Matching strings in switch statements

Question

I'd like to use a switch statement to branch according to the first two characters of a string. In the example shown below, I'd like to return "Apple" with either "aabb" or "aacc"

test<-c("aabb", "aacc", "bbbb")

foo <- Vectorize(function(a) {
      switch(as.character(a),
            "^aa" = "Apple",
            "bbbb" = "Grape",
            "Unknown")

}, "a")

> foo(test)
aabb      aacc      bbbb 
"Unknown" "Unknown"   "Grape" 

I can get the statement to work if there is an exact match, but I can't figure out how to get a match using only the first two characters. Maybe there is a way using Regex? As shown, I've tried to use ^ to match the beginning of the string (which works with grep ) but it doesn't work.

I think I could use nested ifelse statements with grepl but can it work using switch ?

Answer 1

According to manual, this is impossible with switch because it only allows exact match:

switch(EXPR, ...)

If EXPR evaluates to a character string then that string is matched (exactly) to the names of the elements in ... .

But you can do it with case_when in dplyr :

foo <- function(x){
    dplyr::case_when(
        grepl('^aa', x) ~ 'Apple',
        x == 'bbbb' ~ 'Grape',
        TRUE ~ 'Unknown'
    )
}
foo(test)
# [1] "Apple" "Apple" "Grape"

Answer 2

Use substr() to extract the first two letters; use setNames() to propagate the full names rather than abbreviation to the answer

> X = setNames(substr(test, 1, 2), test)
> sapply(X, switch, aa="Apple", bb = "Grape", "Unknown")
   aabb    aacc    bbbb 
"Apple" "Apple" "Grape" 

Answer 3

this might work:

regex_switch<-function(.v,...)
{
  e<-enexprs(...)
  i<-min(which(str_detect(.v,names(e))))
  eval(e[[i]],parent.frame())
}

> map_chr(
   c("aaaa","bb","aabbaa"),
   regex_switch,
   "^aa"="Apple","^bbbb"="Grape","^*$"="Unknown"
)  
[1] "Apple"   "Unknown" "Apple"  
>