 Register  # 28.1. Limits to Computing¶

## 28.1.1. Limits to Computing¶

By now you have studied many data structures that can be used in a wide variety of problems, and many examples of efficient algorithms. In general, our search algorithms strive to be at worst in $O(\log n)$ to find a record, and our sorting algorithms strive to be in $O(n \log n)$. You might have come across a few algorithms have higher asymptotic complexity. Both Floyd’s all-pairs shortest-paths algorithm and standard matrix multiply have running times of $\Theta(n^3)$ (though for both, the amount of data being processed is $\Theta(n^2)$ since they both act on $n \times n$ matricies).

We can solve many problems efficiently because we have available (and choose to use) efficient algorithms. Given any problem for which you know some algorithm, it is always possible to write an inefficient algorithm to “solve” the problem. For example, consider a sorting algorithm that tests every possible permutation of its input until it finds the correct permutation that provides a sorted list. The running time for this algorithm would be unacceptably high, because it is proportional to the number of permutations which is $n!$ for $n$ inputs. When solving the minimum-cost spanning tree problem, if we were to test every possible subset of edges to see which forms the shortest minimum spanning tree, the amount of work would be proportional to $2^{|{\rm E}|}$ for a graph with $|{\rm E}|$ edges. Fortunately, for both of these problems we have more clever algorithms that allow us to find answers (relatively) quickly without explicitly testing every possible solution.

Unfortunately, there are many computing problems for which the best possible algorithm takes a long time to run. A simple example is the Towers of Hanoi problem which requires $2^n$ moves to “solve” a tower with $n$ disks. It is not possible for any computer program that solves the Towers of Hanoi problem to run in less than $\Omega(2^n)$ time, because that many moves must be printed out.

Besides those problems whose solutions must take a long time to run, there are also many problems for which we simply do not know if there are efficient algorithms or not. The best algorithms that we know for such problems are very slow, but perhaps there are better ones waiting to be discovered. Of course, while having a problem with high running time is bad, it is even worse to have a problem that cannot be solved at all! Such problems (which are called unsolveable problems) do exist. The classic example of such a problem is deciding whether an arbitrary computer program will go into an infinite loop when processing a specified input. This is known as the halting problem.  