Foreach does not perform IF statements - r

Question

I've implemented foreach in one of the multiple for statements of my R code. It returns the main result (the one after all the iterations), however, it does not perform an IF statement within the code.
Below the skeleton of my code (it's too long to put everything).The if statement does not work and variable "Disc_Time" remains the same (as initialized). What I'm doing wrong or missing? I've tried with .export="f" and .export=ls(GlovalEnv) without success.
library(foreach)
library(doParallel)
cores=detectCores()
cl <- makeCluster(cores[1]-1) #not to overload your computer
registerDoParallel(cl)
Disc_Time<-c("UE","Beam_order","Time")
.... MORE VARIABLES
MDP_x<-foreach (d = 1:length(dista),.combine='c')%dopar%
{
for (q in 1:sim)
{
for (ue in 1:n)
{
for (i in 1:length(seq_order_BS))
{
for (j in 1:length(seq_order_UE))
{
if(first==0)
{
Disc_Time<-rbind(Disc_Time,c(ue,i,D_Time))
}
}
}
}
}
stopcluster(cl)

Answer 1

To see if your if statement is working, we need to know how first is set, and what value it has before your loop. It does not look like first changes within your loop anyway, so it really should sit outside your %dopar% statement anyway.
That said, the if statement is not your issue. foreach returns a list of each return of the expression within. For example:
ls <- foreach(d = 1:5) %dopar% {
return(i)
}
gives an list ls that contains the numbers one to five.
The only expression in your function is an assignment call to Disc_Time. This is evaluated within each of your nodes, and never returned to the parent environment. Disc_Time is never changed where the code was called from).
It looks as though you are trying to set a side effect of your parallel function (to change Disc_Time), which to my knowledge is not possible in a parallel context. Perhaps you want:
MDP_x<-foreach (d = 1:length(dista),.combine='c')%dopar%
{
for (q in 1:sim)
{
for (ue in 1:n)
{
for (i in 1:length(seq_order_BS))
{
for (j in 1:length(seq_order_UE))
{
if(first==0)
{
return(rbind(Disc_Time,c(ue,i,D_Time)))
} else {
return(NA)
}
}
}
}
}
stopcluster(cl)
MDP_x should then have the values you want for each d