create flag based on row values in grep()

I have a 10-row data frame of tweets about potatoes and need to flag them based on the punctuation each tweet contains (questions marks or exclamation points). The grep function will return row numbers where these characters appear:

grep("\\?", potatoes$tweet)
grep("!", potatoes$tweet)

I've tried to create the flag variable question with mutate in dplyr as shown...

potatoes$question <- NA
potatoes <- mutate(potatoes, question = +row_number(grep("\\?", potatoes$tweet)))

Error in mutate_impl(.data, dots) : 
Column `question` must be length 10 (the number of rows) or one, not 3

I'm also happy to consider more elegant solutions than conditioning on the output of grep. Any help appreciated!