How to write trycatch in R

Question

I want to write trycatch code to deal with error in downloading from the web   url  lt - c       http   stat ethz ch R-manual R-devel library base html connections html        http   en wikipedia org wiki Xz   y  lt - mapply readLines  con url    These two statements run successfully  Below  I create a non-exist web address   url  lt - 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 until the end of the list of URLs

User · Answer

Since I just lost two days of my life trying to solve for tryCatch for an irr function, I thought I should share my wisdom (and what is missing). FYI - irr is an actual function from FinCal in this case where got errors in a few cases on a large data set.

Set up tryCatch as part of a function. For example:

irr2 <- function (x) {
  out <- tryCatch(irr(x), error = function(e) NULL)
  return(out)
}

For the error (or warning) to work, you actually need to create a function. I originally for error part just wrote error = return(NULL) and ALL values came back null.
Remember to create a sub-output (like my "out") and to return(out).

User · Answer

Well then  welcome to the R world  -  Here you go Setting up the code urls  lt - c       quot http   stat ethz ch R-manual R-devel library base html connections html quot        quot http   en wikipedia org wiki Xz quot        quot xxxxx quot    readUrl  lt - function url        out  lt - tryCatch                          Just to highlight  if you want to use more than one                R expression in the  quot try quot  part then you ll have to                use curly brackets                 tryCatch    will return the last evaluated expression                in case the  quot try quot  part was completed successfully              message  quot This is the  try  part quot                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  quot try quot  part is not wrapped inside a function  unlike that               for the condition handlers for warnings and error below                     error function cond                message paste  quot URL does not seem to exist  quot   url               message  quot Here s the original error message  quot               message cond                Choose a return value in case of error             return NA                     warning function cond                message paste  quot URL caused a warning  quot   url               message  quot Here s the original warning message  quot               message cond                Choose a return value in case of warning             return NULL                     finally             NOTE            Here goes everything that should be executed at the end            regardless of success or error            If you want more than one expression to be executed  then you            need to wrap them in curly brackets          otherwise you could           just have written  finally  lt expression gt                message paste  quot Processed URL  quot   url               message  quot Some other message at the end quot                           return out     Applying the code  gt  y  lt - lapply urls  readUrl  Processed URL  http   stat ethz ch R-manual R-devel library base html connections html Some other message at the end Processed URL  http   en wikipedia org wiki Xz Some other message at the end URL does not seem to exist  xxxxx Here s the original error message  cannot open the connection Processed URL  xxxxx Some other message at the end Warning message  In file con   quot r quot     cannot open file  xxxxx   No such file or directory  Investigating the output  gt  head y  1     1   quot  lt  DOCTYPE html PUBLIC   quot -  W3C  DTD HTML 4 01 Transitional  EN  quot  gt  quot         2   quot  lt html gt  lt head gt  lt title gt R  Functions to Manipulate Connections lt  title gt  quot         3   quot  lt meta http-equiv   quot Content-Type  quot  content   quot text html  charset utf-8  quot  gt  quot   4   quot  lt link rel   quot stylesheet  quot  type   quot text css  quot  href   quot R css  quot  gt  quot                5   quot  lt  head gt  lt body gt  quot                                                             6   quot  quot        gt  length y   1  3   gt  y  3    1  NA  Additional remarks tryCatch tryCatch returns the value associated to executing expr unless there s an error or a warning  In this case  specific return values  see return NA  above  can be specified by supplying a respective handler function  see arguments error and warning in  tryCatch   These can be functions that already exist  but you can also define them within tryCatch    as I did above   The implications of choosing specific return values of the handler functions As we ve specified that NA should be returned in case of error  the third element in y is NA  If we d have chosen NULL to be the return value  the length of y would just have been 2 instead of 3 as lapply   will simply  quot ignore quot  return values that are NULL  Also note that if you don t specify an explicit return value via return    the handler functions will return NULL  i e  in case of an error or a warning condition    quot Undesired quot  warning message As warn FALSE doesn t seem to have any effect  an alternative way to suppress the warning  which in this case isn t really of interest  is to use suppressWarnings readLines con url    instead of readLines con url  warn FALSE   Multiple expressions Note that you can also place multiple expressions in the  quot actual expressions part quot   argument expr of tryCatch    if you wrap them in curly brackets  just like I illustrated in the finally part

User · Answer

Here goes a straightforward example     Do something  or tell me why it failed my update function  lt - function x       tryCatch            This is what I want to do                      y   x   2         return y                           but if an error occurs  tell me what happened           error function error message                message  This is my custom message                message  And below is the error message from R                message error message              return NA                      If you also want to capture a  warning   just add warning  similar to the error  part

User · Answer

tryCatch has a slightly complex syntax structure  However  once we understand the 4 parts which constitute a complete tryCatch call as shown below  it becomes easy to remember   expr   Required  R code s  to be evaluated  error    Optional  What should run if an error occured while evaluating the codes in expr  warning    Optional  What should run if a warning occured while evaluating the codes in expr  finally    Optional  What should run just before quitting the tryCatch call  irrespective of if expr ran successfully  with an error  or with a warning  tryCatch      expr               Your code              goes here                             error   function e               Optional            Do this if an error is caught               warning   function w              Optional            Do this if an warning is caught               finally                Optional            Do this at the end before quitting the tryCatch structure              Thus  a toy example  to calculate the log of a value might look like   log calculator  lt - function x       tryCatch          expr                 message log x               message  Successfully executed the log x  call                       error   function e               message  Caught an error                print e                     warning   function w               message  Caught an warning                print w                     finally                 message  All done  quitting                            Now  running three cases   A valid case  log calculator 10    2 30258509299405   Successfully executed the log x  call    All done  quitting    A  warning  case  log calculator -10    Caught an warning     lt simpleWarning in log x   NaNs produced gt    All done  quitting    An  error  case  log calculator  log me     Caught an error     lt simpleError in log x   non-numeric argument to mathematical function gt    All done  quitting    I ve written about some useful use-cases which I use regularly  Find more details here  https   rsangole netlify com post try-catch   Hope this is helpful

User · Answer

R uses functions for implementing try-catch block   The syntax somewhat looks like this   result   tryCatch       expr    warning   function warning condition        warning-handler-code    error   function error condition        error-handler-code    finally       cleanup-code      In tryCatch   there are two    conditions    that can be handled     warnings    and    errors     The important thing to understand when writing each block of code is the state of execution and the scope   source

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