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

[Rex_Bryan@urscorp.com]

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?

Rex