C++ Neural Networks and Fuzzy Logic by Valluru B. Rao M&T Books, IDG Books Worldwide, Inc. ISBN: 1558515526   Pub Date: 06/01/95

Chapter 3
A Look at Fuzzy Logic

Crisp or Fuzzy Logic?

Logic deals with true and false. A proposition can be true on one occasion and false on another. “Apple is a red fruit” is such a proposition. If you are holding a Granny Smith apple that is green, the proposition that apple is a red fruit is false. On the other hand, if your apple is of a red delicious variety, it is a red fruit and the proposition in reference is true. If a proposition is true, it has a truth value of 1; if it is false, its truth value is 0. These are the only possible truth values. Propositions can be combined to generate other propositions, by means of logical operations.

When you say it will rain today or that you will have an outdoor picnic today, you are making statements with certainty. Of course your statements in this case can be either true or false. The truth values of your statements can be only 1, or 0. Your statements then can be said to be crisp.

On the other hand, there are statements you cannot make with such certainty. You may be saying that you think it will rain today. If pressed further, you may be able to say with a degree of certainty in your statement that it will rain today. Your level of certainty, however, is about 0.8, rather than 1. This type of situation is what fuzzy logic was developed to model. Fuzzy logic deals with propositions that can be true to a certain degree—somewhere from 0 to 1. Therefore, a proposition’s truth value indicates the degree of certainty about which the proposition is true. The degree of certainity sounds like a probability (perhaps subjective probability), but it is not quite the same. Probabilities for mutually exclusive events cannot add up to more than 1, but their fuzzy values may. Suppose that the probability of a cup of coffee being hot is 0.8 and the probability of the cup of coffee being cold is 0.2. These probabilities must add up to 1.0. Fuzzy values do not need to add up to 1.0. The truth value of a proposition that a cup of coffee is hot is 0.8. The truth value of a proposition that the cup of coffee is cold can be 0.5. There is no restriction on what these truth values must add up to.

Fuzzy Sets

Fuzzy logic is best understood in the context of set membership. Suppose you are assembling a set of rainy days. Would you put today in the set? When you deal only with crisp statements that are either true or false, your inclusion of today in the set of rainy days is based on certainty. When dealing with fuzzy logic, you would include today in the set of rainy days via an ordered pair, such as (today, 0.8). The first member in such an ordered pair is a candidate for inclusion in the set, and the second member is a value between 0 and 1, inclusive, called the degree of membership in the set. The inclusion of the degree of membership in the set makes it convenient for developers to come up with a set theory based on fuzzy logic, just as regular set theory is developed. Fuzzy sets are sets in which members are presented as ordered pairs that include information on degree of membership. A traditional set of, say, k elements, is a special case of a fuzzy set, where each of those k elements has 1 for the degree of membership, and every other element in the universal set has a degree of membership 0, for which reason you don’t bother to list it.

Fuzzy Set Operations

The usual operations you can perform on ordinary sets are union, in which you take all the elements that are in one set or the other; and intersection, in which you take the elements that are in both sets. In the case of fuzzy sets, taking a union is finding the degree of membership that an element should have in the new fuzzy set, which is the union of two fuzzy sets.

If a, b, c, and d are such that their degrees of membership in the fuzzy set A are 0.9, 0.4, 0.5, and 0, respectively, then the fuzzy set A is given by the fit vector (0.9, 0.4, 0.5, 0). The components of this fit vector are called fit values of a, b, c, and d.

Union of Fuzzy Sets

Consider a union of two traditional sets and an element that belongs to only one of those sets. Earlier you saw that if you treat these sets as fuzzy sets, this element has a degree of membership of 1 in one case and 0 in the other since it belongs to one set and not the other. Yet you are going to put this element in the union. The criterion you use in this action has to do with degrees of membership. You need to look at the two degrees of membership, namely, 0 and 1, and pick the higher value of the two, namely, 1. In other words, what you want for the degree of membership of an element when listed in the union of two fuzzy sets, is the maximum value of its degrees of membership within the two fuzzy sets forming a union.

If a, b, c, and d have the respective degrees of membership in fuzzy sets A, B as A = (0.9, 0.4, 0.5, 0) and B = (0.7, 0.6, 0.3, 0.8), then A [cup] B = (0.9, 0.6, 0.5, 0.8).