Why won't my "if then" statement keep my variable from dipping below zero?

So, I have a pretty simple code but I need to keep one variable (xg_z) from ever being below zero (the math will allow that to happen but physical reality will not). I thought I just needed an if-then (see in code; last 4 lines), but when I check it, it still lets the variable go below zero ... which screws up all the other calculations I need to make based on the value.

Any idea what I'm doing wrong?

if true
  % code
% Constants: 
  density_1 = 2830;
  zStart = 6000;              % depth in crust (m)
  zEnd = 0;                   % depth in crust (m)
  rhoc = 2650;                % density of the crust (kg m^-3)
  xo = 0.045;                 % initial quantity of dissolved gas 
  rhom0 = density_1;          % Initial density of the magma (or saturated density) 
  Q_in = 2.3;                 % Volume flux of magma into the system [m^3s^-1]
  t_flux = 35.*24.*60.*60;    % Days of magma flux into the system (35) [s]
  mass_initial = density_1./(Q_in.*t_flux); % density / volume to get initial mass of magma 
  volume_final = (Q_in.*t_flux);  % Volume actually cancels and is unecessary  
% Depth range 
  z = zEnd:10:zStart;         % Total range from chamber top to surface 
  z2 =  1200:10:6000;         % Range from chamber to conduit 
  z3 =  0:10:1200;            % Range from conduit to surface 
% Constants: 
  rhog = 0.804;               % density of gas [kg m^-3]
  s = 4.11.*(10.^-6);         % empirical coefficient depending on gas compositions of gas & melt
  n = 0.5;                    % empirical coefficient depending on gas compositions of gas & melt
  Psurface = 101325;          % pressure at Earth's surface [Pa]
  g = 9.8;                    % acceleration due to gravity [m s^-2]
  rhow = 1000;                % density of water [kg m^-3]
% Variables: 
P_z = Psurface + (rhoc.*z.*g);  % Pressure as a function of depth [Pa]
xg_z = xo - (s.*P_z.^n);        % gas in vapor form as a function of z 
if(xg_z<0)
  xg_z=0;
end

Answer 1

[James Tursa]

You have a vector of values in xg_z, so you can use logical indexing to set those values that are less than 0 to 0. E.g.,

xg_z( xg_z < 0 ) = 0;

[Cathina]

Thank you!