MATLAB: How can i solve the error “Undefined function ‘sign’ for input arguments of type ‘sym’.”

sym sign

i have the following function

function [S] = Signfunciton(f,a,b)
v=-a:0.5:b;
S= f(v);
S=sign(S);

then i give it inputs

>>syms x
>>f(x)=3*x+2
>>Signfunction(f,4,5)

but i always get the error "Undefined function 'sign' for input arguments of type 'sym'."

how can i solve this?? thank you

Best Answer

Do you have the Symbolic Toolbox?

And if you do, why aren't you just doing

    syms x;
    f = 3*x+2;
    sign(subs(f,-4:0.5:5))

Related Solutions

MATLAB: How to convert y=f(x) to x=f(y)

Try this:

syms s x 
a=1;
b=1.6;
Eq = x == (b+a)/2+(b-a)/2*s;
s=solve(Eq,s)

producing:

s =
(10*x)/3 - 13/3

Without the numeric substitutions:

syms a b s x 
Eq = x == (b+a)/2+(b-a)/2*s;
s=solve(Eq,s)

produces:

s =
(a + b - 2*x)/(a - b)

MATLAB: I cannot get the error.

a(x)=(1/2)*(cos(x)+3);

That defines a as being a symbolic function

a(b)= vpa(a(b))

That attempts to define the value of the function evaluated at b, but MATLAB does not permit you to define a general function and then assign values to the function at specific locations.

Below is the code rewritten in a way that is acceptable to MATLAB:

 syms f(x) a(x)
f(x)=(cos(x)-(2*x)+3);
a(x)=(1/2)*(cos(x)+3);
da=diff(a,x);
b=pi/2.5;
 s='a                    f(x)';
 
 disp(s);
 
for i=(1:100)
    
     if  abs(vpa(da(b)))<1  
    
    if vpa(f(a(b)))~=0
            
        a(x) = piecewise(x==b, vpa(a(b)), a(x));
        
        f(x) = piecewise(x == b, vpa(f(b)), f(x));
        
            A=[num2str(double(a(b))),'            ',num2str(double(f(b)))];
            
            
            disp(A);
            
           a(x)= piecewise(x==b, vpa(a(a(b))), a(x));
            
            i=i+1;
            
        elseif vpa(f(b))==0
           
            a(x)= piecewise(x==b, vpa(a(b)), a(x));
        
        f(x) = piecewise(x == b, vpa(f(b)), f(x));
            
            A=[num2str(doubble(a(b))),'               ',num2str(double(f(b)))];
            
          disp(A);
          
    end
    
     else 
         
         a(x) = piecewise(x==b, vpa(a(a(b))), a(x));
            
            i=i+1;
        
     end
    
end

I made no attempt at all to correct the algorithm: I just corrected the syntax to make it do exactly what you coded.

Best Answer

Related Solutions

MATLAB: How to convert y=f(x) to x=f(y)

MATLAB: I cannot get the error.

Related Question