Why does polyfit not work?

Hi everyone,

I'm a high school student who has no experience with Matlab whatsoever. I'm working on a project that involves using Fourier analysis to predict the price of an asset (it's a little sketch). Unfortunately, it's due tomorrow and involves a presentation. Since it's a math class, I can explain the math, but the actual plotting on Matlab is killing me. Basically I'm copying the code from https://web.wpi.edu/Pubs/E-project/Available/E-project-022808-142909/unrestricted/FullIQPReport7.pdf

My initial file (LumensP.txt) is 365x1. The problem with the code is on line 9, right below the annotation "&& finding the trend." It also says pp1 is undefined, and there are other errors I can't identify on 15, 44, 29.

y=dlmread('LumensP.txt'); 
for i=1:365
x(i)=i;
end
L=length(x); 
figure(1) 
plot(x,y); hold on; 
%%finding the trend 
p1=polyfit(x,y,3); 
xx=(1:1:365)'; 
pp1=polyval(p1,xx); 
[m,~]=size(y); 
[q,w]=size(pp1); 
%%difference between trend and data 
difference=pp1-y; 
Y=fft(difference); 
[x,z]=size(Y);

for k=8:365 Y1=Y; Y(k)=0; end P=ifft(Y); NewTrend=(abs(P)) + pp1; figure(1) plot(NewTrend); hold on; %% interpolating data for 3 months for k=366:458 t(k)= p1(1)*k^3 + p2(2)*k^2 + p1(3)*k+ p1(4); end %% to interpolate FFT curve for n=366:458 sum=0; for k=1:7 a(k)=real(Y1(k)); b(k)=-imag(Y1(k)); comp_k=(a(k)*cos((2*pi*(k-1)*(n-1))/L) + b(k)*sin((2*pi*(k-1)*(n-1))/L)); sum=(sum+comp_k)/L; end g(n)=sum; end figure(3) plot(abs(g),'r') %% predicting 3 month data NewTrend2 = t + g; monthdata=dlmread('LumensP.txt'); figure(1) plot(monthdata, 'g'); plot(NewTrend2) end

[Walter Roberson]

Duplicates earlier and more complete https://www.mathworks.com/matlabcentral/answers/402498-why-does-polyfit-not-work