Subscript indices must either be real positive integers or logicals

I am getting "Subscript indices must either be real positive integers or logicals" error in line rslossavg=(i-1)*rslossavg/i+rosenbrock(x)/i;.. Can anyone help me? Thanks.

n=2000; 
cases=50; 
x_0=[1 1]';
grad='gradloss'; 
g_0 = grad(x_0); 
a=1;
A=0;
alpha=0.8;
beta=1;
s_0=0;
rslossavg=0;
rslossavgsq=0;
t=2.0096;        
v=0.5;

for i=1:cases
   replication=i;
   x=x_0;
   g=g_0;
   s=s_0;
   ak=(a/(1+A)^alpha);
   for k=1:n
       x=x-ak*g
       gnew=feval(grad,x)'
       if gnew'.*g<0
           s=s+1;
       else
       end
       tk=abs(s/k-1/2);
       if tk>=v
           ak=a/(k+A+1)^alpha;
       else
           ak=a/(k+A+1)^beta;
       end
       g=gnew';
   end
   rslossavg=(i-1)*rslossavg/i+rosenbrock(x)/i;
   rslossavgsq=(i-1)*rslossavgsq/i+(rosenbrock(x)^2)/i;

end
mean=rslossavg;
stdevmean=(((cases/(cases-1))*(rslossavgsq-mean^2))^.5)/(cases^.5);

   % Confidence intervals
  [mean-t*stdevmean,mean+t*stdevmean]

[EDITED, Jan, <http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup code formatted]

Answer 1

[Jan]

rosenbrock seems to be a variable and not a function as one could expect. Then rosenbrock(x) requires a positive integer x as index. What does this reply:

which('rosenbrock', '-all');

There are some other problems:

x_0=[1 1]';
grad='gradloss'; 
g_0 = grad(x_0);

Now grad is the string 'gradloss' and in consequence g_0 is the string 'gg'. Is this intented? I guess you want:

grad = @gradloss