read following csv
with
csv <-read.csv('test.csv',stringsAsFactors=F)
typeof(csv$col1)
csv$col1
typeof(csv$col2)
csv$col2
GNU-R
integer
NA 2
integer
1 NA
fastR
integer
NA 2
character
"1" ""
seems this only happen if missing value after valid value. for example add more row like this:
fastR typeof col1 will also changed to character