9.1.1. Models of Computation¶

So far we have seen a few simple machine types, such as DFA, NFA, PDA, RegEx. And we have seen corresponding grammars: Regular grammars, context-free grammars, etc. There are some differences in the machines. DFAs are deterministic. NFAs add non-determinism, which simply means that there can be multiple choices out of a state for the same input character. The PDA adds the concept of a stack for storage.

These machines all have some similarities. They all take an input string. They all march across the string from left to right, one character at each step. They stop when they reach the end of the string and then make a simple decision: If the machine is in a final state, then it accepts the string, and otherwise it rejects the string. In other words, they are acceptors for some language. All that they can do is determine if a string is a member of the language, and the “more powerful” machines can accept or reject string from more “complicated” languages.

But there is a lot more that we typically expect when we talk about “computation”. Like, well, the ability to “compute” rather than simply accept. What does “compute” actually mean? For example, we would like to be able to take as input the arithmetic expression “\(2 + 3\)” and compute the value 5. This means that we need to be able to output that computed value 5. Or put another way, we can “compute” a value by taking the input string “\(2 + 3\)” and replacing it with the output string “5”.

In its most general form, we can think of everything that a computer does as taking some string as input, and then providing some string as output. Of course, modern peripheral devices like keyboards, mice, and computer monitors give us rich ways to express input strings (perhaps as button presses), and rich ways to interpret output strings (say, as pictures). But it’s not a huge stretch of the imagination to consider computation as converting an input string to an output string.

This concept of converting is far more powerful than simple accepting. A machine that takes an input and provides an output is called a transducer.

In the next section, we will introduce a simple machine, the Turing Machine, that is a transducer. It is only slightly more complicated than the machines that we have seen so far, and only slightly different in its operation. But these differences are significant. Ultimately, we will see that a Turing Machine can do any computation that even the most sophisticated modern computer can do.