Thanks for the response. The second set of SAS code I provided does do what I want. Basically, for each date I want to calculate the same thing - only once. However, because the lag is used in the calculation for subsequent dates, I have to update the lags (the lags are used in the forecast). The first set of data is the output for a PROC REG. Then I use the output model to create the forecast, I want to create a simple DO statement does what the second set of code produces (it works as intended, I just want to simplify the code). There is a DO statement in the first (if date>='31dec2018'd then DO), it seems to do the same thing as the code you provided, It seems to loop through many times. Not just executes once for each date and updates the lags and replace the missing data with the calculated values. I attached the ao.forecast file, that may help make sense of what I want to do. Thanks.
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