5.1: A simple CA
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Cellular automatons1 are governed by rules that determine how the state of the cells changes over time.
As a trivial example, consider a cellular automaton (CA) with a single cell. The state of the cell during time step i is an integer, xi. As an initial condition, suppose x0 = 0.
Now all we need is a rule. Arbitrarily, I’ll pick xi+1 = xi + 1, which says that during each time step, the state of the CA gets incremented by 1. So this CA performs a simple calculation: it counts.
But this CA is atypical; normally the number of possible states is finite. As an example, suppose a cell can only have one of two states, 0 or 1. For a 2-state CA, we could write a rule like xi+1 = (xi + 1) % 2, where % is the remainder (or modulus) operator.
The behavior of this CA is simple: it blinks. That is, the state of the cell switches between 0 and 1 during each time step.
Most CAs are deterministic, which means that rules do not have any random elements; given the same initial state, they always produce the same result. But some CAs are nondeterministic; we will see examples later.
The CA in this section has only one cell, so we can think of it as zero-dimensional. In the rest of this chapter, we explore one-dimensional (1-D) CAs; in the next chapter we explore two-dimensional CAs.
1You might also see the plural “automata”.