MATLAB: How to find cell position in a column based on sum of column.

cell position

Hi,
I have a 365×500 matrix (A). How to calculate another matrix B (3×500) such that each row of matrix B contains:
row1: sum of each column (I did this)
row2: 50% of sum of each column (ie. 0.5*values in row1) (I did this)
row3: positions of the cells in each column of matrix (A) such that cumulative sums of all the previous cells in each column of A is less than or equal to values in row2 (50% of sum of each column).
Thanks in advance.

Best Answer

B(1,:) = sum(A); 
B(2,:) = B(1,:)/2; 
B(3,:) = sum(cumsum(A)<=B(2,:)); 
% sanity check
% Checks that the cumulative sums of each column in A up until the 
% row numbers identified in B(3,:) are <= B(2,:) and that the 
% cumulative sum of each column in A up until the rows B(3,:)+1 exceed
% the values in B(2,:)
Ac = cumsum(A); 
allTrue = all(Ac(sub2ind(size(A),B(3,:),1:size(A,2))) <= B(2,:)); 
allFalse = ~any(Ac(sub2ind(size(A),B(3,:)+1,1:size(A,2))) <= B(2,:));
if ~allTrue || ~allFalse
   error('Something''s wrong.') 
end
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