Anonymous function handle incosistence problem with Fourier functions

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Elad
Elad 2025-7-24
编辑： Matt J 2025-7-24
Ran in:
Hello, during some optics simulation I dared to shorten my code (a .m type) by making fourier transform an anonymous function as such:
F = @(x) fftshift(fft(ifftshift(x)));
iF = @(x) ifftshift(ifft(fftshift(x)));
poow = @(x) abs(x).^ 2;
with these I would Transform, inverse transform and Find the intensity of the signal but for a reason I don't know it only works when I post it in the command window... sometimes.
here is an example of it in action:
L = 15; % mm
F = 100; % mm
f0 = 1/100e-3; % lp/mm
f1 = 1/300e-3; % lp/mm
dx = 4e-3; % mm
t = 0:dx:L-dx; % mm
n = length(t);
f = -1/(2*dx):1/L:(1/(2*dx)-1/L);
object = chirp(t,f0,t(end),f1,"quadratic");
I = poow(object);
%% transform the object and display
object_ = F(object);
Array indices must be positive integers or logical values.
I_object_ = poow(object_);
Now, the problem is as you might see here is that the error this time is:
Array indices must be positive integers or logical values.
but on my machine its:
Error: File: LiveEditorEvaluationHelperE1735339498.m Line: 57 Column: 7
Invalid expression. Check for missing or extra characters.
Error in LiveEditorEvaluationHelperE1735339498>@(x)fftshift(fft(ifftshift(x))) (line 3)
F = @(x) fftshift(fft(ifftshift(x)));
^
this error happens to all previewed anonymous functions.
I have checked if a simpler handle would work and it does only crash when calling these 3 specific ones when running the script.
  1 个评论
Stephen23
Stephen23 2025-7-24
编辑：Stephen23 2025-7-24
"Array indices must be positive integers or logical values."
is caused by these two lines:
F = 100; % mm
..
object_ = F(object);
where you incorrectly think that you are calling a function, but in fact you are indexing into a scalar numeric.
To debug the other error message please upload your code by clicking the paperclip button. Did you copy that code from a website or something similar?

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回答（1 个）

Matt J
Matt J 2025-7-24
编辑：Matt J 2025-7-24
Ran in:
Aside from what Stephen mentions,
(1) the time and frequency axes need to have a common origin at the center of the array.
(2) ifftshift always comes first, in both F and iF, when processing centered signals.
F = @(x) fftshift(fft(ifftshift(x)));
iF = @(x) fftshift(ifft(ifftshift(x)));
poow = @(x) abs(x).^ 2;
L = 15; % mm
%F = 100; % mm
f0 = 1/100e-3; % lp/mm
f1 = 1/300e-3; % lp/mm
dx = 4e-3; % mm
%Set up axes
n=ceil(2*L/dx);
df=1/n/dx;
ax=-ceil((n-1)/2):+floor((n-1)/2); %normalized axis
t=ax*dx; %time axis
f=ax*df; %frequency axis
object=zeros(size(t));
object(t>=0) = chirp(t(t>=0),f0,t(end),f1,"quadratic");
I = poow(object);
%% transform the object and display
object_ = F(object);
I_object_ = poow(object_);
figure; plot(t,object);
●
figure; plot(f,abs(object_))

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