Handle Error In R
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If Error In R
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R Catch Error And Continue
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> inputs = list(1, 2, 4, -5, 'oops', 0, 10) > for(input in inputs) { + print(paste("log of", input, "=", log(input))) + } [1] "log of 1 = 0" [1] "log of 2 = 0.693147180559945" [1] "log of 4 = 1.38629436111989" [1] "log of -5 = NaN" Error in log(input) : Non-numeric argument to mathematical function In addition: Warning message: In log(input) : NaNs produced
The loop handled the negative arguments more or less gracefully (depending on how you feel about NaN), but crashed on the non-numeric argument, and didn't finish the list of inputs. How are we going to handle this? The try block The most straightforward way is to wrap our problematic call in a try block:
> for(input in inputs) { + try(print(paste("log of", input, "=", log(input)))) + } [[1] "log of 1 = 0" [1] "log of 2 = 0.693147180559945" [1] "log of 4 = 1.38629436111989" [1] "log of -5 = NaN" ErroR -- Basic error Handing with tryCatch() Posted on December 7, 2011 by Jonathan Callahan This entry is part 4 of 20 in the series Using RThe R language definition section on Exception r continue loop if error Handling describes a very few basics about exceptions in R but is of little r simpleerror use to anyone trying to write robust code that can recover gracefully in the face of errors. In fact, if you do r trycatch return value a little searching you will find that quite a few people have read through the ?tryCatch documentation but come away just as confused as when they started. In this post we'll try to clarify a https://www.r-bloggers.com/error-handling-in-r/ few things and describe how R's error handling functions can be used to write code that functions similarly to Java's try-catch-finally construct. List of error handling functions Without any simple documentation on the subject, the first thing we need is a list of the functions involved in error handling. With this list in hand we can then start up R and type ?function_of_interest to read associated documentation or function_of_interest [without http://mazamascience.com/WorkingWithData/?p=912 the ‘()'] to see how the function is implemented. Here is a minimal list of functions that anyone writing error handling code should read up on: warning(…) -- generates warnings stop(…) -- generates errors suppressWarnings(expr) -- evaluates expression and ignores any warnings tryCatch(…) -- evaluates code and assigns exception handlers Other functions exist that relate to error handling but the above are enough to get started. (The documentation for these functions will lead to all the other error-related functions for any RTFM enthusiasts.) R does try-catch-finally differently In case you hadn't noticed, R does a lot of things differently from most other programming languages. Java and Python and C and all other languages covered in Wikipedia's excellent page on Exception handling syntax use language statements to enable try-catch-finally. R, needing to be different, uses a function. But the tryCatch() function actually looks a lot like other languages' try-catch syntax if you format it properly: result = tryCatch({ expr }, warning = function(w) { warning-handler-code }, error = function(e) { error-handler-code }, finally = { cleanup-code } 123456789 result = tryCatch({ expr}, warning = function(w) { warning-handler-code}, error = function(e) { error-handler-code}, finally = { cleanup-code} In tryCatch() there are two ‘conditions' that can be handled: ‘warnings'
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company http://stackoverflow.com/questions/12193779/how-to-write-trycatch-in-r Business Learn more about hiring developers or posting ads with us Stack Overflow Questions http://www1.maths.lth.se/help/R/ExceptionHandlingInR/ Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up How to write trycatch in R up vote 121 down vote favorite 86 I want to write trycatch code to deal error in with error in downloading from web. url <- c( "http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html", "http://en.wikipedia.org/wiki/Xz") y <- mapply(readLines, con=url) These two statements run successfully. Below, I create a non-exist web address: url <- c("xxxxx", "http://en.wikipedia.org/wiki/Xz") url[1] does not exist. How does one write a trycatch loop (function) so that: When the url is wrong, the output will be: "web url is wrong, can't get". When the url is wrong, the code does not stop, but continues to download error in r until the end of list of urls? r r-faq share|improve this question edited Sep 10 '14 at 19:56 Andrie 102k19255332 asked Aug 30 '12 at 9:27 Dd Pp 8622714 add a comment| 3 Answers 3 active oldest votes up vote 252 down vote accepted +50 Well then: welcome to the R world ;-) Here you go Setting up the code urls <- c( "http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html", "http://en.wikipedia.org/wiki/Xz", "xxxxx" ) readUrl <- function(url) { out <- tryCatch( { # Just to highlight: if you want to use more than one # R expression in the "try" part then you'll have to # use curly brackets. # 'tryCatch()' will return the last evaluated expression # in case the "try" part was completed successfully message("This is the 'try' part") readLines(con=url, warn=FALSE) # The return value of `readLines()` is the actual value # that will be returned in case there is no condition # (e.g. warning or error). # You don't need to state the return value via `return()` as code # in the "try" part is not wrapped insided a function (unlike that # for the condition handlers for warnings and error below) }, error=function(cond) { message(paste("URL does not seem to exist:", url)) message("Here's the original error message:") message(cond) # Choose a return value in case of error return(NA) }, warning=function(cond) { message(paste("URL cause