Error code line 19 with my Y=f(x), equation used is x^3+x^2-4x-4

function [x,y,err]=muller(f,x0,x1,x2,delta,epsilon,max1)

%Input   - f is the object function 
%	        - x0, x1, and x2 are the initial approximations 
%           - delta is the tolerance for x0, x1, and x2
%	        - epsilon the the tolerance for the function values y 
%           - max1 is the maximum number of iterations
%Output- x is the Muller approximation to the zero of f
%           - y is the function value y = f(x)
%           - err is the error in the approximation of x.

%Initalize the matrices X and Y
X=[x0 x1 x2];
Y=f(X);

%Calculate a and b in formula (15)

for k=1:max1
   h0=X(1)-X(3);h1=X(2)-X(3);e0=Y(1)-Y(3);e1=Y(2)-Y(3);c=Y(3);
   denom=h1*h0^2-h0*h1^2;
   a=(e0*h1-e1*h0)/denom;
   b=(e1*h0^2-e0*h1^2)/denom;
   
   %Find the smallest root of (17)
   if b < 0
      disc=-disc;
   end

   z=-2*c/(b+disc);
   x=X(3)+z;
   
   %Sort the entries of X to find the two closest to x
   if abs(x-X(2))<abs(x-X(1))
      Q=[X(2) X(1) X(3)];
      X=Q;
      Y=f(X);
   end
   if abs(x-X(3))<abs(x-X(2))
      R=[X(1) X(3) X(2)];
      X=R;
      Y=f(X);
   end
   
   %Replace the entry of X that was farthest from x with x
   X(3)=x;
   Y(3) = f(X(3));
   y=Y(3);
   
   %Determine stopping criteria
   err=abs(z);
   relerr=err/(abs(x)+delta);
   if (err<delta)||(relerr<delta)||(abs(y)<epsilon)
      break
   end
end

[Steven Lord]

That's not the full and exact text of the error message. Please show us all the text displayed in red in the Command Window (and if there are any warning messages displayed in orange, please show us those too.) The exact text may be useful and/or necessary to determine what's going on and how to avoid the warning and/or error.

Also, please indicate which line of your code is line 19 and show exactly how you call your function (including how any variables you use in that call are defined.)

[ky win]

line 19 Y=f(X);

muller(f,1,1.5,2,0.001,0.001,10)

Index exceeds the number of array elements. Index must not exceed 1.

Error in indexing (line 956)

R_tilde = builtin('subsref',L_tilde,Idx);

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Error in muller (line 19)

Y=f(X);

^^^^

[Walter Roberson]

Exactly how have you defined f before calling

muller(f,1,1.5,2,0.001,0.001,10)

?

Answer 1

[Walter Roberson]

equation used is x^3+x^2-4x-4

X=[x0 x1 x2];
Y=f(X);

You are passing a vector of X values to f. You have coded x^3+x^2-4x-4 expecting scalar inputs x . x^3 is x*x*x where * is algebraic matrix multiplication. So you are trying to take [1 by 3] * [1 by 3] * [1 by 3] -- which is going to fail. Also, 4x is just invalid syntax in MATLAB: MATLAB has absolutely no implied multiplication.

Learn how to use the element-wise versions of operators -- x.^3 instead of x^3