Continuing a while loop after error in R

Question

I am trying to use a while loop to download a bunch of JSON files.

They are numbered from 255 to 1. However, some of them are missing (for example, 238.json does not exist).

scheduleurl <- "http://blah.blahblah.com/schedulejsonfile="
i <- 255
while ( i > 0) {

    last = paste0(as.character(i), ".json")
    path = "/Users/User/Desktop/Temp"
    fullpath = paste0(path, last)
    ithscheduleurl <- paste0(scheduleurl, as.character(i))
    download.file(ithscheduleurl, fullpath)
    i <- i - 1
    }

I basically want to write my while loop such that if it encounters a nonexisting file (as it will when i = 238), it basically continues to 237 instead of stopping.

I tried the tryCatch() function this way, but it didn't work (keeps trying the same URL) :

while ( i > 0) {
    possibleError <- tryCatch({
    last = paste0(as.character(i), ".json")
    path = "/Users/dlopez/Desktop/Temp"
    fullpath = paste0(path, last)
    ithscheduleurl <- paste0(scheduleurl, as.character(i))
    download.file(ithscheduleurl, fullpath)
    i <- i - 1}
    , error=function(e) e) 

    if(inherits(possibleError, "error")) {
            next
    }
}

Any help would be appreciated!

Answer 1

url.exists from the RCurl package should do the trick.

library(RCurl)

while ( i > 0) {

last = paste0(as.character(i), ".json")
path = "/Users/User/Desktop/Temp"
fullpath = paste0(path, last)
ithscheduleurl <- paste0(scheduleurl, as.character(i))
if (url.exists(ithscheduleurl)) {
download.file(ithscheduleurl, fullpath)
}
i <- i - 1
}