r - Using filter with count

Question

I'm trying to filter row using the count() helper. What I would like as output are all the rows where the map %>% count(StudentID) = 3.

For instance in the df below, it should take out all the rows with StudentID 10016 and 10020 as they are only 2 instances of these and I want 3.

StudentID StudentGender Grade     TermName      ScaleName TestRITScore
100             M     9    Fall 2010    Language Usage          217
100             M    10    2011-2012    Language Usage          220
100             M     9    Fall 2010    Reading                 210
10016           M     6    Fall 2010    Language Usage          217
10016           M     6    Fall 2010    Mathematics             210
10020           F     7    Fall 2010    Language Usage          210
10020           F     7    Fall 2010    Mathematics             213
10022           F     8    Fall 2010    Language Usage          232
10022           F     9    2011-2012    Language Usage          240
10022           F     8    Fall 2010    Mathematics             242

if I do:

count(df, StudentID)

then it only gives me a df with 2 columns, but I want to keep all the columns of my df. thats's why I think I should use filter.

Answer 1

I don't think count is what you looking for. Try n() instead:

df %>% 
  group_by(StudentID) %>%
  filter(n() == 3)

# Source: local data frame [6 x 6]
# Groups: StudentID
# 
#   StudentID StudentGender Grade  TermName      ScaleName TestRITScore
# 1       100             M     9 Fall 2010 Language Usage          217
# 2       100             M    10 2011-2012 Language Usage          220
# 3       100             M     9 Fall 2010 Reading                 210
# 4     10022             F     8 Fall 2010 Language Usage          232
# 5     10022             F     9 2011-2012 Language Usage          240
# 6     10022             F     8 Fall 2010 Mathematics             242