array = [3 4 2 1];
matrix = [0 2 4 5; 1 3 2 6; 1 0 3 5; 5 6 3 1];
% first, you need a vector with the same length of array that tells you
% how often each element must be repeated
reps = diag(matrix(array, :));
% now you can use 'repelem' with that vector
result = repelem(array, reps);
