This could be done without a loop, but it gets clumsy and awkward. I'll just use a loop.
A = [0 0;
0 0;
0 0;
2 2;
0 0;
0 3;
0 -5;
0 0;
0 0;
0 0;
-4 0;
0 1;
0 9;
0 0;
0 0;
0 0;
-1 0;
0 0;
0 0;
0 0;
0 0;
0 0;
0 0;
0 0];
nahead = 5; % number of elements to look ahead
% add new column
A = [A,zeros(size(A,1),1)];
% find nonzero elements of col1
c1occurrence = find(A(:,1)~=0);
for occ = 1:numel(c1occurrence)
% make sure to limit the index so it doesn't run off the end
regionahead = A(min(c1occurrence(occ)+(0:nahead-1),size(A,1)),2);
lastvalloc = find(regionahead~=0,1,'last'); % location of last nz value
if isempty(lastvalloc)
A(c1occurrence(occ),3) = A(c1occurrence(occ),1);
else
A(c1occurrence(occ)+lastvalloc-1,3) = A(c1occurrence(occ)+lastvalloc-1,2);
end
end
A
gives
A =
0 0 0
0 0 0
0 0 0
2 2 0
0 0 0
0 3 0
0 -5 -5
0 0 0
0 0 0
0 0 0
-4 0 0
0 1 0
0 9 9
0 0 0
0 0 0
0 0 0
-1 0 -1
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
