map algebra with ascii grid

Dear R Users,

could you please advise me on doing map algebra with spatial grids? It's
the first time I am using spatial objects in R. I have imported an ascii
grid file and wanted to round the values in it or sum with a value, but it
gives an error message:

require(maptools)
x <- readAsciiGrid(fname="xxxx.asc")
x+3

Error in x + 3 : non-numeric argument to binary operator

round(x,2)

Error in round(x, digits) : Non-numeric argument to mathematical function

Arithmetic operations are not defined for SpatialGridDataFrame operations, 
as the columns of the data frame (here a single column) may contain data 
of different classes. Do the operations on the columns directly:

names(x)

If there is a column called "band1", then

x$band1 + 3

will print "band1" + 3, and

x$band1 <- x$band1 + 3

will add 3 to the "band1" column. Think what would happen if band1 was 
categorical or logical to see why doing arithmetic directly isn't such a 
good idea.

Hope this helps,

Roger

Thank you for any suggestions.

Best wishes,

Michal

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