obtain hour from DateTime vector - r

Question

I have a DateTime vector within a data.frame where the data frame is made up of 8760 observations representing hourly intervals throughout the year e.g.
2010-01-01 00:00
2010-01-01 01:00
2010-01-01 02:00
2010-01-01 03:00
and so on.
I would like to create a data.frame which has the original DateTime vector as the first column and then the hourly values in the second column e.g.
2010-01-01 00:00 00:00
2010-01-01 01:00 01:00
How can this be achieved?

Answer 1

Use format or strptime to extract the time information.
Create a POSIXct vector:
x <- seq(as.POSIXct("2012-05-21"), by=("+1 hour"), length.out=5)
Extract the time:
data.frame(
date=x,
time=format(x, "%H:%M")
)
date time
1 2012-05-21 00:00:00 00:00
2 2012-05-21 01:00:00 01:00
3 2012-05-21 02:00:00 02:00
4 2012-05-21 03:00:00 03:00
5 2012-05-21 04:00:00 04:00
If the input vector is a character vector, then you have to convert to POSIXct first:
Create some data
dat <- data.frame(
DateTime=format(seq(as.POSIXct("2012-05-21"), by=("+1 hour"), length.out=5), format="%Y-%m-%d %H:%M")
)
dat
DateTime
1 2012-05-21 00:00
2 2012-05-21 01:00
3 2012-05-21 02:00
4 2012-05-21 03:00
5 2012-05-21 04:00
Split time out:
data.frame(
DateTime=dat$DateTime,
time=format(as.POSIXct(dat$DateTime, format="%Y-%m-%d %H:%M"), format="%H:%M")
)
DateTime time
1 2012-05-21 00:00 00:00
2 2012-05-21 01:00 01:00
3 2012-05-21 02:00 02:00
4 2012-05-21 03:00 03:00
5 2012-05-21 04:00 04:00

Answer 2

Or generically, not treating them as dates, you can use the following provided that the time and dates are padded correctly.
library(stringr)
df <- data.frame(DateTime = c("2010-01-01 00:00", "2010-01-01 01:00", "2010-01-01 02:00", "2010-01-01 03:00"))
df <- data.frame(df, Time = str_sub(df$DateTime, -5, -1))
It depends on your needs really.

Answer 3

Using lubridate
library(stringr)
library(lubridate)
library(plyr)
df <- data.frame(DateTime = c("2010-01-01 00:00", "2010-01-01 01:00", "2010-01-01 02:00", "2010-01-01 03:00"))
df <- mutate(df, DateTime = ymd_hm(DateTime),
time = str_c(hour(DateTime), str_pad(minute(DateTime), 2, side = 'right', pad = '0'), sep = ':'))

Answer 4

On a more general note, for anyone that comes here from google and maybe wants to group by hour:
The key here is: lubridate::hour(datetime)
p22 in the cran doc here: https://cran.r-project.org/web/packages/lubridate/lubridate.pdf