My attempt to use a "connection" while trying to read in input causes R to freeze or crash

Question

Sorry, but the terminology I use in the title may not be used correctly. Whenever I try to run this code, it seems like it is trying to run it but never completes the command. When I click the stop command sign (red), it doesn't do anything. I cannot close out of R. So why is this taking forever to run?

con <- file('stdin', open = 'r')

inputs <- readLines(con)

Answer 1

When working in RStudio, you need to use readLines(stdin()) rather than readLines(file('stdin')), though you can use either if running R in the terminal.

However, there is also an issue from not specifying the number of lines of input since you are using RStudio. When reading input from stdin, Ctrl+D signals the end of input. However, if you are doing this from RStudio rather than from the terminal Ctrl+D is unavailable, so without specifying the lines of input there is no way to terminate the reading from stdin.

So, if you are running R from the terminal, your code will work, and you signal the end of input via Ctrl+D. If you must work from RStudio, you can still use readLines(stdin()) if you know the number of lines of input; e.g.,

> readLines(stdin(), n=2)
Hello
World
[1] "Hello" "World"

An alternate workaround is to use scan(), e.g.:

> scan(,'')
1: Hello
2: World
3: 
Read 2 items
[1] "Hello" "World"

(On the third line I just pressed Enter to terminate input). The advantage there is that you don't need to know the number of lines of input beforehand.

Answer 2

RStudio has a somewhat indirect connection to R (At least 4 years ago it redirected stdin to nowhere). It is probably, for our purposes, embedded. This is probably part of why stdin() can work when paired with readLines (It creates a terminal connection rather than a file connection). @duckmayr's scan() solution is quite nice and is documented to be the kind of thing that works in this situation...

the name of a file to read data values from. If the specified file is "", then input is taken from the keyboard (or whatever stdin() reads if input is redirected or R is embedded).

In case you want to consider a blank input 'okay', you can also loop over getting the data from a single line with some sentinel value, i.e. thing that makes the loop stop (here 'EOF').

input <- function() {
  entry <- ''
  while (!any(entry == "EOF")) {
    entry <- c(readline(), entry)  
  }
  return(entry[-1])
}