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I have a list of SpatialPolygons I created using lapply and the raster::buffer function. I would like to convert the files in the list back to a list of SpatialPolygonsDataFrame objects. I tried the answer provided here (although note my issue is slightly different because I would like to end up with a LIST of SpatialPolygonsDataFrame objects): R - SpatialPolygonsDataFrame from a list of SpatialPolygons, but i don't know how to get unique IDs for each object in the list. When I apply that code, I get the following error msg:

#Getting polygon IDs
IDs <- sapply(list_of_SPols, function(x)
 slot(slot(x, "polygons")[[1]], "ID"))

#Checking
length(unique(IDs)) == length(list_of_SPols) #output = [1] FALSE

#Making SpatialPolygons from list of polygons
Spol <- SpatialPolygons(lapply(list_of_SPols,
                            function(x) slot(x, "polygons")[[1]]))

#output = Error in validObject(res) : 
  #invalid class “SpatialPolygons” object: non-unique Polygons ID slot 
  #values

My SpatialPolygons list elements have unique index values, but when I check the output of the IDs object created above, a subset of it looks like this:

#[1] "1" "1" "1" "1" "1" "1" "1"....

So all list elements have the same IDs. So I think what I need to do is create unique IDs equal to the index numbers?

How do I do this, then how do I go on to create a list of SpatialPolygonsDataFrame objects?

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  • you have been asking several questions over the past days; but you are not really following the rules. You need to provide a self-contained example. That is, include some code and data generated by code, or using the examples. A part of the answer to your questions is that you can use match.ID = FALSE. But there is more that you do wrong, I think, but it is hard to answer without data. Commented Oct 22, 2018 at 16:11

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One of the reasons why it is important to show data is that you might be asking the wrong question. If you want to use raster::buffer with a SpatialPolygonsDataFrame and keep that object type, you should use the option dissolve=FALSE

library(raster)
p <- shapefile(system.file("external/lux.shp", package="raster"))
b1 <- buffer(p, .1)
class(b1)
#[1] "SpatialPolygons"

b2 <- buffer(p, .1, dissolve=FALSE)
class(b2)
#[1] "SpatialPolygonsDataFrame"

That probably solves your problem. To answer your question about creating SpatialPolygonDataFrame objects from a list of SpatialPolygon objects

Example list of SpatialPolygon objects

library(raster)
p <- shapefile(system.file("external/lux.shp", package="raster"))
s <- as(p, 'SpatialPolygons')
x <- lapply(seq(1,12,3), function(i) s[i:(i+2),])

Presumably you already have data.frame objects you want to match, but I just create them here. This gives an error

z <- lapply(x, function(i) SpatialPolygonsDataFrame(i, data.frame(id=1:length(i)))) 
#Error in SpatialPolygonsDataFrame(i, data.frame(id = 1:length(i))) : 
#row.names of data and Polygons IDs do not match

This works

z <- lapply(x, function(i) SpatialPolygonsDataFrame(i, data.frame(id=1:length(i)), match.ID = FALSE))

In most cases, you would want to combine the objects. To keep track of them, you could do

zz <- lapply(1:length(x), function(i) SpatialPolygonsDataFrame(x[[i]], data.frame(id=rep(i, length(x[[i]]))), match.ID = FALSE))    
sp <- bind(zz)
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2 Comments

Thank you so much! I am new to stackoverflow and will provide an example in future. Everything works for me except the "sp <-bind(zz)" argument. When applied to my data it returns the following error: Error in (function (classes, fdef, mtable) : unable to find an inherited method for function ‘bind’ for signature ‘"list", "missing"’. I realize this may be difficult for you to address without access to my data, but if you are familiar with the source of this type of error please reply.
perhaps try raster::bind ?

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