R: replace a value of a vector with multiple values - r

Question

Let's say, I have a vector a with length(a) = l and l >= 1.
The element "x" occurs at least one time in a, but we don't know an exact position.
I want to replace every "x" in a with the values c(1,2,3)
For example: a = ("y","x","z"), then I want the result after the replacement to be a = ("y",1,2,3,"z").
I thought of doing it this way:
l <- length(a)
pos.x <- which(a == "x")
if(l == 1L & pos.x == 1L) {
a <- c(1,2,3)
} else if (l > 1L & pos.x == 1) {
a <- c(1,2,3,a[-1])
} else if (l > 1L & pos.x == l) {
a <- c(a[-l],1,2,3)
} else if (l >= 3 & pos.x != 1 & pos.x != l) {
a <- c(a[1:(pos.x - 1)],1,2,3,a[(pos.x + 1):l])
}
While this code does work, my question would be wether there is a more 'elegant' way to solve this problem, that needs less processing power, and that could replace more than one "x".
Thank you!

Answer 1

Here's a simple vectorized solution with base R -
a <- c("y","x","z","y","x","z") # vector to search
b <- 1:3 # replacement values
a <- rep(a, 1 + (length(b) - 1)*(a == "x")) # repeat only "x" length(b) times
a[a == "x"] <- b # replace "x" with replacement values i.e. b
[1] "y" "1" "2" "3" "z" "y" "1" "2" "3" "z"

Answer 2

Here is an option using a for loop
a <- c("y","x","z","y","x","z")
b <- c(1,2,3)
The 'trick' is to create a list first, then replace all "x" with b and finally call unlist.
a_list <- as.list(a)
for(i in which(a_list == "x")) {
a_list[[i]] <- b
}
Result
unlist(a_list)
#[1] "y" "1" "2" "3" "z" "y" "1" "2" "3" "z"
Please consider #Shree's answer!
Here is why:
n <- 1e6
set.seed(1)
a <- sample(c("x", "y", "z"), size = n, replace = TRUE)
b <- 1:3
library(microbenchmark)
benchmark <- microbenchmark(
markus = markus(a, b),
IceCreamToucan = IceCreamToucan(a, b),
Shree = Shree(a, b)
)
autoplot(benchmark)
#Unit: milliseconds
# expr min lq mean median uq max neval
# markus 403.38464 467.03277 615.8078 556.74067 754.5117 1095.7035 100
#IceCreamToucan 401.34614 462.92680 602.1556 526.08280 687.8436 1422.0629 100
# Shree 52.33867 65.32323 157.6680 97.34066 162.0638 650.2571 100
functions
markus <- function(a, b) {
a_list <- as.list(a)
for(i in which(a_list == "x")) {
a_list[[i]] <- b
}
unlist(a_list)
}
Shree <- function(a, b) {
a <- rep(a, 1 + (length(b) - 1)*(a == "x"))
a[a == "x"] <- b
a
}
# from the comments
IceCreamToucan <- function(a, b) {
a_list <- as.list(a)
w <- which(a_list == "x")
a_list[w] <- rep(list(b), length(w)) # changed your answer slightly here
unlist(a_list)
}