Producing 2D array from a 1D array in MATLAB

Question

Does anyone know if there is a way to produce a 2D array from a 1D array, where the rows in the 2D are generated by repeating the corresponding elements in the 1D array.

I.e.:

1D array      2D array

  |1|       |1 1 1 1 1|
  |2|       |2 2 2 2 2|
  |3|  ->   |3 3 3 3 3|
  |4|       |4 4 4 4 4|
  |5|       |5 5 5 5 5|

Answer 1

You can do this using the REPMAT function:

>> A = (1:5).'

A =

     1
     2
     3
     4
     5

>> B = repmat(A,1,5)

B =

     1     1     1     1     1
     2     2     2     2     2
     3     3     3     3     3
     4     4     4     4     4
     5     5     5     5     5

EDIT: BONUS ANSWER! ;)

For your example, REPMAT is the most straight-forward function to use. However, another cool function to be aware of is KRON, which you could also use as a solution in the following way:

B = kron(A,ones(1,5));

For small vectors and matrices KRON may be slightly faster, but it is quite a bit slower for larger matrices.

Answer 2

repmat(a, [1 n]), but you should also take a look at meshgrid.

Answer 3

You could try something like:

a = [1 2 3 4 5]'
l = size(a)
for i=2:5
    a(1:5, i) = a(1:5)

The loop just keeps appending columns to the end.

Answer 4

In the spirit of bonus answers, here are some of my own:

Let A = (1:5)'

1. Using indices [faster than repmat]:

   B = A(:, ones(5,1))
   

2. Using matrix outer product:

   B = A*ones(1,5)
   

3. Using bsxfun() [not the best way of doing it]

   B = bsxfun(@plus, A, zeros(1,5))
   %# or
   B = bsxfun(@times, A, ones(1,5))