You should really show us all of the actual code in question, not just a wordy description of the code. A better description and your actual code would help us to advise you. The following might work for you, but it might leave old data in spots not set which might not be good.
comp.post(1:size(ch1,1),B) = ch1(:,1);
Maybe you would have to do something like this also:
comp.post(size(ch1,1)+1:end,B) = 0;
Again, hard to say what will actually work for you without knowing more about your problem.
