r - How to get the average (or sum, count, etc.) of variables for each group ID? -

this question has answer here:

• how average columns based on id in r? [duplicate] 3 answers
• getting mean value every id in data frame 5 answers

i have data frame list of ids , other variables so:

student_id       score 6                94 2                63 6                100 7                44 6                97 2                67 

i create data frame consists of student_id , average score this:

student_id       avg_score 2                65 6                97 7                44  

the actual data set larger, of course.

you can use dplyr package:

df %>% group_by(student_id) %>% summarise(avg_score = mean(score))   # # tibble: 3 x 2  #   student_id avg_score  #        <int>     <dbl>  # 1          2        65  # 2          6        97  # 3          7        44 

you can use aggregate in base r:

aggregate( score ~ student_id, df, mean) #column name remain "score"  #   student_id score  # 1          2    65  # 2          6    97  # 3          7    44