D,

I think the correct answer has been given but it may be hard to decipher.
The below might do what you want (not tested; as you provide no example
data which would have been easy to include starting from the examples in
'extract')


for(j in 1:3) {
polys <- get(paste0('SpatiallPolygonsDataFrame_', j, 'km'))
    for(k in 1:17){
out <- paste0('extract_', j, '_', k, 'km')
assign(out, extract(stack, polys, layer=[k], nl=1, df=TRUE))
}
}


However, it is VERY inefficient to call extract separately for each layer
in a RasterStack. In stead you could do something like this:


res <- list()
for(j in 1:3) {
polys <- get(paste0('SpatiallPolygonsDataFrame_', j, 'km'))
e <- extract(stack, polys, layer=1, nl=17, df=TRUE)
names(e) <- paste0('extract_', j, '_', 1:17, 'km')
res <- c(res, e)
}

# you could add the 1:17 loop after extract to do the assignments, but
# in the example below you get a list of data.frames which is much more
convenient

# A simpler approach might be:

polys <- list(a, b, c)
res <- list()
for(j in 1:3) {
e <- extract(stack, polys[[j]], layer=1, nl=17, df=TRUE)
names(e) <- paste0('extract_', j, '_', 1:17, 'km')
res <- c(res, e)
}


Best, RH

On Sun, Dec 16, 2012 at 2:18 PM, dcd <dennis.duro@gmail.com> wrote:

> As asked  here
> <
> http://stackoverflow.com/questions/13897455/pass-variable-name-to-a-function-and-output-inside-of-loop-in-r
> >
> on stackoverflow:
>
>
> for(j in 1:3) {
>     for(k in 1:17){
>        extract_[j]km <- extract(RasterStack,
> SpatialPolygonsDataFrame_[j]km,
> layer=[k], nl=1, df=TRUE)
>     }
> }
>
> While the Rasterstack remains constant, the SpatialPolygon and layer within
> the RasterStack will iterate. From the posts I've read I'm now getting the
> hint that loops and R aren't the best combination, but what approach would
> achieve something similar to  the above pseudo-loop code? I've looked at
> get, paste, and lapply, but I can't get them to work with extract. Any help
> would be most appreciated.
>
> Many thanks in advance!
>
> D
>
>
>
>
>
>
> --
> View this message in context:
> http://r-sig-geo.2731867.n2.nabble.com/raster-How-to-loop-extract-function-tp7581978.html
> Sent from the R-sig-geo mailing list archive at Nabble.com.
>
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