[R] [newbie] aggregating table() results and simplifying code with loop

[Davide Rizzo]

Dear all,
I'm looking for primary help at aggregating table() results and at
writing a loop (if useful)

My dataset ( http://goo.gl/gEPKW ) is composed of 23k rows, each one
representing a point in the space of which we know the land cover over
10 years (column y01 to y10).

I need to analyse it with a temporal sliding window of 5 years (y01 to
y05, y02 to y06 and so forth)
For each period I'm looking for specific sequences (e.g., Maize,
-noMaize, -noMaize, -noMaize, -noMaize) to calculate the "return time"
of principal land covers: barley (2BC), colza (2Co), maize (2Ma), etc.
I define the "return time" as the presence of a given land cover
according to a given sequence. Hence, each return time could require
the sum of different sequences (e.g., a return time of 5 years derives
from the sum of [2Ma,no2Ma,no2Ma,no2Ma,no2Ma] +
[no2Ma,no2Ma,no2Ma,no2Ma,2Ma]).
I need to repeat the calculation for each land cover for each time
window. In addition, I need to repeat the process over three datasets
(the one I give is the first one, the second one is from year 12 to
year 24, the third one from year 27 to year 31. So I have breaks in
the monitoring of land cover that avoid me to create a continuous
dataset). At the end I expect to aggregate the sum for each spatial
entity (column WS)

I've started writing the code for the first crop in the first 5yrs
period (http://goo.gl/FhZNx) then copying and pasting it for each crop
then for each time window...
Moreover I do not know how to aggregate the results of table(). (NB
sometimes I have a different number of WS per table because a given
sequence could be absent in a given spatial entity... so I have the
following warning msg: number of columns of result is not a multiple
of vector length (arg 1)). Therefore, I'm "obliged" to copy&paste the
table corresponding to each sequence....

FIRST QUEST. How to aggregate the results of table() when the number
of columns is different?
Or the other way around: Is there a way to have a table where each row
reports the number of points per time return per WS? something like

WS1    WS2    WS3    WS4    ...    WS16    crop    period
23    15    18    43    ...    52       Ma5    01
18    11    25    84    ...    105       Ma2    01
...    ...    ...    ...    ...    ...    ...    ...
...    ...    ...    ...    ...    ...    Co5    01
...    ...    ...    ...    ...    ...    ...    ...
...    ...    ...    ...    ...    ...    Ma5    02
...    ...    ...    ...    ...    ...    ...    ...
In this table each row should represent a return time for a given land
cover a given period (one of the 6 time window of 5 years)?

SECOND QUEST. Could a loop (instead of a modular copy/paste code)
improve the time/reliability of the calculation? If yes, could you
please indicate me some entry-level references to write it?

I am aware this are newbie's questions, but I have not be able to
solve them using manuals and available sources.
Thank you in advance for your help.

Greetings,
Dd

PS
R: version 2.14.2 (2012-02-29)
OS: MS Windows XP Home 32-bit SP3


*****************************
Davide Rizzo
post-doc researcher
INRA UR055 SAD-ASTER
website :: http://sites.google.com/site/ridavide/