R: How to get the average in a dataframe up to each observation? : I want to get the average score by Name going up to the date.

Here is an example dataframe

    Date    Name  Score

    1/1/16  Bill   5
    1/1/16  Hank   5 
    1/1/16  Aaron  10
    1/2/16  Hank   15
    1/2/16  Bill   10
    1/3/16  Bill   20
    1/3/16  Aaron  20
    1/4/16  Aaron  20

How can I add this column?


    Date    Name  Score Average_to_date
    
    1/1/16  Bill   5     NA
    1/1/16  Hank   5     NA
    1/1/16  Aaron  10    NA
    1/2/16  Hank   15    5
    1/2/16  Bill   10    5
    1/3/16  Bill   20    7.5
    1/3/16  Aaron  20    10
    1/4/16  Aaron  20    15

