First think of forming an index vector [1 1 1 1 1 2 2 3 3 3 4 4 5]. Noticing the regular increments here makes me think of cumsum: we can get these steps by putting ones at the correct location in a zeros vector: [1 0 0 0 0 1 0 1 0 0 1 0 1]. And that we can get by running another cumsum on the input list. After adjusting for end conditions and 1-based indexing, we get this:
B(cumsum(r) + 1) = 1;
idx = cumsum(B) + 1;
idx(end) = [];
A(idx)
