
Dear list,
I have a problem with the cumsum function.
I have a data frame like the following one
variable        Year       value
EC01             2005         5

EC01             2006         10

AAO1            2005          2 

AAO1           2006          4
what I would like to obtain is
variable        Year       value   cumsum


EC01             2005         5           5


EC01             2006         10         15


AAO1            2005          2            2


AAO1           2006          4            6


if I use the by function or the aggregate function the result is a list or something else, what I want is a data frame as I showed above...
anyone knows how to get it???
THANKS A LOT





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