[R] testing column data against criteria, point by point

Kara Przeczek przeczek at unbc.ca
Thu Mar 5 09:46:17 CET 2009

I am fairly new to R and I would like to do the following, but do not know where to start. Any help or direction would be appreciated.
I have a time series of snow depth measurements. I would like to determine the depth of snowfall for each snowfall event. There is noise in the data so I only want to add data values if the subsequent depth is greater than the previous by a certain margin. I am only interested in calculating snow accumulation events.
Example data:
Time	 depth	
1	 84.3	
2	 84.5	
3	 86	
4	 86.1	
5	 85.8	
6	 86.7	
7	 87.9	
8	 89.1	
9	 90	
10	 89	
11	 88	
12	 88	
13	 89.1	
14	 90	
15	 91.2	
16	 89.9	
...	 ...	
I would like to create a second data frame from the data that looks something like this:
Event	 InitialDepth	 FinalDepth	 Accumulation	 InitialTime	 FinalTime	
1	 84.3	 90	 5.7	 1	 9	
2	 88	 91.2	 3.2	 11	 15	
I would like to write a program that progresses through the depth column, point by point, to test if (i+1) - i > x. (where I will set x to exlude the noise in the data). As long as i+1 is greater than or equal to i, then the initial depth stays at the first data point and the final value changes to that in i+n. Once the test is false, this indicates the end of the event, the accumulation is calculated, all values are saved as event X and a new event is started.  
I tried using ifelse(), but I do not know how to move through the data and then save the initial and final values and time stamps in another table.
Thank you very much for your time.

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