How would you implement 'var = ...' while respecting that 'VecAt(...)' could return 0? I am currently working on a similar problem and can't seem do seem figure

How would you implement 'var = ...' while respecting that 'VecAt(...)' could return 0? I am currently working on a similar problem and can't seem do seem figure out how to handle these for-loops and setting the iterator properly.