[R] Doing things with POSIXt

[Vadim Ogranovich]

The catch is that the daytime of "1992-01-14 01:03:30 PST" is 1.058333 and
not 7.058333 as your system produces. On my system it is 9.058333 hours,
which makes me think it has something to do with time zones.

-----Original Message-----
From: Dirk Eddelbuettel [mailto:edd at debian.org]
Sent: Monday, November 26, 2001 6:32 PM
To: Vadim Ogranovich
Cc: 'r-help at stat.math.ethz.ch'
Subject: [R] Doing things with POSIXt



  "Vadim" == Vadim Ogranovich <vograno at arbitrade.com> writes:
  Vadim> * Suppose 'datetime' is a vector of POSIXct. I want to index all
  Vadim> elements that are after 1am. Apparently there is no easy way of
  Vadim> doing this (for example there is no access function
  Vadim> daytime(POSIXt)), or hour(POSIXt), or anything like this).  To
write
  Vadim> my own function I tried to follow this line: datetime <-
  Vadim> as.POSIXct("1992-01-14 01:03:30 PST") midnight <-
  Vadim> trunc.POSIXt(datetime, "days") isAfter1am <- difftime(datetime,
  Vadim> midnight, "hours") >= 1 # should yield TRUE
  Vadim> 
  Vadim> This didn't work since difftime(datetime, midnight, "hours")
  Vadim> returned 'Time difference of 9.058333 hours' which is wrong

You need to convert it to numeric

> datetime <- as.POSIXct("1992-01-14 01:03:30 PST")
> midnight <- trunc.POSIXt(datetime, "days")
> difftime(datetime, midnight, "hours")
Time difference of 7.058333 hours
> as.numeric(difftime(datetime, midnight, "hours"))
[1] 7.058333
> as.numeric(difftime(datetime, midnight, "hours")) >= 1
[1] TRUE

Dirk

-- 
Better to have an approximate answer to the right question 
than a precise answer to the wrong question. -- John Tukey

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