MATLAB: Negating every second entree in a matrix column

Question

My goal is pretty simple: I have an arbitrary sized matrix holding only positive real numbers and there will not be anymore than two nonzero elements per column at any time, for example:

b = [1 0 1; 0 1.2 2; 1 3 0];

or

b = [1 2 1 0; 0 1.2 0 0; 0 0 0 3; 1.6 0 4 2.2];

Now what I want is to negate every second nonzero entree of a column. The solution I came up with works, but is probably not the most elegant and/or efficient:

for n=1:numel(b(1,:))
  isPositive = 0;
  for m=1:numel(b(:,n))
      if (isPositive == 1 && b(m,n) ~= 0)
          b(m,n) = b(m,n)*-1;
          break;
      end
      if (abs(b(m,n)) == b(m,n) && b(m,n) ~= 0)
          isPositive = 1;
      end
   end
end

I am quite new to MATLAB, so if anybody knows a more elegant solution, perhaps not involving any for loops, please share.

Thanks in advance

Answer 1

idx = find(b)
b(idx(2:2:end)) = - b(idx(2:2:end))

more variant

[i1 j1] = find(b)
[m m] = unique(j1,'first')
k = sub2ind(size(b),i1(m+1) ,j1(m+1))
b(k) = -b(k)