MATLAB: Sum of vector by group

Question

i have a vector of numbers

a=[1 4 1 -3 -6 9 9 1 1 1 -1 -1 -4]

and i group them by their sign (i don't have zero, only positive and negative number); 3 positive, 2 negative, ext. so:

b=[3 2 5 3]

now i want to find the sum of each group:

res=[6 -9 21 -6]

how can i do it, without using a loop.

Note: the vector "a" can be empty, contain only positive number, only negative number, or both of them when the first number can be positive or negative. in any case it don't contain zeros.

Note2: the way i find "b" is:

b = diff([1 find(diff([0 sign(a)]==1)~=0) length(a)+1]);
b=b(b~=0);

Answer 1

a=[1 4 1 -3 -6 9 9 1 1 1 -1 -1 -4];
idx = cumsum([0 abs(diff(sign(a)))==2])+1;
res = accumarray(idx.',a.')