Invalid Index in position 2

When I run, Matlab says "Index in position 2 is invalid. Array indices must be positive integers or logical values." I cannot find the error in the following code (ignore that plot function is missing in the end):

clc
clear all
N=50;
M=100;
B=11;
n=50;
s=zeros(N,M);
volat=ones(n,N);
s=zeros(N,M);
rng(11);

figure (1);
hold on
for k=1:N
 eps=zeros(n,M); 
 eps(1,1)=volat(1,k)*randn(1,1);
 auxmtx=zeros(M,B,2);
 pstar=zeros(M,1);
 for i=2:n
  if eps(i-1,1)<0
  volat(i,1)=0.01+0.89*volat(i-1,1)+0.2*(eps(i-1,1)^2);
  else volat(i,j)=0.01+0.89*volat(i-1,1);
  end
 eps(i,1)=sqrt(volat(i,1))*randn(1,1);
 end
 s(k,1)=sum(eps(:,1))/sqrt(n);
 for j=2:M
  for l=1:n
   if eps(l,1)>0
   eps(l,j)=sqrt(volat(l,1))*abs(randn(1,1));
   else 
   eps(l,j)=eps(l,1);
   end
  end
  
 s(k,j)=sum(eps(:,j))/sqrt(n);
  
 end

end

Answer 1

[Walter Roberson]

else volat(i,j)=0.01+0.89*volat(i-1,1);

At that point you have not assigned to j so it is the default sqrt(-1)

Answer 2

[Daniel M]

First, rename the variable eps to something else, because you are overloading the built-in Matlab function eps() which is bad practice and could cause problems.

As a prime example of why you shouldn't use Matlab function names as variable names, your Invalid Index error is actually a result of doing this. In the first else statement, you haven't yet defined j, so it is resorting to its default complex value:

clear
j
ans = 
    0.0000 + 1.0000i