Facing difficulty in finding the error saying Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters."

Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters." here is the code. can't find where the mistake is. Need help.

function f=secant(xi)

f = x(i)^6-x(i)-1;

%f = @(x) x(i)^6-x(i)-1;

x(1)=2;

x(2)=1;

e=10^-3;

n=50;

for i=1:n-2

x(i+2)=x(i+1)-(f(x(i+1))*(x(i+1)-x(i)))/(f(x(i+1))-f(x(i)));

fprintf('x%d=%.10f\n',i,x(3))

if abs(x(3)-x(2))<e

break

end

x(i)=x(i+1);

x(i+1)=x(i+2);

end

plot(x(i),f(x(i)),'b')

disp([i x(i) f(x(i)) abs(x(i)-x(i+1))])

end

Answer 1

[Torsten]

f = @(x)x.^6-x-1

instead of

f = x(i)^6-x(i)-1;

Answer 2

[Cris LaPierre]

You are trying to index a function input variable in the function declaration. This is not allowed. Instead, create a new variable name in the function declaraion,

function f = secant(xIn)

and when calling your function, index you input variable.

out = secant(x(i))

To make this change, you will likely have to update how you use x inside your function.

[Cris LaPierre]

You have updated your problem description. Your original description gave the function declaration and code as pasted below, which does give the error message you provided.

% calling syntax (I added this to recreate the error message)
x = rand(1,5);
out = secant(x)

% original function code
function f=secant(x(i))

Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters.

f = x(i)^6-x(i)-1;
x(1)=2;
x(2)=1;
e=10^-3;
n=50
for i=1:n-2
    x(i+2)=x(i+1)-(f(x(i+1))*(x(i+1)-x(i)))/(f(x(i+1))-f(x(i)));
    fprintf('x%d=%.10f\n',i,x(3))
    if abs(x(3)-x(2))<e
        break
        
    end
    x(i)=x(i+1);
    x(i+1)=x(i+2);
end
plot(x(i),f(x(i)),'b')
disp([i x(i) f(x(i)) abs(x(i)-x(i+1))])
end