Why are lubridate functions so slow when compared with as.POSIXct?

Question

As the title goes. Why is the lubridate function so much slower?

library(lubridate) library(microbenchmark)  Dates <- sample(c(dates = format(seq(ISOdate(2010,1,1), by='day', length=365), format='%d-%m-%Y')), 50000, replace = TRUE)  microbenchmark(as.POSIXct(Dates, format = "%d-%b-%Y %H:%M:%S", tz = "GMT"), times = 100) microbenchmark(dmy(Dates, tz ="GMT"), times = 100)  Unit: milliseconds expr                                                            min         lq          median      uq          max 1 as.POSIXct(Dates, format = "%d-%b-%Y %H:%M:%S", tz = "GMT")   103.1902    104.3247    108.675     109.2632    149.871 2 dmy(Dates, tz = "GMT")                                        184.4871    194.1504    197.8422    214.3771    268.4911 

Answer 1

For the same reason cars are slow in comparison to riding on top of rockets. The added ease of use and safety make cars much slower than a rocket but you're less likely to get blown up and it's easier to start, steer, and brake a car. However, in the right situation (e.g., I need to get to the moon) the rocket is the right tool for the job. Now if someone invented a car with a rocket strapped to the roof we'd have something.

Start with looking at what dmy is doing and you'll see the difference for the speed (by the way from your bechmarks I wouldn't say that lubridate is that much slower as these are in milliseconds):

dmy #type this into the command line and you get:

>dmy function (..., quiet = FALSE, tz = "UTC")  {     dates <- unlist(list(...))     parse_date(num_to_date(dates), make_format("dmy"), quiet = quiet,          tz = tz) } <environment: namespace:lubridate> 

Right away I see parse_date and num_to_date and make_format. Makes one wonder what all these guys are. Let's see:

parse_date

> parse_date function (x, formats, quiet = FALSE, seps = find_separator(x),      tz = "UTC")  {     fmt <- guess_format(head(x, 100), formats, seps, quiet)     parsed <- as.POSIXct(strptime(x, fmt, tz = tz))     if (length(x) > 2 & !quiet)          message("Using date format ", fmt, ".")     failed <- sum(is.na(parsed)) - sum(is.na(x))     if (failed > 0) {         message(failed, " failed to parse.")     }     parsed } <environment: namespace:lubridate> 

num_to_date

> getAnywhere(num_to_date) A single object matching ‘num_to_date’ was found It was found in the following places   namespace:lubridate with value  function (x)  {     if (is.numeric(x)) {         x <- as.character(x)         x <- paste(ifelse(nchar(x)%%2 == 1, "0", ""), x, sep = "")     }     x } <environment: namespace:lubridate> 

make_format

> getAnywhere(make_format) A single object matching ‘make_format’ was found It was found in the following places   namespace:lubridate with value  function (order)  {     order <- strsplit(order, "")[[1]]     formats <- list(d = "%d", m = c("%m", "%b"), y = c("%y",          "%Y"))[order]     grid <- expand.grid(formats, KEEP.OUT.ATTRS = FALSE, stringsAsFactors = FALSE)     lapply(1:nrow(grid), function(i) unname(unlist(grid[i, ]))) } <environment: namespace:lubridate> 

Wow we got strsplit-ting, expand-ing.grid-s, paste-ing, ifelse-ing, unname-ing etc. plus a Whole Lotta Error Checking Going On (play on the Zep song). So what we have here is some nice syntactic sugar. Mmmmm tasty but it comes with a price, speed.

Compare that to as.POSIXct:

getAnywhere(as.POSIXct)  #tells us to use methods to see the business methods('as.POSIXct')    #tells us all the business as.POSIXct.date          #what I believe your code is using (I don't use dates though) 

There's a lot more Internal coding and less error checking going on with as.POSIXct So you have to ask do I want ease and safety or speed and power? Depends on the job.

Answer 2

@Tyler's answer is correct. Here's some more info including a tip on making lubridate faster - from the help file:

" Lubridate has an inbuilt very fast POSIX parser, ported from the fasttime package by Simon Urbanek. This functionality is as yet optional and could be activated with options(lubridate.fasttime = TRUE). Lubridate will automatically detect POSIX strings and use fast parser instead of the default strptime utility. "