Apply diff to function handle

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William Alberg 2020-5-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/524282-apply-diff-to-function-handle

评论： William Alberg 2020-5-10

采纳的回答： Walter Roberson

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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!

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Walter Roberson 2020-5-10

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https://ww2.mathworks.cn/matlabcentral/answers/524282-apply-diff-to-function-handle#answer_431575

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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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William Alberg 2020-5-10

This solves all my problems. Thank you!

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Steven Lord 2020-5-9

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https://ww2.mathworks.cn/matlabcentral/answers/524282-apply-diff-to-function-handle#answer_431482

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>> syms x t x(t)
>> dxdt = diff(x, t)
dxdt(t) =
 
diff(x(t), t)

To substitute a function in for x(t):

>> subs(dxdt, x, sin(t))
 
ans(t) =
 
cos(t)

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Walter Roberson 2020-5-10

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ch1 = children(eq_sym);
ch2 = children(eq_sym(2));
symFunType(ch2(2))

ch2(2) displays as D(x2) but symFunType shows that it is D_Var that is the function.

Walter Roberson 2020-5-10

I am running into some difficulties in generalizing the solution. Something like this cries out for a recursive function, but you have to be able to do the subs() at arbitrary nesting level then, which is a problem because the subs() has to be done in the current workspace...

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