vector - swapping user defined consecutive values in R -

given vector, wanted swap values when particular elements consecutive.

x <- c("a", "b", "c") 

i want make rule, if "b" comes before "c", swap values.

x <- c("a", "b", "c") res [1] "a" "c" "b" 

if "t" comes before "u",

y <- c("t", "u", "x", "a", "u") res2 [1] "u" "t" "x" "a" "u" 

more broadly multiple matches also. here if "d" directly before "b",

z <- c("a", "d", "b", "x", "d", "b") res3 [1] "a" "b" "d" "x" "b" "d" 

a function isn't elegant, should work

swap<-function(x, pair) {     stopifnot(length(pair)==2)     a1 <- which(x==pair[1])     a2 <- which(x==pair[2])     if (length(a1<-a1[a1 %in% (a2-1)])) {         x[a1] <- pair[2]         x[a1+1] <- pair[1]     }     x }  z <- c("a", "d", "b", "x", "d", "b") swap(z, c("z","d"))    #no change # [1] "a" "d" "b" "x" "d" "b" swap(z, c("a","d"))    #one change # [1] "d" "a" "b" "x" "d" "b" swap(z, c("d","b"))    #multi-change # [1] "a" "b" "d" "x" "b" "d"