[R] how to create three new variables? Thanks a lot!

Dear R experts: I have following data structure: student id, exam time and score. I want to create three new columes: 1 st before, 2nd before and 3rd before. For example, for student 1's 4th exam score is assumed to be influenced by his previous three cloest scores, which are 6, 9, and 10 in that

[R] how to manipulate this data? Thank you very much!

Hi, R experts: I am asking for helps with manipulating this data: we have different groups: A,B, C, and so on. In each group, we have different students, for example, we have 6 students in A and 5 students in B. Each student gives a rating, which ranges from 1 to 3. We want to manipulate this dat

[R] Data Manipulation, add frequency index

Hi, there, I have a simple data manipulation question for you. Thank you for your help! Suppose that I have this data about people appearing in a class Mary Mary Mary Sam Sam John John John John Then I want to find out what exact time(s) the student appears at the moment such as Mary 1 Mary