[R] Round down to earliest hour or half hour

[Phil Hurvitz]

I wanted to round rather than truncate timestamps

# create a data set to work with
s <- 30 * rnorm(10, 1, 0.1)
gps.timestamp <- Sys.time() + s*1:10
gps.timestamp <- c(round(gps.timestamp[1], "min"), gps.timestamp, 
as.POSIXct("2012-06-08 17:32:59"), as.POSIXct("2012-06-08 17:32:30"))

f <- function(gps.timestamp=gps.timestamp, interval=30){
    # is the interval a multiple of 60? if so, just round
    if(interval %% 60 == 0){
        return(data.frame(gps.timestamp, 
gps.timestamp.round=round(gps.timestamp, "min")))
    }
    # a POSIXlt timestamp for fiddling with seconds
    lt <- as.POSIXlt(gps.timestamp)
    # extract rounded seconds from the timestamp
    gps.seconds <- round(lt$sec, 0)
    # the number of segments in a minute is 60/interval
    seg <- 60/interval
    # the cutpoints
    rounds <- c(0, 1:seg * interval)
    # the "midpoints" for cutting
    cuts <- c(-1, 1:seg * 60/seg - interval/2, 60)
    # and a numeric representation of the cut position
        # (the factor position, really)
    numcut <- as.numeric(cut(gps.seconds,cuts))
    # the rounded seconds (*not* truncated)
    round.seconds <- rounds[numcut]
    # truncate the date to the nearest low minute
    lt <- lt1 <- trunc(lt, "min")
    # add the rounded seconds to the truncated minutes
    lt1 <- lt1 + round.seconds
    # how does that look?
    return(data.frame(gps.timestamp, gps.timestamp.round=lt1))
}


# try it:
f(gps.timestamp, 15)
f(gps.timestamp, 30)
f(gps.timestamp, 10)

-P.

**************************************************************
Philip M. Hurvitz, PhD | Research Assistant Professor | UW-CBE
Urban Form Lab  | 1107 NE 45th Street, Suite 535  | Box 354802
University of Washington, Seattle, Washington  98195-4802, USA
phurv...@u.washington.edu | http://gis.washington.edu/phurvitz
"What is essential is invisible to the eye." -de Saint-Exupéry
**************************************************************

I would use package chron and trunc()....

One example from the help of trunc.times (modified):

lirary(chron)
tt <- times(c("12:13:14", "15:46:17"))
trunc(tt, times("00:30:00"))


HTH
Jannis

--- Schatzi <adele_thompson at cargill.com> schrieb am Mo, 9.5.2011:

Von: Schatzi <adele_thompson at cargill.com>
Betreff: [R] Round down to earliest hour or half hour
An: r-help at r-project.org
Datum: Montag, 9. Mai, 2011 14:06 Uhr
I have times and would like to round
down to the earliest 30 minute
increment. For instance, a time of
2011-04-28 09:02:00
(the as.numeric value = 1303999320)
I would like it to be rounded down to:
2011-04-28 09:00:00
(the as.numeric value = 1303999200)

Any ideas of how to do this?

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