Tuesday, April 3, 2012

Inductive reasoning, and why you should care

Inductive reasoning, according Wikipedia:

Inductive reasoning, also known as induction, is a kind of reasoning that constructs or evaluates propositions that are abstractions of observations of individual instances of members of the same class.

That probably conveys absolutely no value to you. The best way to understand what inductive reasoning is, is to apply it.

In order to do that, we need some kind of problem which we can solve using a bit of induction.

I'm going to a pick a math problem. Why, you say? Math is almost always the easiest way to understand reasoning, since math's primary device of "progress", so to speak, is reasoning.

So, here's the problem:

The plane is divided into regions by drawing a finite number of straight lines. Show that it is possible to color each of these regions red or green in such a way that no two adjacent regions have the same color.

If at first this doesn't strike you as a math problem, you probably haven't met this chap called graph theory, or maybe haven't been introduced to bipartite graphs.

So, how do we go about solving this?

Before reading on, try out some cases; one line, two lines, ten lines, and so on. Try and figure out some patterns.

Now, I'll show you the solution.

First we consider one line. So, we can color the regions on opposite sides of the line opposite colors (i.e. red and green), and, we're done.

So, what happens when we add one line? Think about this for a moment. Can we try the same trick?

Yes! What we can do is take one side of the new line we just put in, and swap all the colors on that side (so, red goes to green, green goes to red), and, we get a complete solution! So, every time we add a line, we can simply follow this algorithm and get the correct coloring. Of course, this isn't a fully rigorous proof, but, it shows what we're after.

Inductive reasoning is taking a method you used to solve a problem for a set constraint (we knew how to do it for one line), and then extending that method to solve the problem without the constraint.

If write code, you probably do this all the time (and just didn't know what this was called). If you've got a function that's misbehaving, you pass in a couple of values that you think are forming an edge case, and you've noticed that its going wrong for *one* of these values, you change the function definition so that the errors don't happen.

But, you should use this kind of reasoning much more often. In nearly all problems where you are stuck on something going wrong on a constraint, try inductive reasoning. Of course, none of this actually happens consciously, but, if you try doing it consciously a couple of times, it "just happens" eventually.


The Travelling salesman problem

The travelling salesman problem is possibly the most famous and probably the most easy to understand of the so-called "NP-hard" problems computer scientists (and mathematicians) have found.

The problem statement is quite simple.

Consider you are a salesman, and you have, say, an $n$ number of cities you've got to cover by the end of the day. But, it would be best if you could minimize the distance that you have to travel between all of the cities, and, you know the coordinates of all the cities in reference to some point known as the *origin*. Given these coordinates, define an algorithm that would give you shortest possible path between the cities.

Of course, this doesn't seem like much of a problem at all when you first start at it (like most other difficult problems); you could just try all the routes between the cities, find the route with the minimum distance covered, and, you're done!

Let's see what that entails in terms of performance/scaling. With $n=5$ (i.e. five cities to visit), we have a total of $5! = 5 \cdot 4 \cdot 3 \cdot 2 \cdot 1 = 120$ ways we can trace out the route between the cities, and we have to check all of these. That doesn't seem too bad for a computer.

How about for $n=10$? Well, $10! = 3,628,800$, so, its quite a big jump from $120$, but, still manageable.

Let's go up by another five; $n=15$. That gives: $15! = 1,307,674,368,000$. Holy cow. That's a lot. But, how did it jump that quickly?!

To answer that question, we'll need a bit of math (if you understand big Oh notation, you can skip the next couple of paragraphs).

Consider this situation. You have one car that's accelerating from a low speed, whereas another car is going at a higher speed, but, the second car's speed will not change. What do you think will happen if we let the cars go on forever? Of course, the car that's accelerating will overtake the car that's at a constant speed, right?

That's what scaling algorithms is all about. You might have one algorithm that performs better at lowever "sizes" of the input, but, a second algorithm may overtake it if it can "scale better" (i.e. accelerate). So, when we say that an algorithm is $O(n)$, what we mean is that the algorithm performs *linearly*, i.e. for each unit increase in $n$, we get a certain unit increase in the time spent (or, whatever else you're measuring). Similarly, $O(n^2)$ means that for every unit increase in $n$, we get that much of change, but, squared in output.

But, all of this still seems on some shaky ground, so, let's break out the equations (warning: if you don't know how limits work, skip this). This won't be a *completely* rigorous argument (which would require more math), but, it will be much better than the argument we have in place.

How can we say that a linear time algorithm (i.e. $O(n)$) will scale better than $O(n^2)$? Well, what we're informally saying is that the $O(n^2)$ grows much quicker (i.e. worse) than $O(n)$ as $n \to +\infty$. If you remember that bit about relative rates of growth from first semester calculus; $\lim_{n \to +\infty} \frac{n^2}{n} = \lim_{n \to +\infty} n = +\infty$

So, we've proved that $O(n^2)$ grows faster than $O(n)$, and therefore performs worse than $O(n)$.

What does that mean in terms of the traveling salesman problem? The traveling salesman problem brute force solution is in fact $O(n!)$ (since the number of routes goes up like that).

Why is this so bad?

Consider a polynomial time solution (i.e. $O(a_n x^n + a_{n-1} x^{n-1} ... + a_o)$). Using a bit of calculus/limits, we can say, for certain, that factorial algorithms are much worse than polynomial time solutions as $n \to \infty$.

That's what makes the travelling salesman problem interesting, because it has a non-polynomial solution, and, its something called a NP-Hard problem. What NP-Hard means is actually very complicated and heavily theoretical, but, it consitutes a very important part of the P vs NP problem, which, informally, asks (quoted from Wikipedia): "whether every problem whose solution can be quickly verified by a computer can also be quickly solved by a computer" (s/quickly/polynomial time/g).

So, that's what the travelling salesman problem is, and why people care.