I'm experiencing trouble with creating a new rank column. I have the following data:

```
Title             Cat. Rank     
The Da Vinci Code Book 19
The Da Vinci Code Book 19
The Da Vinci Code Book 19
Sisterhood....    Book 21
Book X            Book 34
Book X            Book 34

```

I need to create a new rank column based on the current rank column where Da Vinci code gets a 1 in all rows with da vinci code. When a new title appears the rank should change to a 2 etc. 

I've been trying a lot of variations of rank, e.g.:

```
  dt.reviews.books.new[,sales_rank_cat:=rank(title, ties.method="first"),by=title]

```

But this assigns a 1 to every title.