Problem with ploting involving function handle

Question

Tsz Tsun 2024-10-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2159295-problem-with-ploting-involving-function-handle

编辑： Matt J 2024-10-15

Hi all, I encounter the following error when I want to plot a function involving function handle with symsum, I cannot figure out the error. My code is as follows. In my matlab 2023b, I receive the following error:

Warning: Error updating FunctionLine.

The following error was reported evaluating the function in FunctionLine update: Undefined function 'symsum' for input arguments of type 'double'.

N= 600

N = 600

sym ii

COMx_k = @(k) symsum(1.*exp(-2*pi*1i*(k/N)*ii), ii,1,N)

COMx_k = function_handle with value:

@(k)symsum(1.*exp(-2*pi*1i*(k/N)*ii),ii,1,N)

%k=0:0.1:300

fplot(COMx_k)

xlabel('k')

ylabel('COMx_k')

Warning: Error in state of SceneNode.
The following error was reported evaluating the function in FunctionLine update: Unrecognized function or variable 'ii'.

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Answer 1

Matt J 2024-10-13

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https://ww2.mathworks.cn/matlabcentral/answers/2159295-problem-with-ploting-involving-function-handle#answer_1531330

编辑：Matt J 2024-10-15

在 MATLAB Online 中打开

Perhaps this is what you meant?

N= 600;

COMx_k = @(k) real(sum(1.*exp(-2*pi*1i*(k/N).*(1:N)' ),1));

%k=0:0.1:300

fplot(COMx_k,[0,300])

xlabel('k')

ylabel('COMx_k')

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Paul 2024-10-13

在 MATLAB Online 中打开

I'm not sure how fplot works in this instance, and that warning sounds ominous. Maybe fplot detects the problem with array inputs and then does only scalar evaluations? The doc page states that:

"The function must accept a vector input argument and return a vector output argument of the same size."

Hmm, "a vector input" is requred in the doc, but the warning is about more general "array inputs".

Furthermore, it looks like COMx_k is supposed to be a DFT-like computation, but can't say for sure w/o more information from @Tsz Tsun.

I suspect that the function is supposed to be (using scalar expansion):

N = 600;
COMx_k = @(k) reshape(real(sum(1.*exp(-2*pi*1i*(reshape(k,1,[])/N) .* (1:N)'),1)),size(k));

Can k really take on non-integer values?

%k=0:0.1:300
%fplot(COMx_k,[0,300])

If this problem is really related to the DFT, then the function should be

COMx_k = @(k) reshape(real(sum(1.*exp(-2*pi*1i*(reshape(k,1,[])/N) .* (0:N-1)'),1)),size(k));

But this is all just speculation w/o more information from the OP as to the what COMx_k is supposed to represent mathematically.

Tsz Tsun 2024-10-15

Yes, in fact I am trying to do a DFT. I just notice that there is a function package called fft(X) in matlab, but I am not sure if fft(X) is doing the job of DFT.

Walter Roberson 2024-10-15

fft() does a two-sided numeric discrete fourier transform.

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Answer 2

Walter Roberson 2024-10-13

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https://ww2.mathworks.cn/matlabcentral/answers/2159295-problem-with-ploting-involving-function-handle#answer_1531575

在 MATLAB Online 中打开

sym ii

That is equivalent to

ans = sym('ii');

which creates the symbolic symbol ii but throws away the reference to the symbolic symbol.

What you probably wanted was

 N= 600
 syms ii
 COMx_k = @(k) symsum(1.*exp(-2*pi*1i*(k/N)*ii), ii,1,N)
 fplot(COMx_k)
 xlabel('k')
 ylabel('COMx_k')

However, this takes a fair amount of time to plot !!

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Walter Roberson 2024-10-13

在 MATLAB Online 中打开

@(k) symsum(1.*exp(-2*pi*1i*(k/N)*ii), ii,1,N)

That is complex-valued when k is not an integer, and is 0 when k is an integer.

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Problem with ploting involving function handle

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