How can i build a for loop for a subset of data having repetitions?

I would like to do a regression analysis with some data in matrix A1 (variables indexed). The variable IDstop1 is repeated many times and i want to do the regression for each subset of Rmean_pred1, Rmean_obs1 and Zero1 where IDstop1 is the same (for example for IDstop1 = 20160008 and then for the next one etc..). I tried this loop but i receive only one value for y, x1, x2, and an error for X. I expect to have the same length of (for example) 20160008 for these variables.

Can someone help me?

IDstop1 = A1(:,1);
Rmean_pred1 = A1(:,15);
Rmean_obs1 = A1(:,14);
Zero1 = A1(:,3)

k = unique(IDstop1);

for i= 1:length(k)
  y = Rmean_pred1(i);  
  x1 = Rmean_obs1(i);
  x2 = Zero1(i);
  X = [ones(size(x1(i))) x1(i) x2(i)];
  b = regress(y,X)
end

Answer 1

[Jan]

Maybe you want:

k      = unique(IDstop1);
result = cell(1, numel(k));
for i = 1:numel(k)
    index = (IDstop1 == k(i));
    y  = Rmean_pred1(index);  
    x1 = Rmean_obs1(index);
    x2 = Zero1(index);
    X  = [ones(size(x1)), x1, x2];
    b  = regress(y, X);
    result{i} = b;
end

[EM geo]

thank you very very much Jan!

[EM geo]

Jan maybe you can help me again, i'm pretty new i'm matlab... if i want to access to the first rows of the first cell in result how can i have to index it? I tried with:

result{1,1}

but it shows me all the values in that cell.

Thank you so much for saving me!! :)

[madhan ravi]

result{1,1}(1,:)

[EM geo]

perfect! Thanks Madhan Ravi!!