[R-sig-Geo] difficult to subtract two column of my shapefile

[Alexander Brenning]

Hi there,

to throw in two more alternatives, there are functions read.dbf and 
write.dbf in packages shapefiles and foreign (see help files for 
examples; the functions differ between the two packages despite having 
the same name).

Remember that operations on the shapefile attributes only affect the dbf 
file of the shape"file" (which is not really a file but a set of files). 
So this will work as long as you don't change the number or order of the 
rows.

This may also be more efficient if the shapes have complex geometry 
(e.g. complex polygons, contour lines...), because the geometry is not 
imported by the functions I mentioned.

Cheers
  Alex


Alessandro wrote:
> Thanks for all help
> 
>  
> 
> I wish two subtract two column of my shapefile but It look like a impossible
> operation with R:
> 
>  
> 
> 1.  In SAGA+R module: "Table calculator for shapes", It runs but doesn't
> save the result (there is bug)
> 
> 2.   I can't use software "GeoDa" because It works only in Windows xp and
> not in Windows Vista
> 
> 3.  Library maptools go down for a memory problems
> 
> 4.  library(rgdal) give an empty column
> 
>  
> 
> is there suggestion? please.
> 
>  
> 
> I attach a small part of my data-set for 
> 
>  
> 
> Thanks for Help and suggestion
> 
>  
> 
>  
> 
> library(maptools)
> 
> my_SPDF <- readShapeSpatial("theme1.shp") 
> 
> my_SPDF$ab <- my_SPDF$a - my_SPDF$b 
> 
> writeSpatialShape(my_SPDF, "mynewfile")
> 
>  
> 
> or:
> 
>  
> 
> library(rgdal)
> 
> my_SPDF <- readOGR(".", "theme1")
> 
> my_SPDF$ab <- my_SPDF$a - my_SPDF$b
> 
> writeOGR(my_SPDF, ".", "mynewfile", driver="ESRI Shapefile")
> 
>  
> 
> 
> 
> 
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-- 
Alexander Brenning
brenning at uwaterloo.ca - T +1-519-888-4567 ext 35783
Department of Geography and Environmental Management
University of Waterloo
200 University Ave. W - Waterloo, ON - Canada N2L 3G1
http://www.fes.uwaterloo.ca/geography/faculty/brenning/