[R] Posix Problem, difftime

[Thomas Lumley]

On Thu, 17 Oct 2002, Matthew Pocernich wrote:

>  When I try to combined POSIX columns or manipulate them like matrices,
> I have problems such as
>
> > cbind(aa$posit, aa$posit)
> Error in cbind(...) : cannot create a matrix from these types

As it says, you can't create matrix from these types.  Matrices can only
hold numbers or characters. You can create a dataframe.

> Ultimately I would like to check to make certain the hourly data is not
> missing.  I tried doing this using vectors such as difftime(aa$posit,
> lag(aa$posit)).  This didn't work.  Ultimately I used a for loop, which

No, lag() just adjusts the time series attributes of a vector.

You could do
  n<-length(a$posit)
  difftime(a$posit[1:(n-1)],a$posit[2:n])

> is very slow and it produces the following error.  The last column is
> the difference in times between subsequent posit values.  The fourth row
> shows a 2 hour gap between events.  (This did not happen in any of the
> other days.  )
>
>
>      tide               posit diffh y365 diftime
> 7223 -1.25 1901-10-28 22:00:00  -2.7  301       1
> 7224 -2.75 1901-10-28 23:00:00  -1.5  301       1
> 7225 -2.25 1901-10-29 00:00:00   0.5  302       1
> 7226 -0.25 1901-10-29 01:00:00   2.0  302       2
> 7227  2.65 1901-10-29 02:00:00   2.9  302       1
>
> Any help or suggestions are greatly appreciated.

At least in my time zone this is correct.  There was a 2 hour interval
between 1901-10-29 00:00:00 and 1901-10-29 01:00:00.  Note that 1901-10-28
is the last Saturday in October: it was the end of summer time.

> as.POSIXlt(ISOdate(1901,10,29))$isdst
[1] 0
> as.POSIXlt(ISOdate(1901,10,28))$isdst
[1] 1


	-thomas

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