error with the trapmf function

Dear Sir/ Madam

I would like to ask for a sloution fot this problem please.

What should I do? Thank you

Answer 1

[John D'Errico]

You wrote this:

x1 = 0:1:25;
y1 = trapmf(x, [0 5 15 25]) ;

Unrecognized function or variable 'x'.

So you created the variable x1. Then you called the function trapmf. What is x? Should MATLAB somehow know what it should do here? What is x? Should it read your mind, that somehow, it should know that MAYBE, you wanted to pass in x1, instead of the undefined variable x?

Software does what you tell it to do. It does not read your mind.

[Ameri]

Ok many thanks. really i didn't notice this. May I ask you how could I make it as a symmetric, please? Its tilted a bit

[John D'Errico]

Why would it not be tilted? First, what is trapmf?

help trapmf

 TRAPMF Trapezoidal membership function.
    TRAPMF(X, PARAMS) returns a matrix which is the trapezoidal
    membership function evaluated at X. PARAMS = [A B C D] is a 4-element
    vector that determines the break points of this membership function.
    We require that A <= B and C <= D. If B >= C, this membership
    function becomes a triangular membership function that could have
    a height less than unity. (See the example below.)
 
    For example:
 
        x = (0:0.1:10)';
        y1 = trapmf(x, [2 3 7 9]);
        y2 = trapmf(x, [3 4 6 8]);
        y3 = trapmf(x, [4 5 5 7]);
        y4 = trapmf(x, [5 6 4 6]);
        plot(x, [y1 y2 y3 y4]);
        set(gcf, 'name', 'trapmf', 'numbertitle', 'off');
 
    See also DSIGMF, EVALMF, GAUSS2MF, GAUSSMF, GBELLMF, PIMF, PSIGMF,
    SIGMF, SMF, TRIMF, ZMF, LINSMF, LINZMF.

    Documentation for trapmf
       doc trapmf

Ok a trapezoid.

x1 = 0:1:25;
y1 = trapmf(x1, [0 5 15 25]);
plot(x1,y1)

So, a trapezoid, where the top edge is not centered. Where does that top live? It is completely controlled by the second and third break points, and where they lie, compared to the first and 4th breakpoint.

Note that the distance from 0 to 5, is NOT the same as the distance from 15 to 25.

Would you expect the result to be symmetrical? Again, computers follow your orders. They do what you tell them to do.

If you insist on symmetry, then you need to choose a set of breakpoints that would be symmetrical. Ergo, you might decide to adjust one of the numbers 5, 15 or 25 in that call. Where you want to put them is your choice. For example, what would happen if you used [0 5 20 25] as the second argument to trapmf?

[Ameri]

Thank you so much for your response. I have changed the distance from 15 to 25. To be 20 25 and now looks fine. Many thanks