Error: Conversion to logical from sym is not possible

Hello there. I get the following error for my code: "Conversion to logical from sym is not possible".

I need the if statement due to the root in my equation: if my d gets negativ, it can't solve the equation anymore. that is why i would like to have the following condition that in case of a negativ number it will put d equal to zero.

How can I include if statements in a solver (so having values that are saved as syms)? I need the solver function because the real equation is way more complex but having a root.

Also how can I export the solution to a excel file? I don't understand as the solution is still saved as sym. is there the possibility to convert a sym to a double in the end of the code?

syms x

a = 30 
b = 20           
c = 10  
if d > 0
   d = (x-c).*2
else d = 0
end


eqn = a + b - 200.*x - d^0.5 == 0

sol_x = vpasolve(eqn, x)

Answer 1

[MULI]

Hi Delia,

I understand that you are getting an error since symbolic variable is compared to a numeric value. To resolve this issue, you may follow these steps.

• Define `a`, `b`, and `c` are defined as numeric values. This ensures that these parameters are treated as ordinary numbers rather than symbolic variables.
• Define the equation is defined as an anonymous function `eqn` of `x` and solve it
• The `fzero` function is employed to find a numerical solution `sol_x` for the equation `eqn`.
• These modifications transform the original symbolic approach into a numerical one, effectively addressing the conversion error.

Below attached code incorporates above suggestions and solves the value.

% Define parameters
a = 30;
b = 20;
c = 10;

% Define the equation as an anonymous function
eqn = @(x) a + b - 200 * x - sqrt(compute_d(x, c));

% Solve equation numerically
sol_x = fzero(eqn, 0);

% Display solution
disp(['Solution for x: ', num2str(sol_x)]);

% Export the solution to an Excel file
filename = 'solution.xlsx';
xlswrite(filename, sol_x); % Write the solution directly

% Define the function to compute d
function d = compute_d(x, c)
    if x > c
        d = (x - c) * 2;
    else
        d = 0;
    end
end

You may refer these below documentation links more information on ‘fzero’ function

https://www.mathworks.com/help/releases/R2021a/matlab/ref/fzero.html