Wrapping a while loop inside a try and repeat function to run the while loop again if a specific error occurs

Question

I have a series of functions which goes to a website and collects data. Sometimes the website returns a 404 error and my code breaks. It could take 10 minutes of processing until I get a 404 error, or the code (more often then not) runs without the 404 error.

I have the following code:

linkToStopAt = as.character(unique(currentData$linkURL)[1])
myLinksToSearchOver = as.character(unique(currentData$page))
tmp = NULL
i <- 1
out_lst = list()
while(i <= length(myLinksToSearchOver)){
  print(paste("Processing page: ", i))
  tmp <- possible_collectPageData(myLinksToSearchOver[i]) %>% 
    add_column(page = myLinksToSearchOver[i])
  if(linkToStopAt %in% tmp$linkURL)
  {
    print(paste("We stopped at: ", i))
    break
  }
  out_lst[[i]] <- tmp
  i <- i + 1
}

Broken down as:

linkToStopAt = as.character(unique(currentData$linkURL)[1]) gives me a single URL where the while loops will break if it see this URL

myLinksToSearchOver = as.character(unique(currentData$page)) gives me multiple links in which the while loop will search over, once it finds the linkToStopAt on one of these links, the while loop breaks.

tmp <- possible_collectPageData(myLinksToSearchOver[i]) %>% add_column(page = myLinksToSearchOver[i]) This is a big function, which relies on many other functions...

######################################################

So, the while loop runs until it finds a link linkToStopAt on one of the pages from myLinksToSearchOver. The function possible_collectPageData just does all my scraping/data processing etc. Each page from myLinksToSearchOver is stored in out_lst[[i]] <- tmp.

I recieve a specific error "Error in if (nrow(df) != nrow(.data)) { : argumento tiene longitud cero" in the console sometimes.

What I want to do, is something like:

repeat {
  tmpCollectData <- try(while("ALL-MY-WHILE-LOOP-HERE??")) #try(execute(f))
  if (!(inherits(tmpCollectData, "Error in if (nrow(df) != nrow(.data)) { : argumento tiene longitud cero"))) 
    break
}

Where, if the while loop breaks with that error, just run it all again, setting tmp = NULL, i = 1, out_list = list() etc. (Basically start again, I can do this manually by just re-executing the code again)

langtang · Accepted Answer · 2022-07-20 18:21:44Z

1

You could create a function that does your work, and then wrap the call to that function in try(), with silent=TRUE. Then place that in a while(TRUE) loop, breaking out if get_data() does NOT return an error:

Function to do your work

get_data <- function(links, stoplink) {
  
  i=1
  out_lst=list()
  while(i <= length(links)){
    print(paste("Processing page: ", i))
    
    tmp = possible_collectPageData(links[i]) %>% add_column(page = links[i])
    
    if(stoplink %in% tmp$linkURL) {
      print(paste("We stopped at: ", i))
      break
    }
    
    out_lst[[i]] <- tmp
    i <- i + 1
  }
  return(out_lst)
}

Infinite loop that gets broken if result does not have any error.

while(TRUE) {
  result = try(get_data(myLinksToSearchOver, linkToStopAt), silent=T)
  if(!"try-error" %in% class(result)) break
}

edited Jul 20, 2022 at 18:21

answered Jul 20, 2022 at 17:45

langtang

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2 Comments

user113156 Over a year ago

Thanks for your answer. I am not looking to return a NULL value, if the error occurs, just forget everything and start the while loop again. The error is just a 404 error because something strange occured and the Firefox browser closed.

langtang Over a year ago

Okay, perhaps my edit helps?

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