Sign reverse does not work - algorithm

Question

For some reason when I try to reverse the sign of my current velocity in MATLAB, it just won't do it.
For example, I start off with velocity_x = 3 and velocity_y = 3 (I am drawing circle collisions).
Now inside of checking conditions I need to reverse the sign and I do the following:
% This doesn't work:
velocity_x = -velocity_x;
velocity_y = -velocity_y;
These expressions don't seem to work. Even though in the variable list it still shows as -3, the ball is just twitching and not going in the opposite direction. But when I simply put numbers there, it works fine!
% This works perfectly fine:
velocity_x = -3;
velocity_y = -3;
Here's the whole loop:
velocity_x = 3;
velocity_y = 3;
% While is not commanded to exit the loop
while exit_loop == false
[b1_x_c, b1_y_c] = getCenter(b1);
xMove(b1, velocity_x);
yMove(b1, velocity_y);
if ((b1_x_c + radius + 1) >= WINDOW_WIDTH) || ((b1_y_c + radius + 1) >= WINDOW_HEIGHT)
velocity_x = -1 * velocity_x;
velocity_y = -1 * velocity_y;
elseif ((b1_x_c - radius - 1) <= 0) || ((b1_y_c - radius - 1) <= 0)
velocity_x = (-1) * velocity_x;
velocity_y = (-1) * velocity_y;
end
redraw;
end % of the while loop

Answer 1

When you come in region where if or elseif condition fulfills, sign could change every cycle turn - velocity value 3 -3 3 -3 and so on...
You have to use some flag to indicate that the sign has already been changed and don't change it until that region will be leaved (a kind of hysteresis)