In this loop:
DO 180 J=1,9 K=9*(I-1)+J 180 ZX(K)=0.D0
How many times will
(ZX(K)=0.D0 execute? Once or 9?
Answer: 9 times
DO Loop Range
The range of a DO loop consists of all of the executable statements that appear following the DO statement, up to and including the terminal statement.