R - calculating the average value of a dataframe column from the top row to bottom row

Question

The title may not be that clear, since it was difficult to summarize the problem in a few words, although I don't think the problem is that difficult to solve. To explain the problem, let me share a dataframe for reference:

head(df, n = 10)

     team     score 
1       A        10       
2       A         4        
3       A        10        
4       A        16        
5       A        20        
6       B         5
7       B        11         
8       B         8    
9       B        16         
10      B         5       

I'd like to add a third column, that calculates the average score for each team, with the average score updating as I go down the rows for each team, and then resetting at a new team. For example, the output column I am hoping for would look like this:

head(df, n = 10)

     team     score   avg_score
1       A        10          10 
2       A         4           7
3       A        10           8
4       A        16          10
5       A        20          12
6       B         5           5
7       B        11           8 
8       B         8           8
9       B        16          10 
10      B         5           9


# row1: 10 = 10  
# row2: 7 = (10 + 4)/2  
# row3: 8 = (10 + 4 + 10)/3   
# ... 

with the pattern following, and the calculation restarting for a new team.

Thanks,

Answer 1

library("data.table")
setDT(df)[, `:=` (avg_score = cumsum(score)/1:.N), by = team]

or more readable as per the comment by @snoram

setDT(dt)[, avg_score := cumsum(score)/(1:.N), by = team]

#    team score avg_score
# 1:    A    10        10
# 2:    A     4         7
# 3:    A    10         8
# 4:    A    16        10
# 5:    A    20        12
# 6:    B     5         5
# 7:    B    11         8
# 8:    B     8         8
# 9:    B    16        10
# 10:    B     5         9