[R] Posix Problem, difftime

Matthew Pocernich Matthew.Pocernich at parsons.com
Thu Oct 17 18:51:24 CEST 2002

I am having a series of problems using date time data that has been converted into a POSIXt and POSIXlt classes.  I have  hourly time series data from 1900 that has been converted from text data.
I assume most of my problems come from a mis-underdanding of the POSIX class.  My matrix named (aa) for this year is approx 8700 by 4.   When I try to calculate the length of posit column ( which is the date and time) I get
> length(aa$posit)
[1] 9
When I try to combined POSIX columns or manipulate them like matrices, I have problems such as 
> cbind(aa$posit, aa$posit)
Error in cbind(...) : cannot create a matrix from these types
Ultimately I would like to check to make certain the hourly data is not missing.  I tried doing this using vectors such as difftime(aa$posit, lag(aa$posit)).  This didn't work.  Ultimately I used a for loop, which 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.
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