calculate derivative of a function using handle

Hi, Matlab community

I have two scripts, one is

syms  f(x)
f=@(x)func(x);
df=diff(f,x);
fplot(@(x) df(x),[0 10])

the other one is

function f=func(x)
if x >= 1 && x <= 2;
f=-x*1/4+2;
else
f=0;
end

And the error shows like:

Conversion to logical from sym is not possible.

How to solve ths problem?

Thank you for your attention

[KSSV]

How did you use those functions...show us the full code.

[denial nakayama]

thank you for your quick answer.

I put them in the same folder and ran demo.m.

Answer 1

[Ameer Hamza]

You need to take the derivative of the original function and then apply the limits

syms  x
f(x) = (-x*1/4+2);
df(x) = diff(f,x).*(1<=x&x<=2);
fplot(df,[0 10])

[denial nakayama]

Thank you for your answer. Error shown again shows Error using sum/subindex.

[Ameer Hamza]

Can you paste complete error message?

[denial nakayama]

sure. demo

Error using sym/subsindex (line 663)

Invalid indexing or function definition. When defining a function, ensure that the body of the

function is a SYM object. When indexing, the input must be numeric, logical or ':'.

Error in demo (line 2)

f(x)=(-x*1/4+2);

[Ameer Hamza]

Try your code again after running

clear f x

[denial nakayama]

Thank you for advice. However, it shows

Error using sym/min (line 86)

Input arguments must be convertible to floating-point numbers.

Error in fplot (line 116)

ymin = min(y(J)); ymax = max(y(J));

Error in demo (line 5)

fplot(df,[0 10])

[Ameer Hamza]

Not sure about the issue. I don't have R2014a. Maybe this will work

fplot(matlabFunction(df),[0 10])

[Walter Roberson]

In R2014a, fplot() was not yet able to plot symbolic expressions.

Answer 2

[Walter Roberson]

function f=func(x)
f = piecewise(x >= 1 && x <= 2, -x*1/4+2, 0);

[denial nakayama]

Thank you for your answer. Piece wise is a function introduced after Matlab 2016. Is there a syntax to do it on the version earlier than 2016?

[denial nakayama]

Mine is 2014a and Thank you for your attention.

[Walter Roberson]

function f=func(x)
  syms t
  temp(t) = feval(symengine, 'piecewise', '([t >= 1 and t <= 2, -t*1/4+2], [Otherwise, 0])');
  f = temp(x);