R help - Dec 2002 - Advice on long for loop

I'd appreciate advice on the following:

I've written an R function
that uses 3 vectors (temperature,
precipitation and potential evapotranspiration)
for a given site, calculates a water budget
(which implies few (<4) iterations), and, from
thresults of this water budget, calculates a number of
bioclimatic indices.

Now I want to calculate these indices
for a large number of sites (arranged
as cells in a raster map),  using
monthly raster maps (m rows x n cols)  of temperature,
precipitation and potential evapotranspiration.

I do not see any other way than using
two nested for loops and calling the
aforementioned function for each cell in the
map. But this would be a problem in R
as m and n can be ~ 500.

My question is: would it be any better if
I write a simple python function with the loops
and call the R function for each cell (using
the R-Python interface)? Or, as the calculation
in each cell is independent, should I rather write a
shell script with the loop and submit jobs
with R CMD BATCH?

Thanks

Agus

Dr. Agustin Lobo
Instituto de Ciencias de la Tierra (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona SPAIN
tel 34 93409 5410
fax 34 93411 0012
alobo at ija.csic.es

I think something like the following will let you do the thing you want in R:
> vec1 <- 1:10
> vec2 <- 2:11
> vec3 <- 3:12
> fun <- function(x){
+ vec1[x] * vec2[x] * vec3[x] #whatever
+ }
> tt <- lapply(1:10, fun)
>
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>
>
>I'd appreciate advice on the following:
>
>I've written an R function
>that uses 3 vectors (temperature,
>precipitation and potential evapotranspiration)
>for a given site, calculates a water budget
>(which implies few (<4) iterations), and, from
>thresults of this water budget, calculates a number of
>bioclimatic indices.
>
>Now I want to calculate these indices
>for a large number of sites (arranged
>as cells in a raster map),  using
>monthly raster maps (m rows x n cols)  of temperature,
>precipitation and potential evapotranspiration.
>
>I do not see any other way than using
>two nested for loops and calling the
>aforementioned function for each cell in the
>map. But this would be a problem in R
>as m and n can be ~ 500.
>
>My question is: would it be any better if
>I write a simple python function with the loops
>and call the R function for each cell (using
>the R-Python interface)? Or, as the calculation
>in each cell is independent, should I rather write a
>shell script with the loop and submit jobs
>with R CMD BATCH?
>
>Thanks
>
>Agus
>
>Dr. Agustin Lobo
>Instituto de Ciencias de la Tierra (CSIC)
>Lluis Sole Sabaris s/n
>08028 Barcelona SPAIN
>tel 34 93409 5410
>fax 34 93411 0012
>alobo at ija.csic.es
>
>______________________________________________
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