Matching values from two vectors in R

Question

I have two vectors:

A <- c(1,3,5,6,4,3,2,3,3,3,3,3,4,6,7,7,5,4,4,3) # 7 unique values
B <- c("a","b","c","d","e","f","g")   # 7 different values

I would like to match the values of B to A such that the smallest value in A gets the first value from B and continued on to the largest.

The above example would be:

A:        1 3 5 6 4 3 2 3 3 3 3 3 4 6 7 7 5 4 4 3
assigned: a c e f d c b c c c c c d f g g e d d c

Answer 1

Try this:

A <- c(1,3,5,6,4,3,2,3,3,3,3,3,4,6,7,7,5,4,4,3)
B <- letters[1:7]

B[match(A, sort(unique(A)))]
#  [1] "a" "c" "e" "f" "d" "c" "b" "c" "c" "c" "c" "c" "d" "f" "g"
# [16] "g" "e" "d" "d" "c"

Answer 2

Another option that handles the general case that @JoshO'Brien addresses would be

B[as.numeric(factor(A))]
# [1] "a" "c" "e" "f" "d" "c" "b" "c" "c" "c" "c" "c" "d"
# [14] "f" "g" "g" "e" "d" "d" "c"

A2<-ifelse(A > 4, A + 1, A)
# [1] 1 3 6 7 4 3 2 3 3 3 3 3 4 7 8 8 6 4 4 3
B[as.numeric(factor(A2))]
# [1] "a" "c" "e" "f" "d" "c" "b" "c" "c" "c" "c" "c" "d"
# [14] "f" "g" "g" "e" "d" "d" "c"

However, following benchmark shows that this method is slower than @JoshOBrien's.

library(microbenchmark)
B <- make.unique(rep(letters, length.out=1000))
A <- sample(seq_along(B), replace=TRUE)
unique_sort_match <- function() B[match(A, sort(unique(A)))]
factor_as.numeric <- function() B[as.numeric(factor(A))]
bm<-microbenchmark(unique_sort_match(), factor_as.numeric(), times=1000L)
plot(bm)

Answer 3

To elaborate on the comments in @Josh's answer:

If A does in fact represent a permutation of the elements of B (ie, where a 1 in A represents the first element of B , a 4 in A represents the 4th element in B , etc), then as @Matthew Plourde points out, you would want to simply use A as your index to B :

B[A]

If A does not represent a permutation of B, then you should use the method suggested by @Josh