It’s rare for a CS major to graduate without getting a healthy dose of NP-completeness and Turing machines, two beautiful areas of theory that would be enjoyable if they didn’t end up creating bad instincts. One biologist asked me to solve a problem in DNA sequence matching and I came back to him with the claim that it was NP-complete, a class of problems that can take a very long time to solve. He didn’t care. He needed to solve it anyway. And it turns out that most NP-complete problems are pretty easy to solve most of the time. There are just a few pathological instances that gum up our algorithms. But theoreticians are obsessed with the thin set that confound the simple algorithms, despite being rarely observed in everyday life.Any of my three regular readers will know that I periodically obsess/carp about NP-fixation, so I'm sympathetic to the tenor of this. At the same time, I have a somewhat mixed reaction to it.

- "... NP-complete, a class of problems that can take a very long time to solve." This is certainly factually correct, and the author thankfully said "can" rather than "will". One thing that concerns me in general, though, is that not everyone grasps that problems in class P, for which polynomial time algorithms are known, can also take a very long time to solve. One reason, of course, is that "polynomial time" means run time is a polynomial function of problem size, and big instances will take longer. Another is that $p(n)=n^{1000}$ is a polynomial ... just not one you want to see as a (possibly attainable) bound for solution time. There's a third factor, though, that I think many people miss: the size of the coefficients (including a constant term, if any) in the polynomial bound for run time. I was recently reading a description of the default sorting algorithm in a common programming language. It might have been the one used in the Java collections package, but don't quote me on that. At any rate, they actually use two different sorting algorithms, one for small collections (I think the size cutoff might have been around 47) and the other for larger collections. The second algorithm has better computational complexity, but each step requires a bit more work and/or the setup is slower, so for small collections the nominally more complex algorithm is actually faster.
- "He didn’t care. He needed to solve it anyway." I love this. It's certainly true that users can ask coders (and modelers) for the impossible, and then get snippy when they can't have it, but I do think that mathematicians (and, apparently, computer scientists) can get a bit too locked into theory. <major digression> As a grad student in math, I took a course or two in ordinary differential equations (ODEs), where I got a taste for the differences between mathematicians and engineers. Hand a mathematician an ODE, and he first tries to prove that it has a solution, then tries to characterize conditions under which the solution is unique, then worries about stability of the solution under changes in initial conditions or small perturbations in the coefficients, etc., ad nauseum. An engineer, faced with the same equation, tries to solve it. If she finds the solution, then obviously one exists. Depending on the nature of the underlying problem, she may or may not care about the existence of multiple solutions, and probably is not too concerned about stability given changes in the parameters (and maybe not concerned about changes in the initial conditions, if she is facing one specific set of initial conditions). If she can't solve the ODE, it won't do her much good to know whether a solution exists or not.</major digression> At any rate, when it comes to optimization problems, I'm a big believer in trying a few things before surrendering (and trying a few optimization approaches before saying "oh, it's NP-hard, we'll have to use my favorite metaheuristic").
- "And it turns out that most NP-complete problems are pretty easy to solve most of the time. There are just a few pathological instances that gum up our algorithms." I find this part a bit misleading. Yes, some NP-complete problems can seem easier to solve than others, but the fundamental issue with NP-completeness or NP-hardness is problem dimension. Small instances of problem X are typically easier to solve than larger instances of problem X (although occasionally the Universe will mess with you on this, just to keep you on your toes). Small instances of problem X are likely easier to solve than large instances of problem Y, even if Y seems the "easier" problem class. Secondarily, the state of algorithm development plays a role. Some NP-complete problem classes have received more study than others, and so we have better tools for them. Bill Cook has a TSP application for the iPhone that can solve what I (a child of the first mainframe era) would consider to be insanely big instances of the traveling salesman problem in minutes. So, bottom line, I don't think a "few pathological instances" are responsible for "gum[ming] up our algorithms". Some people have problem instances of a dimension that is easily, or at least fairly easily, handled. Others may have instances (with genuine real-world application) that are too big for our current hardware and software to handle. That's also true of problems in class P. It's just that nobody ever throws their hands up in the air and quits without trying because a problem belongs to class P.

Very good points! I think it is important for students to know about complexity analysis of algorithms, but I tend to agree that for practice P vs NP has hurt more than it helped. We routinely solve scheduling problems with millions of binaries. We have no right to be able to do that and we can certainly craft instances that we wouldn't be able to solve. But planes fly, teams and umpires show up for games, drivers deliver packages.

ReplyDelete"or solved heuristically, with acceptance of a suboptimal final solution": Let us keep in mind that NP-completeness was originally defined only for feasibility problems. If any feasible suboptimal solution was found, the problem was already solved (concerning NP-completeness). I think proving optimality is even harder than NP-hard, because we have to solve an NP-complete problem (the feasibility problem) and another problem (the proof that there is no better solution). As far as I know, the latter has not been proven to be in NP, because there is no proof (i.e. solution) that we can verify in polynomial time. (For pure linear problems, such a proof - i.e. a farkas certificate - exists.)

ReplyDeleteThat's why I think we should call optimization problems NP-hard, not NP-complete.

Anyway, I agree that this is not very helpful for many practical problems which can be solved very quickly despite being mixed-integer programs.

Best regards,

Thomas