Don’t implement a bad algorithm

That’s a lesson I learned yesterday, the hard way. We’re currently learning about tree-like data structures in my computer science class and one of our four semester projects was writing a balancing method for an AVL-tree. Though the concept is very simple and fairly efficient (the balancing is logarithmic in the size of the tree on average), it’s very easy to come up with algorithms that seem to do the right thing, but really don’t. And that’s true of a lot of algorithms that programmers come up with. If that were the only thing wrong it wouldn’t be too much of a problem: simply discard the algorithm and create a new one. Unfortunately, the tendency for most student programmers is to simply implement the first algorithm that comes to mind and then start bug-hunting. Though I try hard to avoid doing this, I let down my guard this time. The result as you can imagine was not pretty.

If the algorithm is in itself a good one, which will give the correct result if applied right, ironing out the implementation details is not particularly hard. But in this case, I did not care to check if the algorithm was right. When trying to balance an AVL tree, there are a number of cases which need to be considered and dealt with. There are not a lot of them, but each of them needs a slightly different procedure to be taken care of properly. If I had started by breaking up the problem into each of the sub-problems and then writing code to deal with each case, the problem would have been simple (I finally did that and it took me less than an hour to get everything working). But instead, I tried to come up with a generic algorithm which could be altered to fit each of the cases. Now the idea of building a generic abstraction which can act in a number of different ways based on input and other conditions is a very powerful idea and can greatly reduce the amount of redundant code in a large system (closures anyone?). However, it should be kept in mind that a number of times, its actually easier to implement a slightly cluttered redundant system than spend time and effort trying to create a conceptually more complex system to reduce structural complexity. I made the mistake of not keeping this mind. And my algorithm did not work.

Things would still have worked out if I realized at that point that it would be easier to just follow the book and code solutions for the cases separately (abstracting only the most obvious parts into generic solutions). But I didn’t realize that. I tried to hammer and mutilate my broken algorithm to react as I wanted to the different cases and I kept piling on more and more code, desperately hoping that somehow it would also start working. I wasted a good 8 hours trying to get my abysmal mess of a balancing method to work right and I ended in utter and abject failure. In many ways, those have been my darkest hours since I first started programming about 5 years ago.

But yes, I do have a happy ending. 6 hours of sleep and a physics class later, I finally decided that enough was enough and that I was going to start from scratch, this time following the book’s recommendations and keeping excess complexity to a bare minimum. I was done in under an hour. So what’s the moral of this story? Spend more time thinking than you do coding and don’t implement an algorithm until you are fairly sure that it works the way you want it to to.

Sure, some bugs won’t be revealed until you actually have some code to compile and test, but there’s no way that you are going to code your way out of a broken algorithm. It’s very alluring to just sit down at a computer and start typing away. It makes you feel very productivity because there’s something happening on screen. In contrast, simply sitting down and working out an algorithm in your head often makes you feel that you are not really doing anything. It’s unfortunate that we call what we do “programming” when much of our time and effort is actually spent program planning (or should be). As the preface to Structure and Implementation of Computer Programs says, programs should be written primarily for people to read and understand and only incidentally for machines to execute. It follows from that if you can’t understand your own program and prove its correctness, you shouldn’t expect a computer to do the same. That’s a lesson I learned the hard way and I hope I never have to relearn it.

2 thoughts on “Don’t implement a bad algorithm

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