r - Create a new column whose formula depends on a cell value of another row -



r - Create a new column whose formula depends on a cell value of another row -

how create new column formula depends on cell value of row

x y z 1 1 10 2 2 20 3 3 30 4 b 1 40

this sample data. want final output follows

x y z prevy 1 1 10 0 2 2 20 10 3 3 30 20 4 b 1 40 0

where prevy z value x=current_x_val , y=current_y_val-1 0 if not available.

how accomplish this.

my progress far :

data[data$x == "a" & data$y==2-1,3]

i manually come in values , prevy each row. how do rows in single shot ?

or data.table solution (similar mrflick) faster big info set

library(data.table) setdt(dat)[, prevy := c(0, z[-length(z)]), = x]

r

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