R converting dataframe of strings to unique numbers

I have a dataframe that’s very large (let’s say 8 rows by 10,000 columns) that is filled with strings. I want to convert each unique string to a number and replace it with it.

For example, if I had a dataframe:

   X1       X2       X3
1 cat    mouse     rabbit
2 dog   cat, dog    dog

I’d like to convert it to:

   X1        X2     X3
1   1         2       3
2   4         5       4

Note the combined label of "cat,dog" gets its own unique number. The actual numbering of each string is irrelevant as I’m doing this for an inter-rater reliability calculation.

Short of me getting all the unique elements, assigning them a number and replacing is there a more elegant way to do this?

Also, if a value in an element is blank, eg "", it should be converted to an NA in the numeric DF.

>Solution :

You can match on the unique values:

df[] <- sapply(df, match, unique(unlist(df)))

#> df
  X1 X2 X3
1  1  3  5
2  2  4  2

Or, even simpler:

df[] <- match(unlist(df), unique(unlist(df)))