[R] subtract values

[Kara Przeczek]

Dear R-help
I am using R version 2.6.2. I am trying to subtract specific values from a larger data frame. I feel this should be straightforward, but I am struggling.
I have a dataframe "Bk" as follows:
DateTime	 cumPrecip	
01/01/2008 00:00	 348	
01/01/2008 01:00	 348	
01/01/2008 02:00	 348	
01/01/2008 03:00	 347	
01/01/2008 04:00	 348	
01/01/2008 05:00	 348	
01/01/2008 06:00	 349	
01/01/2008 07:00	 349	
01/01/2008 08:00	 349	
01/01/2008 09:00	 348	
01/01/2008 10:00	 349	
...                                    ...
 
I would like to subtract the cumulative precipitation value at 01/01/2008 00:00 from the value at 01/02/2008 00:00 and so on. Thus, I would like to subtract cumPrecip at DateTime i from DateTime i+23, for each 24 hour period.
I tried using for loop:
dPpt=0
for (i in 1:length(Bk[,"cumPrecip"]))
{dPpt[i]=diff(Bk[i,(i+23),"cumPrecip"])}
dPpt=dPpt[seq(1,length(dPpt),24)]
 
But it had many errors. I also tried using diff(Bk$cumPrecip, lag=23) but this moves through the data one step at a time and thus calculates a value for every hour of each day, not just midnight.
 
Thank you very much for your time and assistance!
Kara
 
 
 
Kara Przeczek
M.Sc. Candidate  NRES - Environmental Science
University of Northern B.C.
3333 University Way
Prince George B.C   V2N 4Z9
Phone: (250) 960-5427
przeczek at unbc.ca