Repeating values in loop until error disappears

Question

I am currently using a for loop to geocode a large number of addresses using the Googleway package. Initially, I ran into issues with "500 internal server errors" stopping the execution of the loop. I was able to get around this using tryCatch(). However, since this tends to be a transient error, I would like the function to repeat the address that throws the error until it receives a result or hits some arbitrary number of attempts, let's say 10.

Unfortunately, I've found tryCatch() and the documentation associated with it confusing, so I'm at a loss for how to do anything other than get it to throw an error message and move on. Here is my current code:

rugeocoder.fun <- function(addr){
              require(googleway)
              output <- vector("list", length=length(addr))
              tryCatch({
                for(i in 1:length(addr)){
                  output[[i]] <- google_geocode(address=addr[i], key="myapikey", language="ru", simplify=T)
                  print(i)

                }},error=function(e) output[[i]] <- "Error: reattempt")
              return(output)
              }

Answer 1

You probably want to separate out the logic for calling google_geocode() safely, and for looping over the addresses.

Here's a function that modifies other functions to call them repeatedly until they work, or they fail max_attempts times. Functions that modify other functions are sometimes called "adverbs".

safely <- function(fn, ..., max_attempts = 5) {
  function(...) {
    this_env <- environment()
    for(i in seq_len(max_attempts)) {
      ok <- tryCatch({
          assign("result", fn(...), envir = this_env)
          TRUE
        },
        error = function(e) {
          FALSE
        }
      )
      if(ok) {
        return(this_env$result)
      }
    }
    msg <- sprintf(
      "%s failed after %d tries; returning NULL.",
      deparse(match.call()),
      max_attempts
    )
    warning(msg)
    NULL
  }
}

Try it out on this simple function that generates a random number, and throws an error if it is too small.

random <- function(lo, hi) {
  y <- runif(1, lo, hi)
  if(y < 0.75) {
    stop("y is less than 0.75")
  }
  y
}
safe_random <- safely(random)
safe_random() # will sometimes work, will sometimes return NULL
safe_random(0, 10) # will usually work

In you case, you want to modify the google_geocode() function.

safe_google_geocode <- safely(google_geocode)

Then loop over addresses calling this.

geocodes <- lapply( # purrr::map() is an alternative
  addresses,
  safe_google_geocode,
  key = "myapikey", 
  language = "ru", 
  simplify = TRUE
)