How do I get Derivative of an inserted function ?

I want user insert the function and the program get Derivative of it by using the given code but it wouldn't work

and show me this message :

Undefined function 'diff' for input arguments of type 'inline'.

Error in Untitled (line 8)

m=diff(f)

.........................

my code :

clc
clear
str = input('Give an equation in x: ','s')  ;
x = input('Type in a value of x: ') ; 
f = inline(str,'x') ;
y = feval(f,x) ;
m=diff(f)
disp(['"' str '", for x = ' num2str(x) ', equals ' num2str(y)]) ;

[amir]

I find out the answer but now I have another question , how do I use inserted value for the derivative of func??

error:

Error using feval

Function to evaluate must be represented as a string scalar, character vector, or function_handle object.

Error in Untitled (line 9)

n=feval(df,2)

----------------------------------------------------

clc
clear
str = input('Give an equation in x: ','s')  ;
x = input('Type in a value of x: ') ; 
f = inline(str,'x') ;
y = feval(f,x) ;
syms x
df = diff(f(x),x)
n=feval(df,2)

Answer 1

[Walter Roberson]

str = input('Give an equation in x: ','s')  ;
x = input('Type in a value of x: ') ;
f = str2sym(str);
y = subs(f, sym('x'), x);
m = diff(f);
disp(['"' str '", for x = ' num2str(x) ', equals ' char(y)]) ;

Notice that you take the derivative but you do not display the derivative or calculate anything with it. I suspect that what you want to do is evaluate the derivative at x, not evaluate the function at x.

[amir]

thank u dear Walter ,

and yes ,as u said I want to evaluate the derivative at x and this is my prob too :((

I want to use "feval()" as I mentioned in last comment bu I couldn't...

would u mind helping me ? :)

[Walter Roberson]

str = input('Give an equation in x: ','s')  ;
x = input('Type in a value of x: ') ;
f = str2sym(str);
m = diff(f);
y = subs(m, sym('x'), x);
disp(['"' str '", for x = ' num2str(x) ', equals ' char(y)]) ;

If you must use feval for some reason, then:

str = input('Give an equation in x: ','s')  ;
x = input('Type in a value of x: ') ;
f(sym('x')) = str2sym(str);
m = diff(f);
y = feval(m, x);
disp(['"' str '", for x = ' num2str(x) ', equals ' char(y)]) ;

This adds useless overhead for no purpose,

[amir]

thank u sooooo muchhhh :))))