[R-sig-Geo] substract two columns in a shapefile (but SAGA+R has an bug error)

[Roger Bivand]

On Tue, 24 Feb 2009, Alessandro wrote:

> Dear Researcher,
>
>
>
> I have a shapefile with two columns "a" and "b" and I wish to subtract a-b
> to obtain a new column of difference.  I tried in SAGA + R with the module
> "Table calculator for shapes". This worked without an error, but it didn't
> save the results either. I suspect that this is a bug in the SAGA module.
>
>
>
> Is there an another code or packages (without GRASS+R) to do operation with
> shapefile?


Why not do it the simple way?

Either:

library(maptools)
my_SPDF <- readShapeSpatial("myfile.shp")
my_SPDF$ab <- my_SPDF$a - my_SPDF$b
writeSpatialShape(my_SPDF, "mynewfile")

or:

library(rgdal)
my_SPDF <- readOGR(".", "myfile")
my_SPDF$ab <- my_SPDF$a - my_SPDF$b
writeOGR(my_SPDF, ".", "mynewfile", driver="ESRI Shapefile")

Insert a summary(my_SPDF) to check that a and b are what you think, and 
that ab has been created. It won't work if a and/or b are strings.

Roger

>
>
>
> Thanks
>
>
>
> Ale
>
>
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>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no