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, which I've attached. 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

Answer 1

[Walter Roberson]

p1=polyfit(x(:),y(:),3);