“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

We can do this in data.table

library(data.table)
reference <- setDT(df2)[df, .N, on = .(object), by = .EACHI]$N
df$reference <- reference
df
#   object group reference
#1:  apple     A         3
#2: banana     B         1
#3:   pear     A         0
#4:  robot     C         3