I am pretty sure that ARRAYs are covered in the documentation.
But perhaps you meant the DO OVER?
I find it much each to explain this type of situation by using a concept that matches the application. In this problem the "INDEX" that is required if you do not use the DO OVER construct has no meaning. The purpose of the DO loop is apply the same logic to EVERY member of the array. The relative order of the members of the array is meaningless.
Now if the variables represented something where an index makes sense then it is clearer to use index notation. So if the readings represent results for the months of the year it would make more sense to write:
array reading m1-m12 ;
do month=1 to 12 ;
if reading(month)= 999 then reading(month)=X;
end;
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