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.

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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

Using filter with count

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