[R] Doing things with POSIXt

[Dirk Eddelbuettel]

  "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

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