根据data.table中其他列的计算值来计算一列的滚动总和

Question

I am trying to calculate values in a column from calculated values from another column.

DT <- data.table(X = c("a","a","a", "b","b","b", "c","c","c"),
                 Y = rep(c(1,2,3),3),
                 z1 = c(10, 11,12),
                 z2 = c(1,2,3))

Here in DT, I want to calculate a new column Z which is sum of z1 and z2 for first row for each X. Next row calculation should be done from the previously calculated Z + current row z2 for all rows of "a". Similarly for "b", "c".
I did try with "for" loops but was not successful. The end result looks like this.

X   Y   z1  z2  Z
a   1   10  1   11
a   2   11  1   12
a   3   12  1   13
b   1   10  2   12
b   2   11  2   14
b   3   12  2   16
c   1   10  3   13
c   2   11  3   16
c   3   12  3   19

Answer 1

您可以使用

DT[, Z := cumsum(z2) + z1[1], by= X]