assign to an element of a vector

I am trying to find a way to assign values to elements of a vector
that will be defined by a user. So I don't have the name of the vector
and cannot hard code the assignment in advance. In the example below I
have to get() the vector using its name. When I try to assign to an
element I get an error:

a <- c(1,2,3)
get('a')[1] <- 0

Error: Target of assignment expands to non-language object

Any suggestions?

FS