[R] how to remove "." or replace it with NA?

[Yong Wang]

Hi,all
a quick question
A larger file include many"." (stands for missing 
value) for variable, say, x. after
read in the file and assign it another name such as :

x1<-data$x

but x1 is always a facotor but not numerical. 

so How can I get a numeric vector from the read in data frame.
how can I replace those "." with "NA" to perform analysis.

thank you very much
best regards