How do I print a variable inside a for loop to the console in real time, as the loop is running, in R?

The last one does print in real time, try it like this:

for(i in 1:10){
  Sys.sleep(0.1)
  print(i)
}

This looks fine in Rstudio, but in classic Rgui you have to click the console in order to it to refresh (increasing the sleep for example by Sys.sleep(0.5) helps to see that). You can circumvent that by using flush.console which clears the buffer:

for(i in 1:10){
  Sys.sleep(0.1)
  print(i)
  flush.console() 
}

Or in Windows you can select Misc in the upper toolbar and uncheck the buffered output.

---

If your goal is to track the process of your loop, the above method feels bit akward (at least to my eyes) when you are running through large number of iterations. In that case it might be nicer to use progress bars:

n<- 1000
pb <- txtProgressBar(min = 0, max = n, style = 3) #text based bar
for(i in 1:n){
   Sys.sleep(0.001) 
   setTxtProgressBar(pb, i)
}
close(pb)

Or something even nicer:

library(tcltk)
n<- 1000
pb <- tkProgressBar(title = "Doing something", min = 0, max = n, width = 200)
for(i in 1:n){
   Sys.sleep(0.001) 
   setTkProgressBar(pb, i, label=paste(round(i/n*100,1),"% done"))
}
close(pb)

The cat() function allows you to make useful complex statements to keep track of progress in your script

for(i in 1:10){
  ptm0 <- proc.time()
  Sys.sleep(0.5)  
  ptm1=proc.time() - ptm0
  jnk=as.numeric(ptm1[3])
  cat('\n','It took ', jnk, "seconds to do iteration", i)
}

>It took  0.49 seconds to do iteration 1