How to multiply -1 with specific vector of a matrix ?

Dear all,

I would like to multiply -1 in all elements of column vector of a matrix in the following conditions: 1) if all elements of the column vectors of the matrix are negative. 2) if some elements of the column vectors are negative and rest of them are positive 3) if all elements of the column vectors are positive, don't need to multiply with -1

would you please give me some idea how can i proceed??? for example the matrix is

A=[-0.5 2 3 -5; -0.8 -7 4 6; -0.9 2 3 -9; -0.3 -6 3 1; -0.2 5 3 -7]

result should be like that Result_A=[0.5 -2 3 5; 0.8 7 4 -6; 0.9 -2 3 9; 0.3 6 3 -1; 0.2 -5 3 7]

Thanks in advance.

Answer 1

[Sean de Wolski]

The easiest and likely fastest way to do this would be with a simple for-loop over columns.

C = A;
for ii = 1:size(C,2)
  if any(sign(C(:,ii)) == -1)
      C(:,ii) = -C(:,ii);
  end
end

But just for fun, here's a vectorized way:

A=[-0.5 2 3 -5; -0.5 -2 4 6; -0.5 2 3 -9; -0.5 -2 3 1; -0.5 2 3 -7]

B = bsxfun(@times,A,-((double(any(sign(A)==-1)))-0.5)*2)

[D Bhuiyan]

Thank you so much for four nice answer. Second method works properly but the first method using for loop doesn't works? why...?

thanks

[Sean de Wolski]

Closing parenthesis was in the wrong place. Try it now.

And, Sean, this is why you should always assert!

assert(isequal(B,C))

[D Bhuiyan]

Thanks a lot:)