There's no simple way to do this without at least some duplication of data. With some duplication of data, you could do something simple like this:
aCell = cell(1, size(b,1));
parfor i = 1:size(b,1)
aCell{i} = <stuff>; % return each block in its entirety
end
a = vertcat(aCell{:}); % concatenate all cell entries into the final result
If that is not sufficiently performant, you could consider using parfeval to give you a little more control, but this is more difficult to code, and may not actually save you much. Here's an untested sketch though:
a = zeros(15,1);
for i = 1:size(b,1)
fut(i) = parfeval(@doStuff, 1, b(i,1), b(i,2)); % invoke doStuff(b(i,1),b(i,2))
end
for i = 1:size(b,1)
[idx, result] = fetchNext(fut); % collect the next result
% (note that 'idx' tells you the index into 'fut' that just
% completed)
a(b(idx,1):b(idx,2),:) = result; % push the result into 'a'
end
