There are three aspects to these questions that make them... awkward.
All the myriad ways
One of the aspects that students (lets face, its students that ask these questions, more on that later...) ask is that it gets comments back like:
Its different because inner loop is being incremented by loop variable of outer loop.
The people asking these questions have great difficulty in abstracting the solution to one answer to others.
One of my great fears with this is that we'll have the question:
for(i=1;i<=n;++i) {
x++;
}
What is the Big O?
And then the question:
for(i=1;i<=n;++i) {
for(j=1;j<=n;++j) {
x++;
}
}
What is the Big O?
And then:
while(++i < n) {
x++;
}
What is the Big O?
And so on. As professors seem to love assigning these homework problems, they keep tweaking the actual implementation and people who don't know how to calculate it in the first place also fail at understanding the abstractions.
Do my homework, or at least check it
These are students that are either asking us to do their homework, or check it. In one exchange on a question I recall:
So is it O(n^2)? -- Student
What makes you think it is O(n^2)? -- Expert
So does that mean its O(n)? -- Student
What makes you think it is O(n)? -- Expert
I really want to say that they either need to do the homework or hand it in. The questions that show no understanding of it, attempt at verifying it themselves, and the like are boring. I do acknowledge that not all people find these questions boring and there are indeed interesting Big O questions, just that the "here's a few nested for loops, what is the Big O?" or "here's a few nested loops, is it O(n^2 log n)?" pasted in a text box are boring.
Poor canonical duplicate
Yep. We have a rather poor canonical duplicate at the moment. It is a community wiki and wiki locked question. That means that it can't be voted on and that means its even harder to find (votes are used in search scoring).
The sum of the parts above is that these questions often fail to material for good answers to be created. It's not that it can't, but the majority of cases are just poor questions asking for poor answers.
I, as a pragmatist want to just say:
Write the code and stick it in a main class or function and count the iterations. Run it for 1, 5, 10, 50, 100, 250, 500, 1000, and 5000 iterations. Toss it in gnuplot or excel or your favorite graphing tool and see what the best fit is.
They're asking people who are drawing on expertise of software design and architecture. While it is useful to know, knowing it beyond "sorting something is O(n log n)" and "contains on an unordered list is O(n)", it isn't something that we run into too often (I run into space complexity problems more often... but we never get asked about that - even if it boils down to the same thing).
We really need a good canonical duplicate question for these if they are on topic here. I'd suggest the tag wiki, but as we know the support for placing information in the tag wiki and being able to find it (or "close as a duplicate of the tag wiki") in the Stack Exchange platform as it stands currently is poor at best.
