Derivative in function handle

 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

Answer 1

[José-Luis]

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.

Answer 2

[James Tursa]

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

[Walter Roberson]

No need for the eval()

syms x

f = @(x) x + log(x)
f1 = matlabFunction( diff(f(x)) );
f2 = matlabFunction( diff(f1(x)) );

[James Tursa]

@Walter: +1