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Parallelism, the concurrent activity of many rules, is an important aspect of classifier systems. Parallelism makes it possible for the system to combine rules into clusters that model the environment, providing two important advantages: |
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1. Combinatorics work for the system instead of against it. The system builds a ''picture" of the situation from parts, rather than treating it as a monolithic whole. The advantage is similar to that obtained when one describes a face in terms of component parts. Select, say, 8 components for the facehair, forehead, eyebrows, eyes, cheekbones, nose, mouth, and chin. Allow 10 variants for each component partdifferent hair colors and textures, different forehead shapes, and so on. Then 108 = 100 million faces can be described by combining these components in different ways. This at a cost of storing only 8 X 10 = 80 "building block" components. |
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2. Experience can be transferred to novel situations. On encountering a novel situation, such as a "red car by the side of the road with a flat tire," the system activates several relevant rules, such as those for "red," "car," "flat tire," etc. When "building-block" rules, such as those for "car," have proved useful in past combinations, it is at least plausible that they will prove useful in new, similar combinations. To exploit these possibilities, the rules must be organized in a way that permits clusters of rules to be activated in changing combinations, as dictated by changing situations. Building-block rules then give the system a capacity for transferring experience to new situations. |
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To define a standard classifier system, we first require all messages to be bit-strings of the same length, k, much as one sets the register size for a computer. Formally, then, messages belong to the set {1,0}k. The condition part of a rule is specified by the use of a "don't care" symbol, #, reminiscent of the "don't care" used to define schemata. Thus, the set of all conditions is the set {1,0,#}k. For k=6, the condition 1##### is satisfied by any message that starts with a 1, while the condition 001001 is satisfied by one and only one message, the message 001001. It is worth noting that a condition's specificity (the reciprocal of the number of messages that satisfy it) depends directly upon the number of #s in the conditionthe more #s, the lower the specificity. |
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In the standard system, all rules consist of two conditions and a single outgoing message, which is sent when the two conditions are satisfied. Rules are specified in the format |
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