Parse error at x

Question

Abul Yasa Hasan 2024-4-2

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2101586-parse-error-at-x

回答： Steven Lord 2024-4-2

I was trying to solve the question attacjed and was writing the code for it. But I keep getting the parse error. I went through instances where poeple had the same issue but every other one seems to be different. (Line 10)

% Define the objective function

function f = objective_function(x)

f1 = x(1) ^ 3 + x(1) ^ 2 * x(2) - x(1) * x(3) + 6;

f2 = exp(x(1)) + exp(x(2)) - x(3);

f3 = x(2) ^ 2 - 2 * x(1) * x(3) - 4;

f = f1 ^ 2 + f2 ^ 2 + f3 ^ 2;

end

% Initial guess for x

x = [0.5; 0.5; 0.5];

% Tolerance value

TOL = 0.05;

% Perform steepest descent

solution = steepest_descent(x_initial, TOL);

fprintf('Approximate solution: [%f, %f, %f]\n', solution);

fprintf('Objective function value at solution: %f\n', objective_function(solution));

% Define the gradient of the objective function

function df = gradient(x)

df_dx1 = 2 * (x(1) ^ 3 + x(1) ^ 2 * x(2) - x(1) * x(3) + 6) * (3 * x(1) ^ 2 + 2 * x(1) * x(2) - x(3)) ...

+ 2 * (exp(x(1)) + exp(x(2)) - x(3)) * exp(x(1)) ...

+ 2 * (x(2) ^ 2 - 2 * x(1) * x(3) - 4) * (-2 * x(3));

df_dx2 = 2 * (x(1) ^ 3 + x(1) ^ 2 * x(2) - x(1) * x(3) + 6) * (x(1) ^ 2) ...

+ 2 * (exp(x(1)) + exp(x(2)) - x(3)) * exp(x(2)) ...

+ 2 * (x(2) ^ 2 - 2 * x(1) * x(3) - 4) * (2 * x(2));

df_dx3 = -2 * (x(1) ^ 3 + x(1) ^ 2 * x(2) - x(1) * x(3) + 6) * x(1) ...

+ 2 * (exp(x(1)) + exp(x(2)) - x(3)) ...

+ 2 * (x(2) ^ 2 - 2 * x(1) * x(3) - 4) * (-2 * x(1));

df = [df_dx1; df_dx2; df_dx3];

end

% Steepest descent algorithm

function solution = steepest_descent(x_initial, tolerance)

x_current = x_initial;

while true

gradient_current = gradient(x_current);

if norm(gradient_current) < tolerance

break;

end

% Choose a step size (you may need to adjust this)

alpha = 0.01;

x_next = x_current - alpha * gradient_current;

x_current = x_next;

end

solution = x_current;

end

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Answer 1

Steven Lord 2024-4-2

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2101586-parse-error-at-x#answer_1434636

If all of the code you posted is in the same file, the line where you define the initial value occurs after the end keyword that ends your function. If you were to try to run that function, that line of code cannot be executed. MATLAB doesn't understand what you're trying to do with that code and so it would error.

Either move the code that follows the definition of the function to a separate file or move the function definition to the end (or if you're using release R2024a, to anywhere but the start) of the file. Since you've already got other functions defined after the commands that call your steepest_descent function, moving objective_function to the end of the file would probably be the easiest.