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

Thank you for any suggestions.

Best wishes,

Michal

Michal Gallay

Postgraduate Research Student
School of Geography, Archaeology and Palaeoecology
Queen's University
Belfast BT7 1NN
Northern Ireland

Tel: +44(0)2890 273929
Fax: +44(0)2890 973212
email: mgallay01 at qub.ac.uk
www: www.qub.ac.uk/geog