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