Writing functions for use in dplyr::mutate() can be done using the following steps:

1. Use a function that returns a single value:

return_single_value <- function(x){
 sum(x) 
}
iris %>% mutate(
 result = return_single_value(Petal.Length)
)

2. Use a function that returns the same number of values as given:

return_length_values <- function(x){
 paste0("result_", 1:length(x))
}
iris %>% mutate(
 result = return_length_values(Petal.Length)
)

3. Use a function that returns neither a single value nor the same number of values as given:

return_three_values <- function(x){
 c("A", "b", "C")
}
iris %>% mutate(
 result = return_three_values(Petal.Length)
)

4. Force repetition of the function to fit the length of the vector:

rep_until <- function(x){
 rep(c("A", "b", "C"), length.out = length(x))
}
iris %>% mutate(
 result = rep_until(Petal.Length)
)