Use tryCatch with a while loop in R

As a general note - your code is kind of weird. I would recommend either a for loop, or possibly better, a function that does this stuff. But you can make your loop work.

# A minimal working version
library(RJSONIO)
options(stringsAsFactors = FALSE)

# Create a data frame with the example data
geocode_area <- data.frame(X = c("-122.425891675136","-122.42436302145","-122.426995326766","-122.438737622757","abc"),
                           Y = c("37.7745985956747","37.8004143219856","37.8008726327692","37.7715411720578","zsads"),
                           Block = c(0,0,0,0,0))

# Your old function, unchanged
getUrl <- function(x,y) {

    root <- "http://geocoding.geo.census.gov/geocoder/geographies/coordinates?"
    u <- paste0(root,"x=", x,"&y=", y,"&benchmark=4&vintage=4&format=json")
    return(URLencode(u))
}

# Getting the length parameter
length <- nrow(geocode_area)
i <- 1
while(i <= length) {

    x_cord <- geocode_area$X[i]
    y_cord <- geocode_area$Y[i]
    target <- getUrl(x_cord,y_cord)

    # Here be new code
    # Do a try(), with silent = TRUE, which suppresses outputs to STDERR
    # In principle, this is dangerous - a better approach is to strip out the offending data before invoking it
    # Errors are, after all, there for a reason
    dat <- try(fromJSON(target),silent = TRUE)

    # Now, we conditionally complete the next steps
    # If the class of dat is not a try-error, perform your normal operations
    # Otherwise, bypass and print a note to the console
    if(class(dat) != "try-error") {

        geocode_area$Block[i] <- dat$result$geographies$`2010 Census Blocks`[[1]]$BLOCK
        print(paste(i/length*100,"% completed",sep=""))
        print(dat$result$geographies$`2010 Census Blocks`[[1]]$BLOCK)
    } else if (class(dat) == "try-error") {print("Error encountered, bypassing")}
i <- i + 1
}

EDITED TO ADD: Obviously, this uses try() instead of tryCatch(). However, as the poster ended up using try() and this may represent a different way to do it, I thought I'd leave it up.

With some help from the kind individuals who commented I got to an answer. Essentially avoid the while loop altogether and use a for loop instead.

I am not sure entirely why a while loop would not work, I think that it might be something to do with a difficulty in updating the loop counter in the finally block.

Indeed, for the specific problem I had I did not even need a tryCatch at all. I would have used the error block to set the value I am updating to 0, however it is possible to just reset the input variable to 0 in the for loop. This is required, because otherwise the value of the previous iteration would be kept. It is also worth nothing that I needed to wrap the try expression in {}, because there were multiple expressions.

Code below. I hope this helps someone!

for(i in 1:length) {
try(
{x_cord <- geocode_area$X[i]
y_cord <- geocode_area$Y[i]
target <- getUrl(x_cord,y_cord)
dat <- fromJSON(target)
geocode_area$Block[i] <- dat$result$geographies$`2010 Census Blocks`[[1]]$BLOCK
print(paste(i/length*100,"% completed",sep=""))
print(dat$result$geographies$`2010 Census Blocks`[[1]]$BLOCK)
dat$result$geographies$`2010 Census Blocks`[[1]]$BLOCK <- 0}
)
}