Seeking help re: ? single apply command or an alternative

Dear R-help list,

Sorry to trouble everyone. This seems like it could be achieved with a
single command (? variant of apply), but I am not quite seeing it.

I am trying to multiply one vector (a1 below) by corresponding values in a3
(as determined by matching element in a vector a2). The example below might
be clearer.

a1 <- c(4,3,4,6,8,9,2,3,4,6,7,4)
a2 <- c("A","A","A","A","B","B","C","C","C","D","D","D")
a3 <- c(2,4,1,0.5)
names(a3) <- c("A","B","C","D")

I would like the vector (8,6,8,12,32,36,2,3,4,3,3.5,2) as my final output
result  i.e. multiply each element of a1 by the matching a3 as determined by
the corresponding element of a2 - that is, for the first 4 elements of a1,
multiply by the value 2 found in a3 ("A"), the next 2 by 4 ("B"), the next 3
by 1.0 ("C"), and the last 3 by 0.5 ("D")

I am trying to avoid the use of loops, because the actual dataset is much
larger.

Thanks very much!

Min-Han

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