find ordering in two R vector
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If I have a character vector `a`,

    a <- c("bob","jim","joe","fred")

and I have another character vector `b`, with the same elements but in a different order,

    b <- c("joe", "jim", "fred", "bob")

is there a function that tells what element of `a` each element of `b` equates to? In other words, I would like a function that would return `c(3,2,4,1)` (because `b` is equivalent to  `c(a[3], a[2], a[4], a[1])`. Thanks!
