Matlab gives NaN when i use the name of a variable

Greetings,

I have been working on an optimization code with a lot of iterations.

I wrote an equation with syms in it to use. But when i try to use eval for one of the equations, it gives me NaN.

For example at the following part of the code;

"Ccurr = eval(subs(F,alph,alpha));

Cnew = eval(subs(F,alph,alphanew));

erralpha = abs((Cnew-Ccurr)*100/Ccurr);

alpha = alphanew;"

Here, Cnew is seen as NaN. But when i write "Cnew = eval(subs(F,alph,58473000))" directly with the value of alphanew; it gives a numerical result.

How can i fix this?

Thanks in advance.

[Yagmur Gencoglu]

To give more insight, here is the part of the code where i do the calculation of alpha:

 %%Calculation of Alpha using Secant Method
     erralpha = 3
     alpha = 0;
     B = 100;
     k=0;
    while ge(erralpha,1)
        
syms alph       
alt0 =0; 
SRH0 = eval(subs(SRH,alt,alt0));
FF = alt0-alph*SRH0;
F = subs(A,alt,FF);

dF = diff(F,alph);
ddF = diff(dF,alph);
step_size = dF/ddF;
ss = eval(subs(step_size,alph,alpha));

alphanew = alpha-ss;
Ccurr = eval(subs(F,alph,alpha));
Cnew = eval(subs(F,alph,alphanew));

erralpha = abs((Cnew-Ccurr)*100/Ccurr);

alpha = alphanew;

    end

[Stephen23]

What is the intent in using eval for this, as opposed to one of the functions that is actually from the symbolic toolbox and designed to handle symbolic equations (e.g. double or subs):

https://www.mathworks.com/help/symbolic/referencelist.html

If you are doing numeric optimization "with a lot of iterations", why are you using symbolic equations? This does not sound very efficient.

[Yagmur Gencoglu]

Thanks for the reply. I am actually doing this as a project for a class that i am taking. So the main purpose is not efficiency but usage of some particular methods. I did not find a better way than using symbolic functions for what i am doing.

I used eval because i actually need a numeric value for the solution of that equation; i dont get that when i use double or subs. And eval actually works when i write:

Cnew = eval(subs(F,alph,10))

instead of

alphanew = 10
Cnew = eval(subs(F,alph,alphanew))

That is the problem i am trying to solve but maybe i phrased it wrong.

[Walter Roberson]

Undefined function or variable 'SRH'.

[Yagmur Gencoglu]

SRH is actually calculated before the while loop starts. Again, when i put anything else in place of "alphanew" i get a result.

Thanks for your answer.

[Walter Roberson]

We cannot test without knowing the values of all variables you use in the code.