specify the number of decimal numbers

This happens also when you use C's fprintf and sprintf (at any rate
in my gcc):
  

r's printing routines (e.g., print, sprintf, cat, anything else?) seem
to rely on the underlying c sprintf, with no prior r-implemented
rounding.  hence they import into r whatever standards-related problems
may the c counterpart have.

  #include <stdio.h>
  #include <math.h>
  

why math.h?

  main(argc,argv) int argc; char **argv;
  {
  fprintf(stdout, "%.1f\n", 0.15);
  fprintf(stdout, "%.1f\n", 0.05);
  fprintf(stdout, "%.2f\n", 0.15);
  fprintf(stdout, "%.2f\n", 0.05);
  }

  cc -o testprint3 testprint3.c
  ./testprint
  0.1
  0.1
  0.15
  0.05

(with similar output when printing a string formatted by sprintf).
  

indeed.  i posted a similar example in reaction to the original 'excel
bug' response.

So, in so far a R relies on the compiler's implementation of the
*printf functions, this can hardly be put right withbout re-writing
[g]cc!

it does (seems to), but probably shouldn't, if excel bugs are to be
avoided.  however, passing the input from sprintf('%.1f', x) through
round to do the rounding would require some work to first parse the
format and decide the number of decimal digits.  not that it's undoable.

vQ