MATLAB: Flip negative value as shown bellow

complexdcdspDSP System ToolboxfftflipifftMATLABMATLAB and Simulink Student Suiteofdmsignal

x=[1 2 -5 7 -9 4 -6 7 -6 8 -5 2 -1 5 -6 -8 -9 7 4 -1 ]

x + = if x(k) ≥ 0

0 otherwise

x-= if x(k) < 0

0 otherwise

. The positive signal x +(k) is transmitted in the first subframe .

the second subframe is used to carry the flipped (inverted polarity) signal −x −(k)

then send full signal

You question is not very clear to me, but for the first part of it, I belive you want to this:

x=[1 2 -5 7 -9 4 -6 7 -6 8 -5  2 -1 5 -6 -8 -9 7 4 -1];
y=x+(x>=0)*2-1

Start with "If an entry in B is greater than or equal to zero": You can either run a loop over all elements of B or use "logical indexing":

% Linear indexing:
for k = 1:numel(B)
   if B(k) >= 0
     ...
   end
end

Or two indices for rows and columns:

for r = 1:size(B, 2)
  for c = 1:size(B, 1)
    if B(r, c) >= 0
       ...
    end
  end
end

Or "logical indexing":

index = (B >= 0);
disp(B(index))

Now try to use this to modify B. Use the mentioned terms to search in the documentation and in the internet.