I need help with an error in my symsum code

So I make this code (for a trapezoid numeric integration). But I keep getting the same error no matter what I do. Can anyone tell me where my mistake is and how to fix it? Thanks beforehand. MATLAB Code:

a=-3.4;
b=2.1; 
fun=inline('(x^2)*sin(x)-exp(-2*x)');
for m=2:2:50 
    h=(b-a)/m;
    x=a:h:b;
    for l=1:m+1
        y(l)=fun(x(l));
    end
    k=sym('k');
    s2=symsum(h/2*(y(k)+y(k+1)),k,1,m); 
end

The Error:

Error using sym/subsindex (line 737) Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and the body http://de.mathworks.com/matlabcentral/answers/questions/new/?s_tid=gn_mlc_ans_ask#code of the function is a SYM expression. When indexing, the input must be numeric, logical, or ':'.

Answer 1

[Abhishek]

Hi,

It seems like you're encountering an issue with symbolic indexing in MATLAB. I tried this in MATLAB R2024b, and I faced the same issue.

The error arises because the code attempts to use symbolic variables for indexing numeric arrays, which is not supported. In MATLAB, array indices must be numeric, logical, or the colon operator (:), and symbolic variables can’t be used for indexing numeric arrays. This is a common pitfall when working with symbolic math alongside numeric operations.

For more details, you can refer to the MATLAB documentation on symbolic math: https://www.mathworks.com/help/releases/R2024b/symbolic/index.html

As a workaround, you can use the following code:

a = -3.4; 
b = 2.1; 
fun = @(x) (x.^2).*sin(x) - exp(-2.*x); 

for m = 2:2:50 
    
    h = (b-a)/m; 
    x = a:h:b; 
    y = fun(x); 

    % Trapezoidal rule calculation 
    s2 = h/2 * (y(1) + 2*sum(y(2:end-1)) + y(end)); 

    % Display the result for each m 
    fprintf('m = %d, Integral approximation = %.5f\n', m, s2); 

end 

Hope this solves the issue.