How to increase buffer size in R-function - r

Question

i would like to implement a function in R which is able to increase the size of a buffer in a for-loop.
The overall-plan is to write a package, which uses a test- and a reference-shapefile. It should create a buffer around the reference shapefile and increases the size as long as necessary, to intersect the whole test-shapefile.
Therefore, i already wrote some code snippets to insert the shapefiles and create the first buffer:
require("rgeos")
require("rgdal")
l1=readOGR(dsn="C:/Maps", layer="osm_ms1")
l2=readOGR(dsn="C:/Maps", layer="osm_ms2")
proj4string(l2) = CRS("+init=epsg:31467") ## DHDN / 3-degree Gauss-Kruger zone 3
l2buffer <- gBuffer(l2, width=0.001, capStyle="ROUND")
plot(l2buffer, col="black")
lines(l2, col="red")
lines(l1, col="blue")
Until now, every works fine.
After that, i wanted to transfer this method to a for-loop with a buffer for every step:
i = 0.001
buffergrow = function(shape) {
for (k in 1:10) {
linebuffer[k] <- gBuffer(l2, width=i, capStyle="ROUND")
plot(linebuffer[k])
i = i+0.001
}
}
> buffergrow(l2)
Error in linebuffer[k] <- gBuffer(shape, width = i, capStyle = "ROUND") :
Object 'linebuffer' not found
As you can see, an error occurs when i call the function 'buffergrow' with 'l2' as the argument (shape). Does anybody has an idea why this happens? I tried already some other ideas, but i need some help.
Optionally / Additionally: Do you have some hints for me, regarding the further work for my overall plan?
Best regards,
Stefan

Answer 1

You have to initialize an object before accessing its subelements.
E.g.:
foo <- double(10)
for (i in 1:10) {
foo[i] <- i;
}
# or
linebuffer <- list()
for (i in 1:10) {
linebuffer[[i]] <- i;
}
But you don't need an object linebuffer in your usecase.
Try the following instead:
buffergrow = function(shape) {
for (k in 1:10) {
plot(gBuffer(l2, width=i, capStyle="ROUND"))
i = i+0.001
}
}
EDIT:
If you need to store the gBuffer results:
buffergrow = function(shape) {
i <- 1
linebuffer <- vector("list", 10)
for (k in 1:10) {
linebuffer[[k]] <- gBuffer(l2, width=i, capStyle="ROUND")
plot(linebuffer[[k]])
i = i+0.001
}
return(linebuffer)
}