Given two vectors A and B, what is the best way to generate the vector C such that sum(C==A(i)) = B(i)?

Example

If

A = [1 2 3]
B = [2 4 6]

then

C = [1 1 2 2 2 2 3 3 3 3 3 3]

Obviously, this can be done in a for-loop, but I want to know if there is a vector manipulation trick I am missing that could do this more efficiently.

Answer 1

[Andrei Bobrov]

A = [1 2 3];
B = [2 4 6];

i2 = cumsum(B);
idx = zeros(1,i2(end));
idx(i2 - B + 1) = 1;
C = A(cumsum(idx));

or

C = cell2mat(arrayfun(@(x,y)x(ones(1,y)),A,B,'un',0));

or

i2 = cumsum(B);
i1 = i2 - B + 1;
C = zeros(1,i2(end));
for jj = 1:numel(A)
    C(i1(jj):i2(jj)) = repmat(A(jj),1,B(jj));
end