if value is in vector, perform this function

Hi,

I'm trying to set up R to run a simulation of two populations in which every 3.5 days, the initial value of one of the populations is reset to 1.5. I'm simulation an experiment we did in which we fed Daphnia populations twice a week with algae, so I want the initial value of the algal population to reset to 1.5 twice a week to simulate that feeding. I've use for loops and if/else loops before but I can't figure out how to syntax "if t is in this vector of possible t values, do this command, else, do this command" if that makes sense. Here's what I have (and it doesn't work):

params = c(1, 0.15, 0.164, 1)
init = c(1.5, 0.05)
t=seq(1,60, by=0.5) #all time values, experiment ran for 60 days

#feeding sequence - every "3.5 days"
feed_days = seq(1,60,by=3.5)

Daphnia <- function(t,x,params){
	C_D = x[2];
	C_A = 0;
	for(t %in% feed_days){
		if t == TRUE {
		C_A = 1.5
		}
	else{
		C_A = 0
	 }}
	gamma = params[1]; m_D = params[2]; K_q = params[3]; q_max = params[4];
	M_D = m_D * C_D
	I_A = (C_D * q_max * C_A) / (K_q + C_A)
	r_D = gamma * I_A
	return(
	list(c(
	 - I_A,
	r_D - M_D
	)))
	}

library(deSolve)
results <- ode(init, t, Daphnia, params, method = "lsoda")


Let me know if there's any other info that would be helpful and thanks very much for your help!


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