txt <- "c1,c2,c3,c4,c5
1,0,100,1,alto
1,1,101,0,medio
0,0,200,1,alto
1,1,101,1,bajo
"
df <- as.data.frame(read.table(textConnection(txt), sep = ",", header=TRUE))

> class(df$c5)
[1] "factor"

> class(df$c3)
[1] "factor"