dplyr: filter rows according to aggregated function result

Question

I have a table listing (amount, year and month) and I want to filter the rows corresponding to complete years. I.e. I want to ommit the last 4 rows of the sample dataframe I

Answer 1

tally() is the equivalent of summarise(n=n()). However, in this case you want to keep the original rows of the data frame, but filtered so that rows that are part of incomplete years are removed. @AndresT's answer will work fine, but you can also do it more concisely without an intermediate step of creating a column to count the number of rows for each group:

df %>% group_by(year) %>% filter(n()==12)