I would like to change the start integer for a DO-loop each time the DO runs as a result of the calculations of the previous iteration. For example the first time the DO runs it should start at 1, the second time at 33, the third time at 60. The numerical value for the end of the loop stays the same. I have tried to change the iterator dynamically as can be seen in the code below, but the g77 gives an error. ACS = 1 NUME = 0 DO 12, AC = ACS,NH IF (VI(AC) .NE. 100.0) THEN VF(AC) = VI(AC) GOTO 12 ELSEIF (VI(AC) .EQ. 100.0) THEN DO 80,AB = AC,AC+24 IF (VI(AB) .EQ. 100) NUME = NUME + 1 IF (VI(AB) .NE. 100) GOTO 81 80 CONTINUE 81 IF ((VI(AC-1)- VI(AC+NUME)) .GT. 0.0) THEN DO 77, VAL = AC,AC+NUME+1 VF(VAL)=VI(AC)-((VI(AC-1)-VI(AC+NUME))/(NUME+1)) 77 CONTINUE ELSEIF ((VI(AC-1)- VI(AC+NUME)) .LT. 0.0) THEN NUM = 0 DO 99, VAL = AC,AC+NUME-1 NUM = NUM + 1 VF(VAL)=VF(AC+NUM-2)+((VI(AC+NUME)-VI(AC-1))/(NUME+1)) AC = VAL -1 <=== compiler error 99 CONTINUE How can I modify AC without getting a compiler error or is an alernative logic necessary??