[R] NA problem

[n.vialma at libero.it]

Dear R list
I have a problem with NA, which should be a string, but R seems that it 
doesn't recognize it.  What I do is first give the format command to my data 
frame:

format.data.frame(mydata,big.mark=" ")

so I give a blank as thousand separator. All my records in my data frame 
become strings, so instead of having NA I have "NA". I try to convert "NA" in 
".",but it seems that R doesn't recognize "NA".

Someone knows why and how to treats those "NA"??


Thanks for your attention