Cover by Toby Segaran

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Programs As Trees

In order to create programs that can be tested, mutated, and bred, you'll need a way to represent and run them from within your Python code. The representation has to lend itself to easy modification and, more importantly, has to be guaranteed to be an actual program—which means generating random strings and trying to treat them as Python code won't work. Researchers have come up with a few different ways to represent programs for genetic programming, and the most commonly used is a tree representation.

Most programming languages, when compiled or interpreted, are first turned into a parse tree, which is very similar to what you'll be working with here. (The programming language Lisp and its variants are essentially ways of entering a parse tree directly.) An example of a parse tree is shown in Figure 11-2.

Example program tree

Figure 11-2. Example program tree

Each node represents either an operation on its child nodes or an endpoint, such as a parameter with a constant value. For example, the circular node is a sum operation on its two branches, in this case, the values Y and 5. Once this point is evaluated, it is given to the node above it, which in turn applies its own operation to its branches. You'll also notice that one of the nodes has the operation "if," which specifies that if its leftmost branch evaluates to true, return its center branch; if it doesn't, return its rightmost ...

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