R help - Dec 2010 - how to use diff() with different variables?

Hi,

I first should say I am new to R. I have searched without success the R-archives
to see if I could find an answer to what I am about to ask you.

My dataset is like:

  x                fine
1 A 2010-12-09 07:57:33
2 B 2010-12-09 08:05:00
3 C 2010-12-08 20:42:00
...

that is:

'data.frame':	3 obs. of  2 variables:
 $ x   : Factor w/ 3 levels "A","B","C": 1 2 3
 $ fine: POSIXct, format: "2010-12-09 07:57:33" "2010-12-09
08:05:00" "2010-12-08 20:42:00"

What I am trying to do is to build another variable fine1 that should contain
the lagged value for "fine", that is:

  x                fine               fine1
1 A 2010-12-09 07:57:33 NA
2 B 2010-12-09 08:05:00 2010-12-09 07:57:33
3 C 2010-12-08 20:42:00 2010-12-09 08:05:00

How can I do that? 

Thanks,
Luca

Luca Meyer
www.lucameyer.com
IBM SPSS Statistics release 19.0.0
R version 2.12.0 (2010-10-15)
Mac OS X 10.6.5 (10H574) - kernel Darwin 10.5.0

Luca wrote on 12/09/2010 09:38:07 AM:
> ...
> What I am trying to do is to build another variable fine1 that 
> should contain the lagged value for "fine", that is:
> 
>   x                fine               fine1
> 1 A 2010-12-09 07:57:33 NA
> 2 B 2010-12-09 08:05:00 2010-12-09 07:57:33
> 3 C 2010-12-08 20:42:00 2010-12-09 08:05:00
> 
> How can I do that? 
A few ways.  If you search for lag in the last couple of weeks, you'll see 
a discussion on this.  One is as follows:

 foo <- data.frame(x=LETTERS, fine=as.Date(0:25,
origin='2010-01-01'))
 foo$fine1<-as.Date(NA)
 foo[2:nrow(foo),]$fine1 <- foo[1:(nrow(foo)-1),]$fine

cur
-- 
Curt Seeliger, Data Ranger
Raytheon Information Services - Contractor to ORD
seeliger.curt@epa.gov
541/754-4638



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