How to add/subtract elements of 2 matrices to create a new matrix of the same size based upon conditional statements?

I have 2 11x11 matrices called Mx and Mxy. I want to add and subtract corresponding elements of each matrix to create a new matrix called Mrx.

I need each element of Mxy to be added or subtracted from the corresponding Mx element based upon if that Mx element is negative or positive. If Mx=0, no addition or subtraction needs to be done and the corresponding element of the new output matrix Mrx will be 0 as well.

The if/else statements below explain what I need. I know I need to indicate that I want all elements of the 11x11 matrices (Mx and Mxy) to be ran through and used. I also think I may need a command that says I want output Mrx to be a 11x11 matrix based on the equations and variables as shown below.. but I am not sure. I thought I might need to add a for loop to do what I need, but again I am not sure.

        if Mx < 0 

            Mrx = Mx - Mxy;

        elseif Mx > 0

            Mrx = Mx + Mxy;

        else 

            Mrx = 0;

        end

Answer 1

[dpb]

Nothing needed but to use the result of the logical array as an addressing expression...

Mrx=Mx;   % build the start point; this'll end up unmodified for the ==0 locations...
ix=Mx>0;  Mrx(ix)=Mrx(ix)+Mxy(ix);  % the >0 elements
ix=Mx<0;  Mrx(ix)=Mrx(ix)-Mxy(ix);  % the <0 elements

You could get clever with the SIGN function, too...

Mrx=Mx+sign(Mx).*Mxy;

[Rylee Drumm]

1st way works as long as I put Mxy(ix) as well, rather than just Mxy. Thank you!

[dpb]

Ah, yes, problem with typing only...when I started the line, that's why I used the temporary variable because need the result more than once but by the time I got to the end forgot about it already! :) Old age does things like that to one... :(

Fixed up Answer for posterity.