Question

How to repeat

A = [ 1 2 ; 
      3 4 ]

repeated by

B = [ 1 2 ; 
      2 1 ]

So I want my answer like matrix C:

C = [ 1 2 2; 
      3 3 4 ]

Thanks for your help.

Answer 1

To make this simple, I assume that you’re only going to add more columns, and that you’ve checked that you have the same number of columns for each row.

Then it becomes a simple combination of repeating elements and reshaping.

EDIT I’ve modified the code so that it also works if A and B are 3D arrays.

%# get the number of rows from A, transpose both
%# A and B so that linear indexing works
[nRowsA,~,nValsA] = size(A);
A = permute(A,[2 1 3]);
B = permute(B,[2 1 3]);

%# create an index vector from B 
%# so that we know what to repeat
nRep = sum(B(:));

repIdx = zeros(1,nRep);
repIdxIdx = cumsum([1 B(1:end-1)]);
repIdx(repIdxIdx) = 1;
repIdx = cumsum(repIdx);

%# assemble the array C
C = A(repIdx);
C = permute(reshape(C,[],nRowsA,nValsA),[2 1 3]);

C =
     1     2     2
     3     3     4