MATLAB: Nested Loop, Can someone help me to understand this code

MATLABMATLAB and Simulink Student Suitenestedloops

Inspect the (basic) functionality of the cumsum function, then implement your own (mycumsum). Only implement the functionality of the first 3 documentation lines, not all the special cases! 
function Y = mycumsum(X) 
%MYCUMSUM cumulative sum of elements. 
Y = X;
[r,c] = size(Y); 
for ii = 2:r 
for jj = 1:c 
Y(ii,jj) = Y(ii,jj) + Y(ii-1,jj); 
end 
end 
end
Can someone help me to understand this code?

Best Answer

function Y = mycumsum(X) 
Y = X;
[r,c] = size(Y); %determine size of maxtrix
for ii = 2:r %1st row will stay constant, start at 2nd row
    for jj = 1:c %go through all columns
        Y(ii,jj) = Y(ii,jj) + Y(ii-1,jj); %first step Y(2,1)=Y(2,1)+Y(1,1) then Y(3,1)=Y(3,1)+Y(2,1),...  this is the cumulative add of column
    end 
end 
end
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