“longer object length is not a multiple of shorter object length”

Question

I have this dataset -

print(df)

  object    group   
1 apple      A    
1 banana     B    
1 pear       A    
1 robot      C

print(df2)

  object    group   
         


        

Answer 1

From my comment: dplyr functions work on the whole column taken as a vector. Try

df %>%
rowwise() %>% 
mutate(reference = length(df2[df2$object == object,]$object))%>%
ungroup()

As you said, ungroup will be needed, unless you plan on doing further row-wise operations.