tryCatch block in R, change value of outer variable

Question

Here is my code. It produces infinite loop, because value of something variable does not change within captured error. Is it supposed to be this way? How can I fix it so that value of something changes to FALSE?

something <- TRUE
counter <- 1
while(something){
  print(counter)  
  tryTest = tryCatch(
    {
      arima(rep(1,3), order = c(1,0,0))
    }, 
    warning = function(w) {

      print('this is warning')
      print(w)
    },
    error = function(e) {
      something <- FALSE
      print('this is error')
      print(e)
    },
    finally = {}
  )
  counter <- (counter +1)
}

Define your warning function as: `warning = function(w) { something <<- FALSE print('this is warning') print(w) } ` ,where "<<-" - *"This operator assigns the variable to the parent scope. It first looks for the variable in the parent scope and if it can’t find one then it creates a new one in the parent scope."* - http://www.studytrails.com/r/core/assignmentoperator/ — nadizan, Jan 22 '18 at 22:18
@nadizan, I just tried it and it does not work, still infinite loop — user1700890, Jan 22 '18 at 22:32
@nadizan the problem with your suggestion is that the parent scope of the lambda is still inside the tryCatch function and is not the outer scope. — Mosquite, Jan 22 '18 at 22:55
As a side note. On error, the return value of the error function is returned by `tryCatch` on error. you could always use that to stop your loop. Condition on `tryTest` being non NULL to continue your loop. Just make sure to return something non null on a warning. — Mosquite, Jan 22 '18 at 22:59
@user1700890, this works completely fine for me. `something <- TRUE counter <- 1 while(something){ print(counter) tryTest = tryCatch( { arima(rep(1,3), order = c(1,0,0)) }, warning = function(w) { something <<- FALSE print('this is warning') print(w) }, error = function(e) { something <- FALSE print('this is error') print(e) }, finally = {} ) counter <- (counter +1) }` — nadizan, Jan 23 '18 at 10:36
@nadizan you are right. Thank you! Funny how I wrote about catching error, but you put code into warning block. This arima generates both error and warning, and if you add `something <<-FALSE` to error block, it would not work. This is I guess a bit different issue: https://stackoverflow.com/questions/48391287/trycatch-block-in-r-returning-variable?noredirect=1#comment83770717_48391287 — user1700890, Jan 23 '18 at 15:18
See also: https://stackoverflow.com/questions/38482937/variable-scope-in-r-trycatch-block-is-necessary-to-change-local-variable-de — climatestudent, Aug 16 '21 at 12:00

Mosquite · Accepted Answer · 2021-09-16T17:11:23.457

This happens because the environment of something in the outer code is different from the environment of something in your lambda:

function(e) {
    something <- FALSE
    print('this is error')
    print(e)
}

So setting something <- FALSE in your lambda actually sets a different something then in the outer code. To fix this you can either 1) make something a global variable or 2) create an environment to use something in.

1)

assign("something", TRUE, env=globalenv())

to set the variable and

get("something", env=globalenv())

to access the variable.

You can also set something inside your lambda in the same way:

assign("something", FALSE, env=globalenv())

or

2)

First create a new variable:

env=new.env()

Then set and access your variable in a similar way as before:

assign("something", TRUE, env=env)
get("something", env=env)

You can assign something inside your lambda with:

assign("something", FALSE, env=env)

Using env is possible because R copies variables to child environments. However if you set a variable in a child environment (like when you did `somethi

tryCatch block in R, change value of outer variable

1 Answers1