Create an anonymous function after Symbolic differentiation
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I want to differentiate a function f and the calculate the derivative of f at value of x. How can I do that? for example:
syms x
f=@(x) x^3+3*x+1;
g=diff(f(x))
Preferably I would like to create a function handle of the derivative and then calculate the derivative at x.
采纳的回答
Sean de Wolski
2013-5-13
Almost there!
gfun = matlabFunction(g)
g(2)
4 个评论
Giorgos Papakonstantinou
2013-5-13
Sean de Wolski
2013-5-13
Rule #1 of Answers: if you rewrite something from MATLAB, you will make a typo :)
Troy
2018-4-13
But what if the original function is just x? For example:
syms x
f=@(x) x;
g = matlabFunction(diff(f(x))
gives the result g = @()1.0 which is not a useable function. How to resolve?
Walter Roberson
2018-4-13
df = diff(f(x));
if isempty(symvar(df))
g = str2func(['@(x) repmat(', char(df), ', size(x))'])
else
g = matlabFunction(df);
end
Note: this will fail in some cases where the derivative just happens to be an expression involving a syntax that differs between MATLAB and the Symbolic Toolbox. For example,
f(x) = int(coth(cos(x)),x,1,5)*x
The int() part is actually a constant independent of the *x it is being muliplied by, so diff(f(x)) should be just int(coth(cos(x)),x,1,5), but if you emit that into the function handle you will fail when the handle is executed.
The situation could probably be improved by using
g = str2func(['@(x) repmat(', char(vpa(df)), ', size(x))'])
which will still lead to some failures, just perhaps fewer of them.
更多回答（1 个）
Iman Ansari
2013-5-13
Hi.
syms x
f=@(x) x^3+3*x+1;
g=diff(f(x))
h=@(y) subs(g,y)
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