I have a tbl_df with several columns.  I am running a simple mutate on it to update a column, but the column is not being updated.  
mutate(fltr, cat = "xxxxx")
cat is a column that is either empty or NA.  Filter is a tbl_df.  What could be causing this?  I have tried to put text in the cat column so that it is not empty or NA in case that was causing the problem. That still did not work.
mutate doesn't change the tbl_df in place, it just returns the new, changed tbl_df. You need to save the results:
fltr <- mutate(fltr, cat = "xxxxx")
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