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Reputation: 1190

Error handling in R: tryCatch() syntax

I am attempting to get my head around error handling in R. From reading the docs, I cannot find a clear definition of the correct syntax.

Here is a self contained example:

foo <- function(dat) {
    print('Calling foo()')
    print(dat[,'r1'])
    print(dat[,'r2'])
    print(dat[,'r3'])
    print('Finished foo()')
    return(dat$r3)
}

bar <- function(dat) {
    print('Calling bar()')
    print(dat[,'r1'])
    print(dat[,'r2'])
    print('Finished bar()')
    return(dat$r2)
}

d1 <- data.frame(r1 = 1:10, r2 = 11:20, r3 = 21:30)
d2 <- d1[c("r1", "r2")]

print('Starting')
print('trying d1')
myVal1 <- tryCatch(foo(d1), error = bar(d1))
print('finished d1')
print('trying d2')
myVal2 <- tryCatch(foo(d2), error = bar(d2))
print('finished d2')
print('Done')

Expected output:

[1] "Starting"
[1] "trying d1"
[1] "Calling foo()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
 [1] 21 22 23 24 25 26 27 28 29 30
[1] "Finished foo()"
[1] "finished d1"
[1] "trying d2"
[1] "Calling bar()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
[1] "Finished bar()"
[1] "finished d2"
[1] "Done"

Actual output:

[1] "Starting"
[1] "trying d1"
[1] "Calling bar()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
[1] "Finished bar()"
[1] "Calling foo()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
 [1] 21 22 23 24 25 26 27 28 29 30
[1] "Finished foo()"
[1] "finished d1"
[1] "trying d2"
[1] "Calling bar()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
[1] "Finished bar()"
[1] "Calling foo()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
Error in tryCatchOne(expr, names, parentenv, handlers[[1L]]) : 
  attempt to apply non-function

My questions:

  1. Why does this tryCatch() call end up running both functions?

  2. Why do I get the error reported above?

  3. Is my syntax of tryCatch(someFunction(), error = someOtherFunction()) incorrect?

Thanks!

Upvotes: 1

Views: 5678

Answers (1)

Matthew Lundberg
Matthew Lundberg

Reputation: 42649

error wants a function. You have passed an expression that is a result of evaluating a function on an argument. Also, error is not passing the data frame to bar. The error text is passed.

Replacing this:

myVal2 <- tryCatch(foo(d2), error = bar(d2))

with this:

myVal2 <- tryCatch(foo(d2), error = function(e) { cat('In error handler\n'); print(e); e })

results in this behavior:

[1] "Calling foo()"
 [1]  1  2  3  4  5  6  7  8  9 10
 [1] 11 12 13 14 15 16 17 18 19 20
In error handler
<simpleError in `[.data.frame`(dat, , "r3"): undefined columns selected>

Note that the handler is called. And myVal2 is set to the result of the error handler:

> myVal2
<simpleError in `[.data.frame`(dat, , "r3"): undefined columns selected>

Upvotes: 5

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