How can I change the value of a variable during a while loop?

I'm trying to create a matrix that selects 29 pieces of data from a matrix 'ozonewest' that is 156x1 in size.

Then, for each iteration of the loop, I want to move the data selected along by one i.e. 1-29, and then 2-30 etc. This would occur 126 times until there are no more data to use.

My code:

TimeSeries=ones(29,126); % Matrix I'm inputting the data into
dt=28;
nt=126;
i1=1;
for i2=i1+dt;
while i2<=nt;
    tmp=ozonewest(i1:i2,1);
    TimeSeries(:,i1)=tmp;
    i1=i1+1;
end
end

My problem is the last line of code, i1=i1+1; only works once (value of 2 in the workspace) and it gives me the Subscripted assignment dimension mismatch error.

Any help would be great. Thanks.

[Adam]

for i2=i1+dt;

is just going to evaluate to

for 29

isn't it? i.e. not a loop at all. You can't have a for loop based on an 'i1' variable that is constantly changing within that same loop.