Why am I getting Index in position 1 is invalid. Array indices must be positive integers or logical values.

clear

syms t;

Fs = 1.5; % Sampling frequency

T = 1/Fs; % Sampling period

L = 1500; % Length of signal

t = (0:L-1)*T;

Yo=4+4.005*(sin(t)+sin(3*t)/3+sin(5*t)/5+sin(7*t)/7);

n = 2^nextpow2(L);

y = fft(Yo);

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);

plot(0:(Fs/n):(Fs/2-Fs/n),p1(i,1:n/2))

title ('Amplitude Spectrum')

xlabel('time')

ylabel('y(t)')

Answer 1

[the cyclist]

clear 
syms t; 
Fs = 1.5;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = 1500;             % Length of signal
t = (0:L-1)*T;
Yo=4+4.005*(sin(t)+sin(3*t)/3+sin(5*t)/5+sin(7*t)/7); 
n = 2^nextpow2(L);
y = fft(Yo); 
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); 
 
plot(0:(Fs/n):(Fs/2-Fs/n),p1(i,1:n/2))

Index in position 1 is invalid. Array indices must be positive integers or logical values.

title ('Amplitude Spectrum') 
xlabel('time') 
ylabel('y(t)')

In the expression

p1(i,1:n/2)

you have not defined i, so MATLAB is trying to use the complex value sqrt(-1) as an index.

Answer 2

[David Hill]

syms t; 
Fs = 1.5;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = 1500;             % Length of signal
t = (0:L-1)*T;
Yo=4+4.005*(sin(t)+sin(3*t)/3+sin(5*t)/5+sin(7*t)/7); 
n = 2^nextpow2(L);
y = fft(Yo); 
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); 
 
plot(t(1:length(p1)),p1);%what is the i? The length of your t array must be equal to the length of p1 
title ('Amplitude Spectrum') 
xlabel('time') 
ylabel('y(t)')

Answer 3

[Image Analyst]

Full explanation in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22

and in your duplicate post.