need help with the question

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)

[per isakson]

See: Debug a MATLAB Program and Examine Values While Debugging

Which is line 11?

I cannot spot t31 in your code.

What does data.mat contain?

[dulanga]

t31 is the m file name

Index exceeds matrix dimensions.

Error in t31 (line 11)

is F1=Fmag(N/2+1:N);

Data mat contain 1*2000 vector arrays of frequency and time

[per isakson]

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]

hmm i think i am wrong should be indexing but it doesnt work in my case

Answer 1

[Walter Roberson]

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.

[dulanga]

noted now however the graph i get is very weird it should have 4 peaks as the question wants me to find the maximum amplitude

[Walter Roberson]

We do not know what change you made to the code.

[dulanga]

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);

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)