MATLAB: How to alternate the sign in an 1*n array

alternate sign

For example: turn A= [ 12, 1, 10, 33] into B = [12, -1, 10, -33] Thanks for sparing time for a beginner

 B=A; 
  B(2:2:end)=-B(2:2:end);

Here is one way:

[m,n] = size(B);
C = zeros(m,2*n);
C(:,1:2:end) = repmat(A,[1,n]);
C(:,2:2:end) = B;

Another approach:

E=[1:4;6:9;11:14;16:19];
m = (-1).^toeplitz((1:size(E,1)),(1:size(E,2)));        % Multiplier Matrix
Out = E .* m

producing:

Out =
    -1     2    -3     4
     6    -7     8    -9
   -11    12   -13    14
    16   -17    18   -19

Create ‘Out’ by multiplying them in the same line if you want. (I kept them separate for clarity.)