MATLAB: Apply diff to function handle

diff()function handleMATLABsymbolic

Hello!
I have a string input that i transform into a symbolic equation. I need to translate "D(___)" into differentiation, but i can't make it work
syms t x x(t) D(x)
eq_str = 'D(x)'; % input string equation
eq_sym = str2sym(eq_str); % transform equation to symbolic
eq_sym2 = subs(eq_sym,x,x(t)) % make x into a function of time
Dif = @(f) diff(f,t) % make Dif into a function handle
subs(eq_sym2,D,Dif) % returns 0, i want "diff(x(t),t)"
I dont think that "subs" is the correct way to do it, but i have not been able to find any other way
Thanks!

Best Answer

syms t x(t) D(x) x1(t) x2(t)
eq_str = '0 == x1 + D(x2)';  % example of input string equation

eq_sym = subs(str2sym(eq_str)); % transform equation to symbolic

eq_str2 = '0 == x1 + D(D(x2))'; % example of input string equation
eq_sym2 = subs(str2sym(eq_str2)); % transform equation to symbolic
eq_sym = mapSymType(eq_sym, 'D_Var', @fixD);
disp(eq_sym);
eq_sym2 = mapSymType(eq_sym2, 'D_Var', @fixD);
disp(eq_sym2)
function in = fixD(in)
    syms t
    if hasSymType(in, 'D_Var')
      in = diff(mapSymType(children(in), 'D_Var', @fixD),t);
    end
end
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