Why am I getting Index exceeds the number of array elements. Index must not exceed 751.

clear 
syms t; 
Fs = 1.5;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = 1500;             % Length of signal
t = (0:L-1)*T;
x1 = 4+4.005*(sin(t));     % First row wave
x2 = sin(3*t)/3;         % Second row wave
x3 = sin(5*t)/5;         % Third row wave
x4 = sin(7*t)/7;         % Fourth row wave
X = [x1; x2; x3; x4];         
n = 2^nextpow2(L);
dim = 2;
Y = fft(X,n,dim);
f = 2*pi*Fs*(0:(L/2))/L; % <- multiplying
p2 = abs(Y/L); 
p1=p2(1:L/2+1); 
p1(2:end-1) = 2*p1(2:end-1); 
for i=1:4
    plot(0:(Fs/n):(Fs/2-Fs/n),p1(1:n/2))
    title ('Amplitude Spectrum') 
    xlabel('time') 
    ylabel('y(t)')
end

Index exceeds the number of array elements. Index must not exceed 751.

[David Hill]

for i=1:4
    plot(0:(Fs/n):(Fs/2-Fs/n),p1(1:n/2))
    title ('Amplitude Spectrum') 
    xlabel('time') 
    ylabel('y(t)')
end

Above makes no sense. You are repeating the same plot four times (nothing changes). You need to stop your code and look at the sizes of your arrays. (see my previous answer to your other same question)

Answer 1

[Cris LaPierre]

The error is due to the fact that you are inconsitant with the use of n and L. Once you define n, you should use that instead of L (when calculating Y, p2, and p1).

There is another issue. You create X as a 4x1500 but then never distinguish downstream rows and columns when indexing variables created from X. The result is that linear indexing is used, which does not get you the same result. This is likely why you are using a for loop to plot - once for each signal

Try this:

Fs = 1.5;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = 1500;             % Length of signal
t = (0:L-1)*T;
x1 = 4+4.005*(sin(t));     % First row wave
x2 = sin(3*t)/3;         % Second row wave
x3 = sin(5*t)/5;         % Third row wave
x4 = sin(7*t)/7;         % Fourth row wave
X = [x1; x2; x3; x4];         
n = 2^nextpow2(L);
dim = 2;
Y = fft(X,n,dim)';       %                            ###### transposed
f = 2*pi*Fs*(0:(n/2))/n; % <- multiplying             ###### Changed L to n
p2 = abs(Y/n);           %                            ###### Changed L to n
p1=p2(1:n/2+1,:);        %                            ###### Changed L to n
p1(2:end-1,:) = 2*p1(2:end-1,:); 
% for i=1:4                      ###### Modifed so for loop is not needed
    plot(0:(Fs/n):(Fs/2-Fs/n),p1(1:n/2,:))
    title ('Amplitude Spectrum') 
    xlabel('time') 
    ylabel('y(t)')
    legend