Code overwrites results in for loop. Cannot figure out how to index.

I've run into a problem with the following code, which is part of a for loop with index k.

      if strcmp(l(1:5),'alpha') %Get row starting with alpha
          l = fgetl(fid); % get next line
          first_data = str2num(l);
          alpha_beta = [first_data(1) first_data(3)]; % store alpha and beta

      end
      

My issue is that the alpha_beta variable keeps overwriting with each iteration of the for loop. Ideally I'd like to save first_data(1) and first_data(3) from each iteration in alpha_beta. I tried a few things with indexing including

      alpha_beta{k} = [first_data(1) first_data(3)];

but because it's a 'double' it won't allow brace indexing. I can't seem to find any alternatives, so if anyone has a suggestion it would be much appreciated.

Answer 1

[Stephen23]

Either

alpha_beta(k,1) = first_data(1);
alpha_beta(k,2) = first_data(3);

or

alpha_beta(k,1:2) = first_data([1,3]);

And remeber to preallocate alpha_beta before the loop:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html