R extract time components from semi-standard strings
posixct, r, string, time
Solution
`difftime` objects are time duration objects that can be added to either `POSIXct` or `POSIXlt` objects. Maybe you want to use this instead of `POSIXlt`?
Regarding the conversion from strings to time objects, you could do something like this:
data <- data.frame(time.string = c(
"1 d 1 h",
"30 m 10 s",
"1 d 2 h 3 m 4 s",
"2 h 3 m 4 s",
"10 d 20 h 30 m 40 s",
"--"))
f <- function(x) {
x <- as.character(x)
format <- paste(c(if (grepl('d', x)) '%j d',
if (grepl('h', x)) '%H h',
if (grepl('m', x)) '%M m',
if (grepl('s', x)) '%S s'), collapse=' ')
if (nchar(format) > 0) {
if (grepl('%j d', format)) {
# '%j 1' is day 0. We add a day so that x = '1 d' means 24hrs.
difftime(as.POSIXct(x, format=format) + as.difftime(1, units='days'),
cut(Sys.Date(), breaks='years'),
units='hours')
} else {
as.difftime(x, format, units='hours')
}
} else { NA }
}
data$time.span <- sapply(data$time.string, FUN=f)
Problem
Setup I have a column of durations stored as a strings in a dataframe. I want to convert them to an appropriate time object, probably POSIXlt. Most of the strings are easy to parse using this method: ``` > data <- data.frame(time.string = c( + "1 d 2 h 3 m 4 s", + "10 d 20 h 30 m 40 s", + "--")) > data$time.span <- strptime(data$time.string, "%j d %H h %M m %S s") > data$time.span [1] "2012-01-01 02:03:04" "2012-01-10 20:30:40" NA ``` Missing durations are coded `"--"` and need to be converted to `NA` - this already happens but should be preserved. The challenge is that the string drops zero-valued elements. Thus the desired value `2012-01-01 02:00:14` would be the string `"1 d 2 h 14 s"`. However this string parses to `NA` with the simple parser: ``` > data2 <- data.frame(time.string = c( + "1 d 2 h 14 s", + "10 d 20 h 30 m 40 s", + "--")) > data2$time.span <- strptime(data2$time.string, "%j d %H h %M m %S s") > data2$time.span [1] NA "2012-01-10 20:30:40" NA ``` Questions - What is the "R Way" to handle all the possible string formats? Perhaps test for and extract each element individually, then recombine? - Is POSIXlt the right target class? I need duration free from any specific start time, so the addition of false year and month data (`2012-01-`) is troubling. Solution @mplourde definitely had the right idea w/ dynamic creation of a formatting string based on testing various conditions in the date format. The addition of `cut(Sys.Date(), breaks='years')` as the baseline for the `datediff` was also good, but failed to account for a critical quirk in `as.POSIXct()` Note: I'm using R2.11 base, this may have been fixed in later versions. The output of `as.POSIXct()` changes dramatically depending on whether or not a date component is included: ``` > x <- "1 d 1 h 14 m 1 s" > y <- "1 h 14 m 1 s" # Same string, no date component > format (x) # as specified below [1] "%j d %H h %M m %S s" > format (y) [1] "% H h % M %S s" > as.POSIXct(x,format=format) # Including the date baselines at year start [1] "2012-01-01 01:14:01 EST" > as.POSIXct(y,format=format) # Excluding the date baselines at today start [1] "2012-06-26 01:14:01 EDT" ``` Thus the second argument for the `difftime` function should be: - The start of the first day of the current year if the input string has a day component - The start of the current day if the input string does not have a day component This can be accomplished by changing the unit parameter on the `cut` function: ``` parse.time <- function (x) { x <- as.character (x) break.unit <- ifelse(grepl("d",x),"years","days") # chooses cut() unit format <- paste(c(if (grepl("d", x)) "%j d", if (grepl("h", x)) "%H h", if (grepl("m", x)) "%M m", if (grepl("s", x)) "%S s"), collapse=" ") if (nchar(format) > 0) { difftime(as.POSIXct(x, format=format), cut(Sys.Date(), breaks=break.unit), units="hours") } else {NA} } ```