how to use loop to calculate row by row without creating new rows

[rawagah]

Hello,

I have this dataset:

Year	Amount
2016	1000
2017	2000
2018	.
2019	.
2020	.
2021	.

and I want to do a calculation on a row level because the output of the calculation in 2018 for example depends on the output from 2017 and so on. For simplicity let's say I want to add column (Expected), which will be = this year's Amount if this year's Amount is not Null, otherwise it will be the output of last years (Expected) * 0.05.

so it will be like this:

Year	Amount	Expected
2016	1000	1000
2017	2000	2000
2018	.	100
2019	.	5
2020	.	0.25
2021	.	0.0125

[PaigeMiller · Accepted solution]

data want;
    set have;
    retain expected;
    if not missing(amount) then expected=amount;
    else expected=expected*0.05;
run;