Unwanted result from parallel computation with forerach

Question

I want to print a message after every 5 iterations in a loop. This works in a base R loop as follows:

vec = c() # empty vector
for (i in 1:100) {
        
        s = sqrt(i)
        vec = c(vec, s)
        if (i %% 5 == 0) {
                print(paste0("Iteration number: ", i, " finished running"))
        }
        
}

I want to be able to do the same using paralel computation with forerach. So far I have done the following:

vec = c() # empty vector
library(doParallel); library(foreach)
ncores = detectCores() # number of cores in my computer
registerDoParallel(ncores)
foreach (i = 1:100) %dopar% {
        
        s = sqrt(i)
        vec = c(vec, s)
        if (i %% 5 == 0) {
                print(paste0("Iteration number: ", i, " finished running"))
        }
        
}
stopImplicitCluster()

However, it does not work the way I want. It also prints NULLs for some reason in a "list-like" format. How can I achieve the same result from the base R loop in the forerach approach?

Answer 1

If the messages printed on each multiple of 5 are strictly not need, the following will create two identical vectors.
Note that I create vec beforehand and that I use message in the loop.

vec <- numeric(100)
for (i in 1:100) {
  vec[i] <- sqrt(i)
  if (i %% 5 == 0) {
    message(paste0("Iteration number: ", i, " finished running"))
  }
}

library(doParallel)
library(foreach)

ncores <- detectCores() # number of cores in my computer
registerDoParallel(ncores)

vec2 <- foreach(i = 1:100, .combine = c) %dopar% {
  sqrt(i)
}
stopImplicitCluster()

identical(vec, vec2)
#[1] TRUE