If I am parsing your question correctly you're asking how you can remove empty lines from a cell and sum the values in the second column for duplicate indices in the first column. Is this correct? If so, you can use unique to find duplicate indices.
A={'Q7',0;'Q12H',-3;'Q12G',0;[],[];'Q12G',0;[],[]};
%look here: https://www.mathworks.com/matlabcentral/answers/42283-index-non-empty-cells-in-cell-array#comment_86674
empty_idx=cellfun('isempty', A(:,1));
A=A(~empty_idx,:);%remove empty cells
second_col=cell2mat(A(:,2));
[a,~,c]=unique(A(:,1));
for n=1:length(a)%loop over all positions
a{n,2}=sum(second_col(c==n));
end
Please note that the loop will be very inefficient if A gets large.
