Functions as Arguments in R

Sometimes, silly small things about code I write just delight me. There are lots of ways to time things in R. ¹ Tools like microbenchmark are great for profiling code, but what I do all the time is log how long database queries that are scheduled to run each night are taking.

It is really easy to use calls to Sys.time and difftime when working interactively, but I didn’t want to pepper all of my code with the same log statements all over the place. So instead, I wrote a function.

Almost all of timing is straightforward to even a novice R user. I record what time it is using Sys.time, do a little formatting work to make things look the way I want for reading logs, and pass in an optional message.

The form of timing was easy for me to sketch out: ²

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timing <- function(STUFF, msg = '') {
  start_time <- format(Sys.time(), '%a %b %d %X %Y')
  start_msg <- paste('Starting', msg,
                     'at:', start_time, '\n')
  cat(start_msg)
  # Call my function here
  end_time <- format(Sys.time(), '%a %b %d %X %Y')
  end_msg <- paste('Completed', 'at:', end_time, '\n')
  cat(end_msg)
  elapsed <- difftime(as.POSIXlt(end_time, format = '%a %b %d %X %Y'),
                      as.POSIXlt(start_time, format = '%a %b %d %X %Y'))
  cat('Elapsed Time: ', format(unclass(elapsed), digits = getOption('digits')),
      ' ', attr(elapsed, 'units'), '\n\n\n', sep = '')
  result
}

The thing I needed to learn when I wrote timing a few years back was how to fill in STUFF and # Call my function here.

Did you know that you can pass a function as an argument in another function in R? I had been using *apply with its FUN argument all over the place, but never really thought about it until I wrote timing. Of course in R you can pass a function name, and I even know how to pass arguments to that function– just like apply, just declare a function with the magical ... and pass that along to the fucntion being passed in.

So from there, it was clear to see how I’d want my function declartion to look. It would definitely have the form function(f, ..., msg = ''), where f was some function and ... were the arguments for that function. What I didn’t know was how to properly call that function. Normally, I’d write something like mean(...), but I don’t know what f is in this case!

As it turns out, the first thing I tried worked, much to my surprise. R actually makes this super easy– you can just write f(...), and f will be replaced with whatever the argument is to f! This just tickles me. It’s stupid elegant to my eyes.

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timing <- function(f, ..., msg = '') {
  start_time <- format(Sys.time(), '%a %b %d %X %Y')
  start_msg <- paste('Starting', msg,
                     'at:', start_time, '\n')
  cat(start_msg)
  x <- f(...)
  end_time <- format(Sys.time(), '%a %b %d %X %Y')
  end_msg <- paste('Completed', 'at:', end_time, '\n')
  cat(end_msg)
  elapsed <- difftime(as.POSIXlt(end_time, format = '%a %b %d %X %Y'),
                      as.POSIXlt(start_time, format = '%a %b %d %X %Y'))
  cat('Elapsed Time: ', format(unclass(elapsed), digits = getOption('digits')),
      ' ', attr(elapsed, 'units'), '\n\n\n', sep = '')
  x
}

Now I can monitor the run time of any function by wrapping it in timing. For example:

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timing(read.csv, 'my_big_file.csv', header = TRUE, stringsAsFactors = FALSE)`

And here’s an example of the output from a job that ran this morning:

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Starting queries/accounts.sql at: Mon May 29 06:24:12 AM 2017
Completed at: Mon May 29 06:24:41 AM 2017
Elapsed Time: 29 secs