comparing class() -- R=NULL and S+=numeric

"Rex_Bryan at urscorp.com" <RexBryan1 at attbi.com> writes:

I'm just reading Modern Applied Statistics with S, 4th Ed., Venables and
Ribley
I'm typing in their examples in both R and S+.  I need insight in the
difference
in the class() statement shown in Chap. 2.  Example from book:

names(powers.of.pi) <- -2:2
powers.of.pi

       -2        -1         0         1         2
0.1013212 0.3183099 1.0000000 3.1415927 9.8696044

class(powers.of.pi)

R produces an answer:      NULL
S+ produces the answer:  [1]  "numeric"

Any insight in understanding the difference?

I suspect that the answer is actually somewhere in MASS4, but I don't
have it at hand right now. 

Basically there have been two class systems in S(-PLUS), S3 and S4,
and R is (mostly) compatible with the former as represented by S-PLUS
3.x.

In S3, classes are really just text attributes, and if none is
present, the class is NULL. However, you can ask for data.class(x) and
get - essentially - its mode. E.g.,

data.class(powers.of.pi)

[1] "numeric"

S4 classes introduce a stricter system where all objects have a class.
This is what is used in current versions of S-PLUS. In R, we have the
"methods" library, which changes the behaviour of R to be like S4:

library(methods)
class(powers.of.pi)

[1] "numeric"

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