Error : The following error occurred converting from sym to double

So I'm running a programme in which there are two symbolic variables and the rest everything is constant, and hu(x,1) is picking up 100 values from a data set at random. When hu(1,1) > 8 is used in 1st iteartion, the code runs and returns LS1(1,1) = 0, but when hu(2,1) < 8 is used in next iteration, the code stops since it is now a symbolic number for LS1(2,1) and returns this error. I'd appreciate it if someone could assist me with this. (mu, SWm, and roww are all constant)

syms hu1 vk
if hu(x,1) > 8
                                         if hu(x,1)> 10
                                                        BF(x,1) = ( 2*(0.5*(0.5+0.2)*1.2)*L);
                                                       Fd(x,1) = 0.5*Cd(x,1)*(vk^2).*L*s;
                                        else if hu(x,1) < 10
                                                        BF(x,1) = (2*(0.5*(0.5+0.2)*(hu1- 3.1))*L);
                                                        Fd(x,1) = 0.5*Cd(x,1)*roww*(vk^2).*L*(hu1-hb);
                                        end
   end
            LS1(x,1) = (Fd(x,1) - (mu*(SWm - BF(x,1))));
         
else 
LS1(x,1) = 0;

Error : The following error occurred converting from sym to double (for LS1(x,1)

This error is popping out in second iteration. How to deal with this?

[Image Analyst]

Do they need to be symbolic? What if you delete the line

syms hu1 vk

and just assign them (if needed) in advance of your other code where you use them?

[Raj Arora]

Yes actually this symbolic is needed. I have to use it further in my code

Answer 1

[Himanshu]

Hey,

The issue in your code arises from the fact that you're trying to assign symbolic expressions to elements of LS1, which is initially defined as a symbolic array. MATLAB is unable to convert symbolic expressions to double-precision numbers when you attempt to perform arithmetic operations involving both symbolic and numeric variables.

Here are the issues with your current code:

• You're working with symbolic variables hu1 and vk. In your code, you have conditions based on the symbolic variable hu(x,1).
• When hu(x,1) > 8, you perform calculations involving symbolic variables and numeric constants, which is fine.
• However, when hu(x,1) < 8 in the second iteration, the code tries to assign symbolic expressions to elements of LS1, which causes MATLAB to throw an error because it cannot convert these symbolic expressions to double-precision numbers.

To address this issue, you need to ensure that LS1 is defined as a numeric array before assigning values to its elements. You can achieve this by initializing LS1 as a numeric array before the loop or before the conditions that involve symbolic calculations.

Hope this helps!