How to Code two Variable Feval??

I want to code a function that has two varibles in it but one of those variables a function.

Mainly HX = ....

and K1...

I attempted feval but it doesn't work so it put it back into typical math format

Inputs:

f = @(x,y) (x^2)+(y^2);
a = 2;
b = 2.2;
c = x;
d = 2*x;
n = 4;
m = 4;
function [di, i] = doubsimpint(f,a,b,c,d,m,n)

syms x y

h = (b - a)/n;
J1 = 0;
J2 = 0;
J3 = 0; 


for i = 1:n
    x = a + i*h;
    HX = (feval(d,x) - feval(c,x))/m;   %d(x) c(x)
    K1 = feval(f,x,feval(c,x)) + feval(f,x,feval(d,x)); %f(x,c(x)) + f(x,d(x))
    K2 = 0;
    K3 = 0;
    
    
    for j = 1:(m-1)
        y = feval(c,x)+j*HX;
        Q = feval(f,x,y);
        if rem(j, 2) == 0  %if j is even
            K2 = K2 + Q;
        else
            K3 = K3 + Q;
        end
    end
    
    L = (K1 +2*K2 + 4*K3)*HX/3;
    
    if i == 0 || i == n
        J1 = J1 + L;
    elseif rem(i, 2) == 0
        J2 = J2 + L;
    else 
        J3 = J3 + L;
    end
end
J = h*(J1 + 2*J2 + 4*J3)/3;
di = J;


end




        

[Shubham Gupta]

Not sure what is 'x' in your input variables?

Then, inside the function doubsimpint, in 1st loop at line:

K1 = f(x,f(x,c)) + f(f(x,d)); 

has only 1 input for f(f(x,d)) , I think it should be f(x,f(x,d))?

Finally, inside the 2nd "for" loop at line:

y = feval(c,x)+j*HX; % for feval c should be a function, you meant to write f?

[Don]

I believe my x is found from x = a + i*h

[Shubham Gupta]

I am not talking about the 'x' inside the function but which you have used to define "input variables" i.e. 'c' & 'd'

[Don]

So I am doing a double integral

a , b are for the outer integral in terms of y

and c(x), and d(x) are for the inner integral in terms of x.

c = x

d = 2*x

[Don]

also seems to be an error when using f(x,d) and f(x,c)

Error in doubsimpint (line 12)

HX = (f(x,d) - f(x,c))/m; %d(x)

c(x)

[Walter Roberson]

        Q = feval(f,x,y);

is valid syntax for feval a function that takes at least two parameters.

feval() is not used much now; it is only suitable for the case where your f might be a character vector or string object that gives the name of a function. In pretty much all modern cases you should be passing around function handles instead of names of functions.