[R] convert columns of dataframe to same factor levels

[Luigi Marongiu]

Dear all,
I have a data frame with character values where each character is a
level; however, not all columns of the data frame have the same
characters thus, when generating the data frame with stringsAsFactors
= TRUE, the levels are different for each column.
Is there a way to provide a single vector of levels and assign the
characters so that they match such vector?
Is there a way to do that not only when setting the data frame but
also when reading data from a file with read.table()?

For instance, I have:
column_1 = c("A", "B", "C", "D", "E")
column_2 = c("B", "B", "C", "E", "E")
column_3 = c("C", "C", "D", "D", "C")
my.data <- data.frame(column_1, column_2, column_3, stringsAsFactors = TRUE)
> str(my.data)
'data.frame': 5 obs. of  3 variables:
 $ column_1: Factor w/ 5 levels "A","B","C","D",..: 1 2 3 4 5
 $ column_2: Factor w/ 3 levels "B","C","E": 1 1 2 3 3
 $ column_3: Factor w/ 2 levels "C","D": 1 1 2 2 1

Thank you
-- 
Best regards,
Luigi