Create new matrix based on an existing one

I have a big matrix. Now I want to create a new matrix that takes the first value in each column and changes the following 11 values with the fist value. Then I want to take the value of the 13th row for each column and put in in the following 11 rows for each column.

An example for a column:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

I would like to change it to:

1 1 1 1 1 1 1 1 1 1 1 1 13 13 13 13 13 13 13 13 13 13 13 13

This would be an example for a small column. My matrix has the dimension of 253X7690.

Thank you for your help.

Answer 1

[Jan]

A = rand(253, 7690);
B = A(1:12:end, :);     % Take every 12th row
C = repelem(B, 12, 1);  % Needs Matlab >= 2015a

With older Matlab versions:

Index = repmat(1:12:size(A, 1), 12, 1);
C     = A(Index(:), :);

[FC93]

Thank you.