problem with fminunc: why the returned gradient is not a vector

Hi, I am using the fminunc to find a minimum of a multivariable function f(x), where x is a vector. I want to know the value of the gradient at the solution x. So I type "[x,grad] = fminunc(fun,x0)" to get the gradient. However, the returned gradient is a scalar instead of a vector. I think the gradient should be a vector since my x is a vector. Could somebody explain it for me? Thanks!

Answer 1

[Mischa Kim]

Chuoxin, have you set

 options = optimset('GradObj','on')   % gradient outputted as 2nd argument

or the equivalent through optimoptions (new in R2013a)? If not, the gradient is returned by using

 [x,fval,exitflag,output,grad] = fminunc(...)   % gradient is 5th argument

[Matt J]

The gradient should always be returned in the 5th output argument, even when GradObj option is used.