MATLAB: How to fix the ‘Invalid use of operator’ Error in the anonymous function bisect code

invalid

function [root,fx,ea,iter]=bisect( @(x)x^7+3*x-1;0;1;0;3 )
% bisect: root location zeroes
% [root,fx,ea,iter]=bisect(func,xl,xu,es,maxit,p1,p2,...):
% uses bisection method to find the root of func
% input:
% func = name of function
% xl, xu = lower and upper guesses
% es = desired relative error (default = 0.0001%)
% maxit = maximum allowable iterations (default = 50)
% p1,p2,... = additional parameters used by func
% output:
% root = real root
% fx = function value at root
% ea = approximate relative error (%)
% iter = number of iterations
if nargin<3,error('at least 3 input arguments required'),end
test = func(xl)*func(xu);
if test>0,error('no sign change'),end
if nargin<4|isempty(es), es=0.0000001;end
if nargin<5|isempty(maxit), maxit=3;end
iter = 0; xr = xl; ea = 100;
while (1)
 xrold = xr;
 xr = (xl + xu)/2;
 iter = iter + 1;
 if xr ~= 0,ea = abs((xr - xrold)/xr) * 100;end
 test = func(xl)*func(xr);
 if test < 0
 xu = xr;
 elseif test > 0
 xl = xr;
 else
 ea = 0;
 end
 if ea <= es | iter >= maxit,break,end
end
root = xr; fx = func(xr);
Then it says 
"Error: ... Line: 1 Column: 36
Invalid use of operator." while underlining the '@' and last parantheses ')' for bisect in red. I'm not sure why

Best Answer

Your syntax isn't correct. Try
f = @(x)x^7+3*x-1;
xl = 0; xu = 1; es = 10^-4; maxit = 30;
[root,fx,ea,iter]=bisect(f ,xl,xu,es,maxit );
disp(root)
function [root,fx,ea,iter]=bisect(f ,xl,xu,es,maxit )
% bisect: root location zeroes
% [root,fx,ea,iter]=bisect(func,xl,xu,es,maxit,p1,p2,...):
% uses bisection method to find the root of func
% input:
% func = name of function
% xl, xu = lower and upper guesses
% es = desired relative error (default = 0.0001%)
% maxit = maximum allowable iterations (default = 50)
% p1,p2,... = additional parameters used by func
% output:
% root = real root
% fx = function value at root
% ea = approximate relative error (%)
% iter = number of iterations
if nargin<3,error('at least 3 input arguments required'),end
test = f(xl)*f(xu);
if test>0,error('no sign change'),end
if nargin<4||isempty(es), es=0.0000001;end
if nargin<5||isempty(maxit), maxit=3;end
iter = 0; xr = xl; ea = 100;
while (1)
 xrold = xr;
 xr = (xl + xu)/2;
 iter = iter + 1;
 if xr ~= 0,ea = abs((xr - xrold)/xr) * 100;end
 test = f(xl)*f(xr);
 if test < 0
 xu = xr;
 elseif test > 0
 xl = xr;
 else
 ea = 0;
 end
 if ea <= es || iter >= maxit,break,end
end
root = xr; fx = f(xr);
end
Related Question