在MatLab中绘制符号函数

Question

I've some problems in plotting a symbolic function in MatLab: for example when I try to plot the function f with ezplot, where:

f = 9/2 - ((2*x)/5 - 2/5)*(x/3 - 17/6) - x

I get the following error:

Error using findstr
Inputs must be character arrays.

Error in ezplot>ezplot1 (line 442)
    if (isa(f, 'inline') && ~isempty(findstr(char(f), '=')))

Error in ezplot (line 145)
                [hp, cax] = ezplot1(cax, f{1}, vars, labels, args{:});

Error in sym/ezplot (line 61)
   h = ezplot(fhandle(f));

I've tried to convert the symbolic function f in the char form but it returns an analogous error:

Error using findstr
Inputs must be character arrays.

Error in ezplot>ezplot1 (line 442)
    if (isa(f, 'inline') && ~isempty(findstr(char(f), '=')))

Error in ezplot (line 145)
                [hp, cax] = ezplot1(cax, f{1}, vars, labels, args{:});

Thanks for any help!

Answer 1

You must have some problem with you function definition. Perhaps x has been defined incorrectly?

The following works, at least in Matlab 2010b. It defines f as a symbolic function of the symbolic variable x :

>> clear all
>> syms x
>> f = 9/2 - ((2*x)/5 - 2/5)*(x/3 - 17/6) - x;
>> ezplot(f)

The following is also valid. It defines f as a string :

>> clear all
>> f = '9/2 - ((2*x)/5 - 2/5)*(x/3 - 17/6) - x';
>> ezplot(f)

Answer 2

What if you define your function as an anonymous function:

myfun = @(x)  4.5 - (((2*x)/5 - 2/5)*(x/3 - 17/6) - x);

figure
ezplot(myfun)

Answer 3

I really don't know why the ezplot command doesn't work with my Matlab 2012b, so I had to go with a brutal solution like this :(

syms x
f = 9/2 - ((2*x)/5 - 2/5)*(x/3 - 17/6) - x;

k = 0.1;
x_p = 0:k:10;
y_p = subs(f,x,x_p);
plot(x_p,y_p)