extract hours and seconds from POSIXct for plotting purposes in R

Question

Suppose I have the following data.frame foo

           start.time duration 1 2012-02-06 15:47:00      1 2 2012-02-06 15:02:00      2 3 2012-02-22 10:08:00      3 4 2012-02-22 09:32:00      4 5 2012-03-21 13:47:00      5 

And class(foo$start.time) returns

[1] "POSIXct" "POSIXt"  

I'd like to create a plot of foo$duration v. foo$start.time. In my scenario, I'm only interested in the time of day rather than the actual day of the year. How does one go about extracting the time of day as hours:seconds from POSIXct class of vector?

Answer 1

This is a good question, and highlights some of the difficulty in dealing with dates in R. The lubridate package is very handy, so below I present two approaches, one using base (as suggested by @RJ-) and the other using lubridate.

Recreate the (first two rows of) the dataframe in the original post:

foo <- data.frame(start.time = c("2012-02-06 15:47:00",                                   "2012-02-06 15:02:00",                                  "2012-02-22 10:08:00"),                   duration   = c(1,2,3)) 

Convert to POSIXct and POSIXt class (two ways to do this)

# using base::strptime t.str <- strptime(foo$start.time, "%Y-%m-%d %H:%M:%S")  # using lubridate::ymd_hms library(lubridate) t.lub <- ymd_hms(foo$start.time) 

Now, extract time as decimal hours

# using base::format h.str <- as.numeric(format(t.str, "%H")) +                as.numeric(format(t.str, "%M"))/60  # using lubridate::hour and lubridate::minute h.lub <- hour(t.lub) + minute(t.lub)/60 

Demonstrate that these approaches are equal:

identical(h.str, h.lub) 

Then choose one of above approaches to assign decimal hour to foo$hr:

foo$hr <- h.str  # If you prefer, the choice can be made at random: foo$hr <- if(runif(1) > 0.5){ h.str } else { h.lub } 

then plot using the ggplot2 package:

library(ggplot2) qplot(foo$hr, foo$duration) +               scale_x_datetime(labels = "%S:00")