Fill missing data by character value

Question

I have a df that looks like this

ring_no.    CR     
1           BMGG  
1           BMGG  
1
1
1
2           BMYY
2
3           BMRR

I'd like fill the missing data (blank cells in the df) in the CR column with character values based on the ring_no. I.e. 1 = BMGG, 2 = BMYY, 3 = BMRR

If possible I'd like a dplyr approach.

I hope that makes sense?

thanks