fast conversion from string time to milliseconds

Question

For a vector or list of times, I'd like to go from a string time, e.g. 12:34:56.789 to milliseconds from midnight, which would be equal to 45296789.

This is what I do now:

toms = function(time) {
  sapply(strsplit(time, ':', fixed = T),
         function(x) sum(as.numeric(x)*c(3600000,60000,1000)))
}

and would like to do it faster.

Here's an example data set for benchmarking:

times = rep('12:34:56.789', 1e6)

system.time(toms(times))
#   user  system elapsed 
#   9.00    0.04    9.05

Answer 1

You could use the fasttime package, which seems to be about an order of magnitude faster.

library(fasttime)
fasttoms <- function(time) {
  1000*unclass(fastPOSIXct(paste("1970-01-01",time)))
}
times <- rep('12:34:56.789', 1e6)
system.time(toms(times))
#   user  system elapsed 
#   6.61    0.03    6.68 
system.time(fasttoms(times))
#   user  system elapsed 
#   0.53    0.00    0.53
identical(fasttoms(times),toms(times))
# [1] TRUE