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Vector concatenation in for loop (MATLAB)

  1. Jul 11, 2010 #1
    I have a program that calculates the inverse of a matrix. However, in my for loop I generate n vectors y which are nx1 vectors. The vectors are correctly calculated, but I can't figure out how to concatenate them into an nxn matrix.

    The problem I think I'm having is that my vector y is generated but it isn't indexed ie y_1, y_2, or something like that. So I can't just make X=[y_1 y_2 ...]. Each time the loop runs it erases y and replaces it with the new y, so at the end I only have the nth vector left.

    Does anyone know of a way to fix this so the output each loop would be [y_1] -> [y_1 y_2] -> ... n times?

    This is my first program using MATLAB and I am quite stuck, thanks in advance.
     
  2. jcsd
  3. Jul 11, 2010 #2
    One simple way to do it is to define a matrix to hold the vectors before entering the loop; once you're inside the loop you can then concatenate the vector to the matrix.

    Code (Text):

    A = []; % This is the matrix to hold the vectors.

    for i = 1:n
       % Do stuff with the vector.
       A = [A y]; % Append the vector y to A.
    end
     
    This should work as a first approximation but it suffers one major drawback: each time you append the vector to the matrix A in the loop, the matrix A is redefined; this takes a lot of time. If you know in advance how many vectors you want to store, you can preallocate the matrix A and simply redefine the necessary portion in each step of the loop.

    For instance, if you wish to store n vectors, with the dimension of each being mx1, something like the following might be useful.

    Code (Text):

    A = zeros(m,n); % Preallocate A to be a mxn matrix of zeros.

    for i = 1:n
       % Do stuff with the vector.
       A(:,i) = y; % Store the vector y in the ith column of A.
    end
     
     
    Last edited: Jul 11, 2010
  4. Jul 11, 2010 #3
    Awesome! Thank you so much Shoehorn, that worked like a charm. I thought I had fought through all the "hard" stuff in writing my program until I ran into this concatenation problem.
     
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