How would I extract the time from a series of POSIXct objects discarding the date part?
For instance, I have:
times <- structure(c(1331086009.50098, 1331091427.42461, 1331252565.99979,
1331252675.81601, 1331262597.72474, 1331262641.11786, 1331269557.4059,
1331278779.26727, 1331448476.96126, 1331452596.13806), class = c("POSIXct",
"POSIXt"))
which corresponds to these dates:
"2012-03-07 03:06:49 CET" "2012-03-07 04:37:07 CET"
"2012-03-09 01:22:45 CET" "2012-03-09 01:24:35 CET"
"2012-03-09 04:09:57 CET" "2012-03-09 04:10:41 CET"
"2012-03-09 06:05:57 CET" "2012-03-09 08:39:39 CET"
"2012-03-11 07:47:56 CET" "2012-03-11 08:56:36 CET"
Now, I have some values for a parameter measured at those times
val <- c(1.25343125e-05, 0.00022890575,
3.9269125e-05, 0.0002285681875,
4.26353125e-05, 5.982625e-05,
2.09575e-05, 0.0001516951251,
2.653125e-05, 0.0001021391875)
I would like to plot val vs time of the day, irrespectively of the specific day when val was measured.
Is there a specific function that would allow me to do that?
Answer
You can use strftime to convert datetimes to any character format:
> t <- strftime(times, format="%H:%M:%S")
> t
[1] "02:06:49" "03:37:07" "00:22:45" "00:24:35" "03:09:57" "03:10:41"
[7] "05:05:57" "07:39:39" "06:47:56" "07:56:36"
But that doesn't help very much, since you want to plot your data. One workaround is to strip the date element from your times, and then to add an identical date to all of your times:
> xx <- as.POSIXct(t, format="%H:%M:%S")
> xx
[1] "2012-03-23 02:06:49 GMT" "2012-03-23 03:37:07 GMT"
[3] "2012-03-23 00:22:45 GMT" "2012-03-23 00:24:35 GMT"
[5] "2012-03-23 03:09:57 GMT" "2012-03-23 03:10:41 GMT"
[7] "2012-03-23 05:05:57 GMT" "2012-03-23 07:39:39 GMT"
[9] "2012-03-23 06:47:56 GMT" "2012-03-23 07:56:36 GMT"
Now you can use these datetime objects in your plot:
plot(xx, rnorm(length(xx)), xlab="Time", ylab="Random value")
For more help, see ?DateTimeClasses