r - dplyr: put count occurrences into new variable

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/26784361/dplyr-put-count-occurrences-into-new-variable

Answer 1

All you need to do is group your data by both columns, "group" and "var1":

df %>% group_by(group, var1) %>% mutate(count = n())
#Source: local data frame [10 x 3]
#Groups: group, var1
#
#   group var1 count
#1      1    1     4
#2      1    1     4
#3      1    1     4
#4      1    1     4
#5      1    2     1
#6      2    1     1
#7      2    2     3
#8      2    2     3
#9      2    2     3
#10     2    3     1

Edit after comment

Here's an example of how you SHOULD NOT DO IT:

df %>% group_by(group, var1) %>% do(data.frame(., count = length(.$group)))

The dplyr implementation with n() is for sure much faster, cleaner and shorter and should always be preferred over such implementations as above.