At the end of an iterated do loop such as
do I= 1 to 8;
you find the value of I to be one more than the maximum value (unless you have some code to skip out of the loop early).
So in your code (slightly reformatted for legibility)
32 do i=1 to 8;
33 mindt = min(of dat[*]);
34 maxdt = max(of dat[*]);
35 if res{i}^='0' then _res{i}=i;
36 end;
37
38 if _res{i}=. then do;
line 38 uses the value of the index variable I after ending the loop and will have a value of 9. So the array index is out of range as you define the array _res to have 8 elements. Line 44 will have the same issue.
Note that you will get the exact same value for
33 mindt = min(of dat[*]);
34 maxdt = max(of dat[*]);
So these really do not need to be in any sort of loop.