Binary search in particular is surprisingly tricky, which is precisely what makes it useful for telling if someone knows how to program. To a significant extent, though, you can cheat by studying binary search itself, which is a surprisingly beautiful thing.

I like this formulation for finding the first index in a half-open range where p is true, assuming p stays true thereafter:

    bsearch p i j :=
     i                   if i == j else
     bsearch p i       m if p m    else
     bsearch p (m + 1) j
     where m := i + (j - i)//2

Or in Python:

    def bsearch(p, i, j):
        m = i + (j - i) // 2
        return (i if i == j
                else bsearch(p, i, m) if p(m)
                else bsearch(p, m+1, j))

The only tricky thing about this formulation is that m < j if i < j, thus the asymmetric +1 in only one case to ensure progress. If invoked with a p such as a[m] >= k it gives the usual binary search on an array without early termination. The i + (j - i) // 2 formulation is not needed in modern Python, but historically an overflowing (i + j) // 2 was a bug in lots of binary search library functions, notably in Java and C.

(Correction: I said a[m] <= k. This formulation is less tricky than the usual ones, but it's still tricky!)

> Binary search in particular is surprisingly tricky, which is precisely what makes it useful for telling if someone knows how to program.

That's the problem. There are many other ways to do that without risking false negatives and annoying potential candidates (e.g. I would not reapply to places that have rejected me due to skepticism about my programming abilities and using tests blatantly irrelevant to day-to-day work because it's a bad indication of the engineering culture).

Even FizzBuzz is better at accomplishing that task.

FizzBuzz (or equivalent) is actually great IMO. It weeds out the people who lied on their resume, without punishing the people who never learned CS because they were too busy learning things that were actually useful to DS, like statistics or data visualization.

I've actually been given fizzbuzz in a DS interview! Up to that point I thought that fizzbuzz was just a meme because it's obviously too easy.

I tried to make Fizzbuzz on a paper when I first heard of it, and it had a bug printing fizzbuzzfizzbuzz on 15.

If you want a correct program without a compiler/computer I don't think anything is too easy. Maybe like, "make a function returning the sum of two float parameters".

That would just test syntax, though. Fizzbuzz tests logic. Your bug was a logic bug.

To a certain extent you can dispense with mental logic by using a compiler. But the feedback loop is much slower. Thinking your logic through before feeding it to a compiler is like looking at a map when you're driving a car; you can cut off whole branches of exploration.

Binary search is a particularly tricky logic problem in part because it's so deceptively simple. In a continuous domain it's easy to get right, but the discrete domain introduces three or four boundary cases you can easily get wrong.

But the great-great-grandparent is surely correct that many programming jobs don't require that level of thinking about program logic. Many that do, it's because the codebase is shitty, not because they're working in an inherently mentally challenging domain.

Ye I meant running it and then correcting the error.

Concerning binary search I acctually implemented that in an ECU for message sorting. It took like a whole day, including getting the outer boundries one off too big in the first test run. Funnely enough the vehicle ran fine anyway.

I would never pull that algorithm off correctly in an interview without training to, I think.

Take a look at the downvoted-to-0 formulation I gave upthread, then see if you can program it that way tomorrow without looking, and then think it through to see if it could possibly be wrong, and once you're satisfied it's correct, try testing it. Probably you'll never need to implement binary search yourself again, but it's a good exercise for thinking through algorithms. You can probably get it working that way in under an hour instead of a whole day.

There are levels of not knowing how to program that go beyond FizzBuzz. But sure, many programming jobs don't require them.

If that's the case for the DS/ML domain, then a short take-home exam should provide a better example of practical coding ability (the common counterargument that "take-home exams can be gamed" is a strawman that would be more on the interviewer's fault for creating a flawed exam).

In my case, I typically got the "implement binary search" questions in a technical interview after I passed a take-home exam, which just makes me extra annoyed.

Agreed.

If you're gaming the take-home exam by looking up the answer on Stack Overflow, you could game the same exam in person by reading books of interview questions ahead of time, and the interviewer can avoid that by making up new questions. (OTOH if you're gaming the take-home exam by paying someone else to solve the problem for you, that might be harder to tell.)

Why is that gaming the exam? What sort of professional doesn't look up the solutions to potential problems online, even if it is just to verify that you're correct? Outside of incredibly trivial things, I would expect this of everyone.

I suppose it depends on whether the purpose of the exam is to see if you know how to write working code to solve new problems or how to look up known solutions to well-known problems. Both are valuable skills, but they are definitely not the same skill. Perhaps telling the difference is one reason interviews frequently include in-person programming challenges rather than using take-home exams.

In most cases the right way to do a binary search is not to copy and paste a binary search implementation from Stack Overflow (https://stackoverflow.com/a/41956372 is similar to the formulation I gave above), but to call a binary-search library function. If calling a library function isn't the solution the interviewer is looking for, probably they wouldn't be satisfied with you searching for it on Stack Overflow either.