# Linguistic variables and hedges in fuzzy logic

â Zadeh

Linguistic variablesare variables whose values are words or sentences in a natural or artificial language

## Linguistic Variables:

Linguistic variables and hedges are quite useful in representing values in fuzzy sets. In fuzzy systems, variable ranges over states – which are fuzzy numbers rather than real numbers. These fuzzy numbers are often represented as linguistic variables such as HOT, COLD, WARM etc.

Linguistic variables are defined in terms of base variables, whose values are real numbers. The base variable is the one that determines the linguistic variableâs value. Imprecision of linguistic variables makes them useful for reasoning.

That is, each linguistic variable consists of the following elements

- A
*name*, which should capture the meaning of the base variable involved - A
*base variable*with its*range of values*(a closed interval of real numbers) - A
*set of linguistic terms*that refer to values of the base variable - A
*semantic rule*, which assigns to each linguistic term its meaningâan appropriate fuzzy number defined on the range of the base variable

An example of a linguistic variable is shown in the following diagram. Its name is âtemperature,â which captures the meaning of the associated base variableâa variable that expresses the room temperature T (in degree) by real numbers in the interval [0,60], Linguistic values (states) of the linguistic variable are âCold,âââwarm”, “high” and âextreme.â Each of these linguistic terms is assigned one of the fuzzy values using the triangular function.

We can categorize linguistic variables as follow:

**Quantification terms:**all, most, many, about one-fourth, some**Usuality terms**: always, sometimes, seldom, never**Likelihood terms**: certain, likely, possible, certainly not

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## Linguistic Hedges:

A linguistic hedge is an operation that modifies the meaning of a fuzzy set. In other words, hedges are the operators which modify the shapes of fuzzy sets by using adverbs such as *more*, *high*, *less*, and *slightly*.

For example, how high the temperature is? Depending upon we can have answers such as very high or slightly high etc. These are examples of hedges applied to the fuzzy set of the temperature.

**Types of hedges:**

- Intensify a fuzzy set (very, extremely)
- Dilute a fuzzy set (somewhat, sort of)
- Express probabilities (probably, not likely)
- Approximate a scalar or single number (exactly)
- Express vague quantities (most, seldom)

## Example

A = **cold climate** with Îŧ_{A}(x) as the MF

B = **Hot climate** with Îŧ_{B}(x) as the MF

**Not cold climate:** A‘ =1 – Îŧ_{A}(x)

**Not hot climate: ** B‘ =1 – Îŧ_{B}(x)

**Extreme climate: ** A âĒ B = maxâĄ( Îŧ_{A}(x), Îŧ_{B}(x) )

**Pleasant climate: ** A âŠ B = minâĄ( Îŧ_{A}(x), Îŧ_{B}(x) )

## Generalization of membership function:

Given a membership function, we can derive many more MFs representing several other linguistic hedges using the concept of **Concentration **and **Dilation**.

**Concentration:** A^{k} =[ Îŧ_{A}(x) ]^{k}, k > 1

**Dilation:** A^{k} =[ Îŧ_{A}(x) ]^{k}, k < 1

**Example:** Age = {Young, Middle-aged, Old}

- Thus corresponding to
, we have*Young**Not young*,*Very young*,*Not very young*and so on - Similarly, with
**Old**,we can have*Not old*,*Very old*,*very very old*,*Extremely old*etc. - Îŧ
_{ExtremlyOld}(x)= (((Îŧ_{Old}(x))^{2})^{2})^{2}) - Îŧ
_{MoreOrLessOld}(x)= (Îŧ_{old}(x))^{0.5}

## Example:

## Modifier:

Let A be a fuzzy set in X. Then we can define the fuzzy sets âvery Aâ and âmore or less Aâ by

**Example:**

Truth = {Absolutely false, Very false, False, Fairly true, True, Very true, Absolutely true}

One may define the membership function of linguistic terms of truth as

True(u)= u, False(u)= 1 – u, âu â [0, 1]

Very clear explanation of both membership function and Linguistic hedges.

I was checking difference between adjectives and Linguistic hedges

Thanks a lot. You can find the entire video playlist on fuzzy logic at: https://www.youtube.com/playlist?list=PLUVnh0w_cCjIzH0i8B6yQcXs567mST9cQ