How to store the output of a for loop in a matrix?

Hi, I am trying to make a for loop which extracts data from one matrix using another matrix (extracting data from a when b = 1), which works fine but I am finding trouble when trying to store the output of the for loop in a matrix.

 a = [3,4,5,2,1];
 b = [1,1,4,3,1];
 for i = 1:length(b)
    if b(i) == 1
       disp(a(i));
    end
 end
^^
This returns
     3

     4

     1

However when I try and store the output in a matrix, it only stores the last iteration. How can I do this? Thanks, Morgan

Answer 1

[Stephen23]

Why waste time writing an loop as if MATLAB is an ugly low-level language like C++? Using logical indexing is simpler and very efficient:

>> a = [3,4,5,2,1];
>> b = [1,1,4,3,1];
>> c = a(b==1)
c =
   3   4   1

If you really want to use a loop, then there are multiple possible ways to do it. Here is one:

>> V = find(b==1);
>> N = numel(V);
>> c = nan(1,N);
>> for k=1:N, c(k)=a(V(k)); end
>> c
c =
   3   4   1

Also read this:

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