Using tryCatch function of R in a loop

Question

I want to use tryCatch function in a loop which sometimes works but sometimes it does not, and I do not know where does it have errors to solve the problem and create a loop which always works. So I want it just to neglect the error and go to next round. To simplify the problem, for example, I have this x, and I want to have sqrt of each value. I write:

x <- c(1, 2, "a", 4)
for (i in x) {
    y <- tryCatch(print(sqrt(i)) , error = function(e) { return(0) } )
    if (y==0) { 
        print("NOT POSSIBLE")
        next
    }
}

I suppose this code should give me this answer:

[1] 1
[1] 1.414214
[1] "NOT POSSIBLE"
[1] 2

but it gives me this:

[1] "NOT POSSIBLE"
[1] "NOT POSSIBLE"
[1] "NOT POSSIBLE"
[1] "NOT POSSIBLE"

I could not find anywhere explaining that why this happens. Why this function does not apply to each round of the loop separately and what can I do about it?

If you have at least one character element in a vector, all the elements becomes character. Instead you may need a list — akrun
– akrun, Commented Apr 22, 2018 at 13:45

akrun · Accepted Answer · 2018-04-22 13:46:25Z

4

The reason is a that one of the elements in the vector is character and a vector cannot have mixed types. So, it is coerced to character. Instead we should have a list where each element can have different types

x <- list(1,2,"a" , 4)

Now, running the OP' code gives

for (i in x) {
   y <- tryCatch(print(sqrt(i)) , error= function(e) {return(0)}  )
   if (y==0) {print("NOT POSSIBLE")
              next}
 }
#[1] 1
#[1] 1.414214
#[1] "NOT POSSIBLE"
#[1] 2

If we can use only a vector, then there should be a provision to convert it to numeric within the loop, but it would also return an NA for the third element as

as.numeric('a')
#[1] NA

Warning message: NAs introduced by coercion

and ends the for loop

answered Apr 22, 2018 at 13:46

akrun

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