Input argument contains an empty equation or variable.

I am trying to build an mlapp for lagrange's multiplier. I need to solve a system of equations to get the value of x,y,z. So, This is the code for solving.

syms x y lambda;
            f=app.Function1EditField.Value;
            func=str2sym(f);
            g=app.Constrain1EditField.Value;
            cons=str2sym(g);
            L = func + lambda*lhs(cons);
            %L=str2sym(l);
            dL_dx=diff(L,x)==0;
            dL_dy=diff(L,y)==0;
            dL_dlambda=diff(L,lambda)==0;
            system = [dL_dx; dL_dy; dL_dlambda];
            [x_val, y_val,lambda_val] = solve(system, [x y lambda], 'Real', true);

the code doesn't show any error when I use it normally, outside the matlab gui code. But when I run the above code I get an error saying:

Error using sym/solve>getEqns

Input argument contains an empty equation or variable.

Error in sym/solve (line 226)

[eqns,vars,options] = getEqns(varargin{:});

please help!!

[Walter Roberson]

I would want to know what the value of f and g are showing up as.

[Nittya Ananda]

This is my interface, nothing is showing up in the output.

[Walter Roberson]

put a breakpoint at the line

              L = func + lambda*lhs(cons);

and execute. When it gets there show us func and cons

Answer 1

[Sarthak]

Hi,

The error message suggests that one of the equations in your system is empty or contains an empty variable. This can happen if the user does not provide a valid input for the function or constraint, resulting in an empty symbolic expression.

To fix this error, you can add some input validation to your code to ensure that valid expressions are entered by the user. For example, you can check that the input function and constraint are non-empty before proceeding with the Lagrange multiplier calculation:

f = app.Function1EditField.Value; 
if isempty(f) 
    error('Please enter a valid function.'); 
end 

g = app.Constrain1EditField.Value; 
if isempty(g) 
    error('Please enter a valid constraint.'); 
end 

func = str2sym(f); 
cons = str2sym(g); 

Refer to the ‘isempty()’ documentation below:

Determine whether dynamic system model is empty - MATLAB isempty (mathworks.com)