[Rd] AS_NUMERIC and as.numeric - Could someone explain?

[Douglas Bates]

AS_NUMERIC in C more closely corresponds to as.double() in R

All that as.numeric() in R does is assure that the result is of a 
numeric mode.  It does not coerce integer variables to doubles.

Check, for example,
str(as.numeric(1:100))


Daniel Hoppe wrote:

> Dear List,
> 
> I stepped into a strange effect which I can't explain to myself
> (probably due to lack of knowledge on R internals). 
> 
> I have four vectors a,b,c and z of size 10000 each. With these vectors I
> call
> 
>     .Call("hyp2f1forrey", a, b, b, z, PACKAGE = "hyp2f1")
> 
> to access 
> 
> SEXP hyp2f1forrey(SEXP a, SEXP b, SEXP c, SEXP x)
> {
>     int i,n;
>     double *xa, *xb, *xc, *xx, *xresr, *xresi;
>     SEXP resr, resi;
> 
>     n = LENGTH(a);
> 
>     PROTECT(a = AS_NUMERIC(a));
>     PROTECT(b = AS_NUMERIC(b));
>     PROTECT(c = AS_NUMERIC(c));
>     PROTECT(x = AS_NUMERIC(x));
>     PROTECT(resr = NEW_NUMERIC(n));
>     PROTECT(resi = NEW_NUMERIC(n));
> 
>     xa = NUMERIC_POINTER(a);
>     xb = NUMERIC_POINTER(b);
>     xc = NUMERIC_POINTER(c);
>     xx = NUMERIC_POINTER(x);
>     xresr = NUMERIC_POINTER(resr);
>     xresi = NUMERIC_POINTER(resi);
> 
>     for (i = 0; i < n; i++)
>         F77_CALL(hyp)(&xx[i], &xa[i], &xb[i], &xc[i],  &xresr[i],
> &xresi[i]);
>     UNPROTECT(6);
>     return(resr);
> }
> 
> I read the documentation in the way that I could either do the call from
> above or  
> 
>     .Call("hyp2f1forrey", as.numeric(a), as.numeric(b), as.numeric(b),
> as.numeric(z), PACKAGE = "hyp2f1")
> 
> and remove the 
> 
>     PROTECT(a = AS_NUMERIC(a));
>     PROTECT(b = AS_NUMERIC(b));
>     PROTECT(c = AS_NUMERIC(c));
>     PROTECT(x = AS_NUMERIC(x));
> 
> in the C-Code. What happens now is that in my resulting vector of length
> 10000 some values are incorrect depending on the exact way of calling
> the C-Function. 
> 
>     .Call("hyp2f1forrey", a, b, b, z, PACKAGE = "hyp2f1")[11]  -->
> incorrect result
>     .Call("hyp2f1forrey", a[11], b[11], b[11]), z[11]), PACKAGE =
> "hyp2f1") --> works
>     .Call("hyp2f1forrey", as.numeric(a), as.numeric(b), as.numeric(c),
> as.numeric(z), PACKAGE = "hyp2f1")[11] -- works
>     .Call("hyp2f1forrey", a[1:10000], b[1:10000], b[1:10000]),
> z[1:10000]), PACKAGE = "hyp2f1") --> works too!
> 
> 
> Could someone explain to me where the difference in these calls comes
> from?
> 
> Thanks in advance,
> 
> Daniel
> 
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