Mesh grid to deal with cartesian and cylindrical coordinates

General Discussion
1

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, 2pi, 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;