Replicate Element-wise in matrix

Question

Let's say, I have: A=[1 2; 3 4];

I want to use repmat that return:

B = [1 1 2 2; 1 1 2 2; 3 3 4 4; 3 3 4 4]

Kindly need your help. Thank you

Answer 1

I do not know a method using repmat but here is a method using kron

kron([1 2 ; 3 4],[1 1;1 1])

ans =

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

Answer 2

An alternative which uses repmat is

A=[1 2; 3 4];
cell2mat(arrayfun(@(x)repmat(x,2,2),A,'UniformOutput',false))

ans =

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

arrayfun is used to evaluate each element in A using the anonymous function @(x)repmat(x,2,2) which replicates that single element into a 2x2 matrix.

The result of arrayfun is a 2x2 cell array where each element is a 2x2 matrix. We then convert this cell array into a matrix via cell2mat.

Answer 3

Let the data be defined as

A = [1 2; 3 4];
R = 2; %// number of repetitions of each row
C = 2; %// number of repetitions of each column. May be different from R

Two possible approaches are as follows:

1. The simplest method is to use indexing:

   B = A(ceil(1/R:1/R:size(A,1)), ceil(1/C:1/C:size(A,2)));
   

2. If you really want to do it with repmat, you need to play with dimensions using permute and reshape: move original dimensions 1, 2 to dimensions 2, 4 (permute); do the repetition along new dimensions 1, 3 (repmat); collapse dimensions 1, 2 into one dimension and 3, 4 into another dimension (reshape):

   [r c] = size(A);
   B = reshape(repmat(permute(A, [3 1 4 2]), [R 1 C 1]), [r*R c*C]);
   

Example result for R=2, C=3 (obtained with any of the two approaches):

B =
     1     1     1     2     2     2
     1     1     1     2     2     2
     3     3     3     4     4     4
     3     3     3     4     4     4