MATLAB: I cannot get the error.

errorMATLAB

 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(b)= vpa(a(b))
        
        f(b)=vpa(f(b))
        
            A=[num2str(a(b)),'            ',num2str(f(b))];
            
            
            disp(A);
            
           a(b)=vpa(a(a(b)));
            
            i=i+1;
            
        elseif vpa(f(b))==0;
           
            a(b)= vpa(a(b))
        
        f(b)=vpa(f(b))
            
            A=[num2str(a(b)),'               ',num2str(f(b))];
            
          disp(A);
          
    end
    
     else 
         
         a(b)=vpa(a(a(b)));
            
            i=i+1;
        
     end
    
end
a                    f(x)
Error using sym/subsasgn (line 837)
Invalid indexing or function definition. When defining a function, ensure that
the arguments are symbolic variables and the body of the function is a SYM
expression. When indexing, the input must be numeric, logical, or ':'.
MATLAB gave the error at the bottom for the code above it. Where am i doing wrong? Anyone help?

Best Answer

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.
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