Declared variable shown as undefined

I have a cost function hybrid_cost() defined for optimization. Howver, with N defined, the function hybrid_cost() on running shows -

Undefined function or variable 'N'

Even after declaring N as global, the problem persists.

N=4;
function cost=hybrid_cost(x) 
  cost=0;
  T1=0;
  q=zeros(N);
  q(1,1)=x(1);
  q(N,N)=x(2);
  M=create_M(x);
  Ai=M+q;
  A=zeros(N);
  for i=1:P
   A=Ai+eyes(size(M,1))*st(i); 
   c=abs(cof(A,1,N))^2;
   T1=T1+c;
  end
  T1=4/(q1*qn) * T1;
  
  cost=T1;
end

Answer 1

[KSSV]

N=4;
function cost=hybrid_cost(x,N) 
  cost=0;
  T1=0;
  q=zeros(N);
  q(1,1)=x(1);
  q(N,N)=x(2);
  M=create_M(x);
  Ai=M+q;
  A=zeros(N);
  for i=1:P
   A=Ai+eyes(size(M,1))*st(i); 
   c=abs(cof(A,1,N))^2;
   T1=T1+c;
  end
  T1=4/(q1*qn) * T1;
  
  cost=T1;
end

The variable N, should be passed to a function as shown above. Though you have defined the variable outside the function, it cannot be found inside the function unless you declare N as gloabal variable.

[Divyayan Dey]

The problem with passing N as a function input is that,this is a cost function for optimization. And its inputs are all control variables. Passing N as a input will make the optimizer consider N to be an input when it is a constant for optimization.

Even after declaring N as global,the problem still persists.

[Stephen Cobeldick]

"The problem with passing N as a function input is that,this is a cost function for optimization."

That is not a problem: just parameterize the function.

"Passing N as a input will make the optimizer consider N to be an input when it is a constant for optimization."

Not when you parameterize the function:

https://www.mathworks.com/help/matlab/math/parameterizing-functions.html

[Divyayan Dey]

Why does the problem persist with global declaration of variables? I won't be able to parameterize every function with nesting because that would make my code too lengthy.

[Stephen Cobeldick]

"I won't be able to parameterize every function with nesting"

There are two methods shown in the documentation for parameterizing functions: nested functions and anonymous functions. Have you tried using anonymous functions as the documentation shows?

[Divyayan Dey]

Thanks. That worked.