Two variable Cumulative Sum calculation

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frankovaT
frankovaT 2019-6-27
评论： frankovaT 2019-6-29
I am having 5 by 5 matrix. I want to make a cumulative sum from each variable As an example:
The A(3,2) Argument would be the sum of all the arguemts before this argument and this argument, like:
A(3,2)=A(1,1)+A(2,1)+A(3,1)+A(1,2)+A(2,2)+A(3,2)
I want to make a for loop to go on each argument
for i = 1:5
For j=1:5
Any ideas on how I can do that?

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Jon
Jon 2019-6-27
If I understand your description corrrectly, I think this does what you want, no loops needed.
B = cumsum(cumsum(A,1),2)
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Jon
Jon 2019-6-28
As you were interested, I've attached a function which I just wrote that computes the cumulative 2d sum using for loops. I did a little bit of testing to compare run times (using tic,toc) for a large array (10^4 x 10^4) using this double loop compared to the one line B = cumsum(cumsum(A,1),2). It seemed that it took almost twice as long using the double loop.
function B = cumsum2d(A)
%CUMSUM2D Compute cumulative sum over 2d array
% B = cumsum2d(A) computes 2-d matrix B whose elements are
% the cumulative sum over elements of 2-d matrix A
% such that B(i,j) is the sum of all elements of A whose row and
% column indices are less than or equal to i and j respectively
% get the total number of rows and columns, we need these for end condition
% on loops
[numRows,numCols] = size(A);
% preallocate matrix the same size as A to hold result
B = zeros(numRows,numCols);
% loop through rows and columns accumulating totals
for jCol = 1:numCols
% initialize cumulative sum of elements in this column
sumThisCol = 0;
for iRow = 1:numRows
% update the sum of elements in the current column
sumThisCol = sumThisCol + A(iRow,jCol);
% update the cumulative sum of all of the elements to the left and
% above this element (inclusive)
% (need branch to handle special case of first column since there
% isn't anything to the left of that)
if jCol > 1
% in the interior, there is a column to the left of this one
B(iRow,jCol) = B(iRow,jCol - 1) + sumThisCol;
else
% this is the first column, there is nothing to the left of it
% to add
B(iRow,jCol) = sumThisCol;
end
end
end
frankovaT
frankovaT 2019-6-29
Dear Jonathan
Thank you very much for sending me this as well. I truly appreciate your effort. This helps alot.
Best

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