Variable not recognized after program restart

So I've been writing some code for a university project, and while I was doing the exact same thing every time I opened my project (open it and when i tried to run it, i added it to the default matlab path). for some reason I get this error:

Undefined function or variable 'x'.

Error in Ex1 (line 4)
df = matlabFunction( diff(f, x) );

My code is this on the first lines:

%Setting of initial functions and variables. Plotting of our function.
h = 10^-6;
f = @(x) 14*x.*exp(x-2) - 12*exp(x-2) - 7*x.^3 + 20*x.^2 - 26*x + 12;
df = matlabFunction( diff(f, x) );
ddf = matlabFunction( diff(df, x) );
...

I believe the code is correct. Besides I have run the code multiple times without a problem!

Do you have any ideas?

Answer 1

[madhan ravi]

diff recognises symbolic x but not as a function handle

See https://www.mathworks.com/matlabcentral/answers/438016-undefined-function-diff-for-input-arguments-of-type-function_handle#answer_354775

syms x
h = 10^-6;
f = 14*x.*exp(x-2) - 12*exp(x-2) - 7*x.^3 + 20*x.^2 - 26*x + 12;
df = matlabFunction( diff(f, x) );
ddf = matlabFunction( diff(df, x) );

[Fjolfrin]

I get this error when I make this change:

Index exceeds array bounds.

Error in sym/subsref (line 859)
            R_tilde = builtin('subsref',L_tilde,Idx);

Error in Ex1 (line 18)
while abs(f(Bisection)) > h

I'm pretty sure that's not it. As I have told in my initial question, this code WORKS. I have seen it working perfectly fine. I believe the problem lies with what path Matlab accepts, I'm not entirely sure though.

I have seen the method you propose, but it doesn't fullfil my needs for this code.

[madhan ravi]

First of all in your question

while abs(f(Bisection)) > h 

The above code didn't even appear in the question you asked and now you add up there's an error here??

Did you try my answer first (read the link) ?? use CLEAR ALL at the very beginning ,

which x -all % what shows up?
which diff -all

Honestly I don't believe that you say it's not working but worked before when I tried your code :

>> h = 10^-6;
f = @(x) 14*x.*exp(x-2) - 12*exp(x-2) - 7*x.^3 + 20*x.^2 - 26*x + 12;
df = matlabFunction( diff(f, x) );
ddf = matlabFunction( diff(df, x) );
Undefined function or variable 'x'. % SEE!!

[Fjolfrin]

After I implemented your method, I get:

which x -all
%x is a variable
which diff -all
%'All the different paths to a diff function'

Again, based on the provided link, you rpopose I do it like this?:

syms x;
f = 14*x.*exp(x-2) - 12*exp(x-2) - 7*x.^3 + 20*x.^2 - 26*x + 12;
df = matlabFunction( diff(f(x), x) );
ddf = matlabFunction( diff(df(x), x) );

When I do, i get:

Error using sym/subsindex (line 814)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments
must be symbolic variables, and function body must be sym expression.

Error in sym/subsref (line 859)
            R_tilde = builtin('subsref',L_tilde,Idx);

Error in Ex1 (line 5)
df = matlabFunction( diff(f(x), x) );

If I ommit the '(x)' on the diff decleration (meaning I type "df = matlabFunction( diff(f, x) );")I have another problem further down the line:

Index exceeds array bounds.

Error in sym/subsref (line 859)
            R_tilde = builtin('subsref',L_tilde,Idx);

Error in Ex1 (line 18)
while abs(f(Bisection)) > h

The code the compiler refers to is:

tic;
l = lower; Bisection = m; u = upper; noBi = 1;
while abs(f(Bisection)) > h   
    if f(Bisection)*f(l) < 0
        temp = u;
        u = Bisection;
    elseif f(Bisection)*f(u) < 0
        temp = l;
        l = Bisection;
    end
    Bisection = (l + u) / 2.0;
    aer = (Bisection - temp);
    noBi = noBi + 1;
end
toc;
BiTime = toc;

[madhan ravi]

Hey ! where did I use

df = matlabFunction( diff(f(x), x) );
                           ^^^---- is it there in my answer!!!??                           

You have some problem with copying the code properly.

If you my try answer properly you will get:

>> syms x
h = 10^-6;
f =  14*x.*exp(x-2) - 12*exp(x-2) - 7*x.^3 + 20*x.^2 - 26*x + 12;
df = matlabFunction( diff(f, x) )
ddf = matlabFunction( diff(df, x) )

df =

  function_handle with value:

    @(x)x.*4.0e1+exp(x-2.0).*2.0+x.*exp(x-2.0).*1.4e1-x.^2.*2.1e1-2.6e1


ddf =

  function_handle with value:

    @(x)x.*-4.2e1+exp(x-2.0).*1.6e1+x.*exp(x-2.0).*1.4e1+4.0e1

>> 

[Fjolfrin]

It was in the link you provided:

Anyway I have enough material up to now.

Thanks for the advice, I'll figure it out. It seems I have many things mixed up.

[madhan ravi]

Ah I shared the link of sir Walter's answer because he has beautifully explained how to use diff() (link - many examples available there).

By the way just try the below alternative (if it works make sure to accept the answer) :

Code:

clear all
syms x
h = 10^-6;
f(x) = 14*x*exp(x-2) - 12*exp(x-2) - 7*x^3 + 20*x^2 - 26*x + 12;
df(x) = ( diff(f(x), x) )
ddf(x) = ( diff(df(x), x) )

Gives:

df(x) =
 
40*x + 2*exp(x - 2) + 14*x*exp(x - 2) - 21*x^2 - 26
 
ddf(x) =
 
16*exp(x - 2) - 42*x + 14*x*exp(x - 2) + 40

[Fjolfrin]

It does work!

I have some rutnrime problems down the line with some of my functions, but that is another problem, I must have an infinite loop somewhere.

Thanks a lot!

BTW I just realised I've been reading your comments on many occations while I was browsing for various answers for problems I had. Additional thanks for those tips! Keep up!

[madhan ravi]

Anytime :) ,

Thank you