The question shows that you have a hard time to understand the complexity of it. Maybe the reason is that you are an experienced developer who deals with all the complexity quite naturally and automatically.
So why is this task complex?
Well, what happens if there are no numbers in the table to begin with? Is the sum of no numbers a zero or is it supposed to be some error?
And are all the numbers supposed to be natural numbers or can there be e.g. irrational and complex numbers be in there? If so, how much precision do we need when summing them up - and how important is performance?
Also, what if there are not only numbers but other things (dates, text, ...) by accident?
And in a real world scenario, what happens when numbers are added to the table during the calculation? Should they be considered or ignored - should the table be locked somehow?
The problem with these questions is not that they are necessarily hard to answer (often they are) but that people don't even know, that they need to be asked in the beginning.
Many developers are so used to it, they often don't understand that they are doing things that are difficult for normal people.
In the same way mathematicians find it easy to do basic algebra for some more difficult task, it's just part of there toolkit - whereas most people don't even understand this basic part from the beginning, let alone more complex problems.
> The problem with these questions is not that they are necessarily hard to answer (often they are) but that people don't even know, that they need to be asked in the beginning.
Right. And what makes it even more problematic is that answers to these questions affect what answers are possible to other questions. So it is not the sum of the difficulties of the answer to each question but their product, the combinations of different possible answers.
Adding the numbers is not the problem, the problem is what the numbers represent, where do they come from (if they come from only one place...) where and for what are they needed.
Quickly you have to consider different kinds of numeral systems, bases, decimal separators (which may be locale dependant), precision, units, the difference in adding time (or dates), or money in different currencies (with and without different kind of taxes), etc, etc, etc.
Sure most of the times you can abstract yourself from all this, use sensible defaults, whatever, but that complexity is still there and sooner or later it will get you, even if you try to hide it under the rug...
The complexity and misunderstandings also creep into system design itself. Seems many are eternally astounded that transactional data is "duplicate data" stored separately from the "same data", etc. Many of these complexities are non-obvious and unintuitive, until you are forced to think it through step by step yourself. Agile is simply the concession that the complexity cannot always be handled up-front, but that solutions must be developed to be adaptable to new observations and realizations.
You're starting from a solution. What exactly is the problem in this case? Generate a balance statement? You will have to divide it into smaller problems until each problem is small enough that it has an obvious solution. The fact that you only need an addition of some numbers in a table is the result of lots of thinking that you just glossed over.
How many jQuery table plugins are there? Sort, filter, pagination, partial display. And yet it is impossible to find components that meets all requirements.
