I want to convert the following code into Freefem++, but I am not sure how to deal with the theta variable in case i choose a square mesh and periodic boundary conditions.

k = 2;

a = 4;

gaussian_like_tanh = @(x) tanh(k * (x + a)) - tanh(k * (x - a));

theta = linspace(0, 2*pi, 1000); % Angular coordinate
x = linspace(-10, +10, 1000); % Radial coordinate
[Theta, X] = meshgrid(theta, x);
Z = gaussian_like_tanh(X);
% Convert cylindrical coordinates to Cartesian coordinates
R = X;
X = R .* cos(Theta);

Y = R .* sin(Theta);

% Plot the surface

figure(7)

surf(X,Y,Z); shading interp;