error handling

knitr::opts_chunk$set(echo = TRUE)
knitr::opts_chunk$set(message = FALSE)

stderr messages

stop()

• halts execution

logit <- function(x){
  if( any(x < 0 | x > 1) ){
    stop('x not between 0 and 1')
  }
  log(x / (1 - x) )
}
logit(0.4)
# error message without stop(): NaNs produced[1] NaN
# error message with stop(): Error in logit(-1) : x not between 0 and 1

warning()

• still allows function to execute,
• but throws a warning

# using ifelse() + warning
# output NA and warning
logit <- function(x){
  x = ifelse( any(x < 0 | x > 1), NA, x )
  if(any(is.na(x))){
    warning('x not between 0 and 1')
  }
  log(x / (1 - x) )
}
logit(-1)

print(), paste(), cat(), message(), warning(), stop()

https://stackoverflow.com/questions/36699272/why-is-message-a-better-choice-than-print-in-r-for-writing-a-package
print(), cat() sends output to stdout
message(), warning(), stop() sends output to stderr

print() - cannot concatenate, prints [ x ] if multiple elements printed; but prints newline automatically
paste() - allows concatenate
cat() - allows concatenate, no [ x ]; but needs to specify newline manually

message() - allows concat, no [ x ]; also needs to specify newline manually; but can be used with tryCatch()
warning() - have problems when building packages, use message() instead
stop() -

exceptions handling

• http://adv-r.had.co.nz/Exceptions-Debugging.html

try()

• allows execution to continue after error

f1 <- function(x) {
  log(x)
  return(c(log(x),10))
}
# f1("x")
# Error in log(x) : non-numeric argument to mathematical function

f1 <- function(x) {
  t <- try({            # capture try() as a variable (if failure, returns an invisible "try-error" object)
    a <- 1              # multiple expressions in try({})
    b <- a + x
    log(b)
    }, silent = TRUE)
  
  return(c(t,class(t),10))       # return try() results and outside results
}
f1("x")
f1(2)

tryCatch()

• take different actions depend on warnings,
• by mapping conditions to “handlers”
• built-in handlers = error, warning, message, interrupt
• catch-all handler = condition
• handlers return a value, or create a more informative error message

tryCatch(message("hello\n"), message=function(e){cat("goodbye\n")})

withCallingHandlers()

• …

EOF