How to add a field to a shapefile using R

This question is trying to make sense of a post that already exists that looks good, but is failing in practice.

The second ranked answer uses the foreign package to read a shapefile attribute table as a .dbf, then allows @mdsummer to add a field to the table as an additional attribute.

I am using Raster and Shapefiles packages (shapefiles borrows the foreign read and write dbf).

library(raster); library(shapefiles)
shp<-shapefile(ZoneShape);

shp

>class       : SpatialPolygonsDataFrame 

nfeatures   : 213 
extent      : 577829.3, 582592.8, 3837297, 3839300  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=15 +datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0 
nvariables  : 1
names       : OBJECTID 
min values  :    24571 
max values  :    28496

Now I'll read my attribute table:

shp.AT<-read.dbf(gsub(".shp", ".dbf", ZoneShape))


class(shp.AT)
>[1] "list"

shp.AT is a list that contains an index produced by R and the Shapefile's OBJECTID. Their are 213 records, but the length of the list is 2.

I'm not a great programmer, but I know I can't add a field to a list.

shp.AT$GID<-1:nrow(ZoneShape)
>[3] ERROR: nrow(ZoneShape)

nrow(ZoneShape)

>nrow(shp.AT) 

# "nothing is returned!"

@mdsummer includes an "as.is = TRUE" in the read.dbf statement. Not sure what that might have done in the past, but R returns an error when this is included.

If I modify my new field slightly:

shp.AT$GID<- 1:length(ZoneShape)
shp.AT

This time, no error. My list of ID's is again reported along with my new GID field, but:

>$GID

[1] 1

If this worked as expected, I would overwrite the old DBF with the write.dbf command.

So, a couple questions really: Has the source changed, or is @mdsummer's post incorrect? or, am I totally missing something?

and (most importantly) how can I add fields to my shapefile attributes.

I also have a cross-posting on Overflow

Answer

The solution is less complicated than I was making it.

shp<-shapefile(ZoneShape)


shp$GID<-1:nrow(ZoneShape)

write.shapefile(shp, gsub(ZoneShape, ".shp", ""))

_{Thanks to vitale232}