Derivative in function handle

44 vues (au cours des 30 derniers jours)
vincenzo
vincenzo le 11 Sep 2017
Commenté : James Tursa le 12 Sep 2017
f=@(x) x + log(x);
f1=diff(f)
f2=diff(f1)
I want to assign first derivative of 'f' to 'f1', and second derivative for 'f1' to 'f2' But i have this error "Undefined function 'diff' for input arguments of type 'function_handle'". How to fix? Thanks

Connectez-vous pour répondre à cette question.

Réponses (2)

José-Luis
José-Luis le 11 Sep 2017
Modifié(e) : José-Luis le 11 Sep 2017
If you're gonna do this numerically, you need to specify an interval in which to evaluate. Note that diff doesn't really give the derivative, but I'll stick to your nomenclature.
limits = [1,10];
f = @(interval) (interval(1):interval(2)) + log(interval(1):interval(2));
f1 = diff(f(limits));
f2 = diff(f1);
You could also do it symbolically but I can't help you there because I don't have the symbolic math toolbox.
James Tursa
James Tursa le 11 Sep 2017
Modifié(e) : James Tursa le 11 Sep 2017
E.g., if you want function handles you could get at them with the symbolic toolbox
>> syms x
>> f = @(x) x + log(x)
f =
@(x)x+log(x)
>> f1 = eval(['@(x)' char(diff(f(x)))])
f1 =
@(x)1/x+1
>> f2 = eval(['@(x)' char(diff(f1(x)))])
f2 =
@(x)-1/x^2
If you plan on feeding vectors or matrices etc to these function handles, then you could wrap the expressions appropriately with the vectorize( ) function. E.g.,
>> f1 = eval(['@(x)' vectorize(char(diff(f(x))))])
f1 =
@(x)1./x+1
>> f2 = eval(['@(x)' vectorize(char(diff(f1(x))))])
f2 =
@(x)-1./x.^2
  2 commentaires
Walter Roberson
Walter Roberson le 11 Sep 2017
No need for the eval()
syms x
f = @(x) x + log(x)
f1 = matlabFunction( diff(f(x)) );
f2 = matlabFunction( diff(f1(x)) );
James Tursa
James Tursa le 12 Sep 2017
@Walter: +1

Connectez-vous pour commenter.

Catégories

En savoir plus sur Calculus dans Help Center et File Exchange

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by