Array indices must be positive integers or logical values.

This is my code. I'm trying to code an 8 bit butterfly fft. I am getting the error in this line: a=y_fft(pos)+y_fft(pos+Half);

close all
clc

f = 15;
fs = 200;
ts = 1/fs;
t = 0:ts:1-ts;
y = sin(2*pi*f*t);

y_fft = y(1:9);                                                       %I thought the error could be because I wasn't selecting the array properly. But if I take it 1:8, I get an error that says "Index exceeds the number of array elements (8)."

N = length(y_fft);
Half = N/2;

for stage=1:3;                                                              % stages of transformation
    for index=0:2:(N-1);                                    % series of "butterflies" for each stage
        for n=0:(Half-1);                                                   % creating "butterfly" and saving the results
            pos=n+index+1;                                                  % index of the data sample
            a=y_fft(pos)+y_fft(pos+Half);                                           % 1-st part of the "butterfly" creating operation
            b=(y_fft(pos)-y_fft(pos+Half));                                      % 2-nd part of the "butterfly" creating operation
            y_fft(pos)=a;                                                       % saving computation of the 1-st part
            y_fft(pos+Half)=b;                                                  % saving computation of the 2-nd part
        end;
    end;
Half=Half/2;                                                                % computing the next "Half" value
end;

Answer 1

[Image Analyst]

The error, probably the most common one we see here, is very well discussed in the FAQ: "Subscript_indices_must_either_be_real_positive_integers_or_logicals."