How to concatenate two matrices in a loop?

I' m running a for loop say 'n' times,everytime a 5x2 matrix will be generted ,i have to join all these matrices such that at last run i have to get a matrix of size 5x2n .How to solve this?

Answer 1

[James Tursa]

Another way using 3-dimensional syntax:

n = number of individual matrices you have;
R = number of rows per matrix;
C = number of columns per matrix;
z = zeros(R,C,n); % Pre-allocate n pages of R x C matrices
for k=1:n
    m = individual R x C matrix;
    z(:,:,k) = m; % Assign to the k-th page
end
z = reshape(z,R,C*n); % Reshape into R x (C*n) matrix

[Oce@n]

This works for double arrays. For cell arrays you should try something else (see below)

[Image Analyst]

You just need to make 3 changes for cells:

1. Use cell() instead of zeros()
2. Use z{k} instead of z(:,:,k), and
3. Get rid of the reshape().

Try this:

n = 3; % number of individual matrices you have;
R = 4; % number of rows per matrix;
C = 5; % number of columns per matrix;
z = cell(n, 1); % Pre-allocate n cells of R x C matrices
for k = 1 : n
    m = rand(R, C); % individual R x C matrix;
    z{k} = m; % Put into the k-th cell
end
celldisp(z) % Display z in the command window.

Answer 2

[Image Analyst]

Maybe not the most efficient way, but for small numbers it won't make any difference. Try this - I think it's what you asked for, where the number of columns might vary.

n = 6;  % 6 iterations
m = []; % Initialize to null.
for k = 1 : n
  % Get the number of columns for this iteration
  % and make up a sample matrix with that many columns.
  numberOfColumns = randi([5,12], 1); % Random number between 2 and 12
  % Get a matrix with random numbers that is
  % 5 rows tall by numberOfColumns columns wide.
  thisMatrix = randi(9, 5, numberOfColumns);
  % Append onto m
  m = [m, thisMatrix];
end
% Print final m to the command window.
m

[Betha Shirisha]

@ image analyst thanks..

[Oce@n]

This works on cell arrays! Thank you!

Answer 3

[Brendan Hamm]

n = 10; % Change n as necessary
z = zeros(5,2*n); % pre-allocate space
for ii = 1:2:2*n-1
    x = rand(5,2); % Just to make sore data
    z(:,[ii,ii+1]) = x; % Assign to the appropriate location
end

[Betha Shirisha]

@brendan thanks...

Can you make it more generalised because i have just given an example of 5x2,actual matrix is of different size with 12 coloumns,so i have to extend the expression from ii,ii+1,ii+2...

And also i'm applying matrix concatenate in a loop,where should i place the matrix x,outside ii loop (i.e current running loop) or inside ii loop?

[Brendan Hamm]

To generalize this example:

m = 12;
n = 10; % Change n as necessary
z = zeros(5,2*m); % pre-allocate space
for ii = 1:m:m*n-1
    x = rand(5,m); % Just to make sore data
    z(:,ii:ii+m-1) = x; % Assign to the appropriate location
end

If your matrix x is independent of a loop, then you don't need a loop to concatenate it. In your question this is what you ask for.