replacing values in one vector with corresponding index from another vector

A quick question for the gurus...

Given:

a=c(58,73,100,40,70)
b=c(40,70,73,100,58,70,70,58)

How can I replace the elements of "b" with the corresponding index
numbers from "a" that start at 1? All values in "a" are unique. So, I
end up with:

b=c(4,5,2,3,1,5,5,1)

I believe I need to use one of the "apply" functions in combination with
"which" but I cannot make it work.

thank you,
-david