Variable not recognized after program restart

Question

Fjolfrin 2019-1-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/438457-variable-not-recognized-after-program-restart

评论： madhan ravi 2019-1-6

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?

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Answer 1

madhan ravi 2019-1-6

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/438457-variable-not-recognized-after-program-restart#answer_355187

编辑：madhan ravi 2019-1-6

在 MATLAB Online 中打开

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) );

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Fjolfrin 2019-1-6

在 MATLAB Online 中打开

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;

Fjolfrin 2019-1-6

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 2019-1-6

Anytime :) ,

Thank you

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