Sine-wave phase-dependent event?

Question

A sinusoidal wave with frequency $f$, angular frequency $\omega = 2 \pi f$, amplitude $A$ and phase $\phi$ is of the form: $$A \sin(\omega x + \phi)$$

an example with in MATLAB:

T = 5000;         % Total time in ms
dt = 0.04;        % Integration time step in ms 
nt = round(T/dt); % Time steps

A = 5;
f = 1; 
omega =  2*pi*f; 
phase = 0;

x = (1:1:nt) * dt/1000;
y = A * sin(omega*x + phase);

plot(x,y)

I am doing a loop as for i=1:1:nt and would like to do something every $k$ iterations. I would like $k$ to depend on the phase of the wave, so that if I change the frequency $f$, the function is applied at the same part of the curve, regardless of the frequency. Can you define $k$ in terms of the iterate i?