguments to be named, as it
gives an error if TAG returns NIL in the line
name = CHAR(PRINTNAME(TAG(args)));
It might improve the example to introduce here a test "if( TAG(args) !=
R_NilValue )".
May I suggest to the maintainer of the manual to change the example to
the following code? (Changes to original: lines 1, 2, 15, 17, 18 added;
one line deleted after line 13.)
1 #include <R.h>
2 #include <Rinternals.h>
3 #include <R_ext/PrtUtil.h>
4
5 SEXP showArgs(SEXP args)
6 {
7 int i, nargs;
8 Rcomplex cpl;
9 const char *name;
10 SEXP el;
11
12 args = CDR(args); /* skip 'name' */
13 for(i = 0; args != R_NilValue; i++, args = CDR(args)) {
14
15 if( TAG(args) != R_NilValue )
16 name = CHAR(PRINTNAME(TAG(args)));
17 else
18 name = "<unnamed>";
19 switch(TYPEOF(CAR(args))) {
20 case REALSXP:
21 Rprintf("[%d] '%s' %f\n", i+1, name, REAL(CAR(args))[0]);
22 break;
23 case LGLSXP:
24 case INTSXP:
25 Rprintf("[%d] '%s' %d\n", i+1, name,
INTEGER(CAR(args))[0]);
26 break;
27 case CPLXSXP:
28 cpl = COMPLEX(CAR(args))[0];
29 Rprintf("[%d] '%s' %f + %fi\n", i+1, name, cpl.r, cpl.i);
30 break;
31 case STRSXP:
32 Rprintf("[%d] '%s' %s\n", i+1, name,
33 CHAR(STRING_ELT(CAR(args), 0)));
34 break;
35 default:
36 Rprintf("[%d] '%s' R type\n", i+1, name);
37 }
38 }
39 return(R_NilValue);
40 }
41
Best regards
Simon Anders
+---
| Dr. Simon Anders, Dipl. Phys.
| European Bioinformatics Institute, Hinxton, Cambridgeshire, UK
| office phone +44-1223-494478, mobile phone +44-7505-841692
| preferred (permanent) e-mail: sanders at fs.tum.de
