need help with the question

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dulanga
dulanga 2019-4-16
编辑： dulanga 2019-4-16
This code is not working i am using this to calulate the amplitude but i am getting this error
Index exceeds matrix dimensions.
Error in t31 (line 11)
F1=Fmag(N/2+1:N);
l
load('data.mat');
dt=t(2)-t(1);% t vector has equally spaced t so this seprates t values
N=length(t);
F=fft(f,N); %fast fourier transform of f/N
Fshifted=fftshift(F);%place 0 frequency in the centre
Fmag=norm(Fshifted);%magnitude of fshifted
nu_NY=1/dt/2;%nquist frequency
nu=linspace(0,nu_NY,N/2+1);%spectrum of frequncy vals associated with FFT frequencies
%adding postive and negative frequncies
F1=Fmag(N/2+1:N);
F2=Fmag(N/2:-1:1);
a=[F1,0]+[0,F2];
%plotting amplitued vs assoicated frequencies
plot(a,nu)
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per isakson
per isakson 2019-4-16
编辑：per isakson 2019-4-16
Should it be indexing
F1=Fmag(N/2+1:N);
as in the error message or a multiplication
F1=Fmag*(N/2+1:N);
as in the code ???
dulanga
dulanga 2019-4-16
hmm i think i am wrong should be indexing but it doesnt work in my case

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回答（1 个）

Walter Roberson
Walter Roberson 2019-4-16
f is a vector. You fft it asking for an output the same length as it is, so F is a vector. fftshift of that is a vector. You then norm() that vector and assign the result to Fmag . norm() of a vector gives a scalar. Then on line 11, Fmag(N/2+1:N) is an attempt to index the scalar at a location that is not 1.
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Walter Roberson
Walter Roberson 2019-4-16
We do not know what change you made to the code.
dulanga
dulanga 2019-4-16
this is my latest code i do get a plot however the graph below is what i get which is not correct as i should get 4 peaks. The chnage was i multiplied the Fmag by the vector.
F1=Fmag*((N/2)+1:N);
Capture.JPG
load('data.mat');
dt=t(2)-t(1);% t vector has equally spaced t so this seprates t values
N=length(t);
F=fft(f/N); %fast fourier transform of f/N
Fshifted=fftshift(F);%place 0 frequency in the centre
Fmag=norm(Fshifted);%magnitude of fshifted
nu_NY=1/dt/2;%nquist frequency
nu=linspace(0,nu_NY,(N/2)+1);%spectrum of frequncy vals associated with FFT frequencies
%adding postive and negative frequncies
F1=Fmag*((N/2)+1:N);
F2=Fmag*(N/2:-1:1);
a=[F1,0]+[0,F2];
%plotting amplitued vs assoicated frequencies
plot(a,nu)

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