thanks Reeza, what I have is: Num Rev Date 3 45454 Jan-01-2013 4 354353 Feb-01-2013 7 25425 Mar-01-2013 3 254235 Jan-01-2014 8 Feb-01-2014 9 Mar-01-2014 12 Jan-01-2015 45 Feb-01-2015 And what I want is following, with conditions: 1) When Date<=Jan-01-2014, Rev_r=Rev; 2) When Date>Jan-01-2014, Rev_r=lag2(Rev_r) * exp(Num) Num Rev Date Rev_r 3 45454 Jan-01-2013 45454 4 354353 Feb-01-2013 354353 7 25425 Mar-01-2013 25425 3 254235 Jan-01-2014 254235 8 Feb-01-2014 75790856.82 9 Mar-01-2014 2060087542 12 Jan-01-2015 1.23353E+13 45 Feb-01-2015 7.19677E+28
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