How to wait for a keypress in R?

Question

I want to pause my R script until the user presses a key.

How do I do this?

Briones

Have you found any answer which you can accept? – Onto

Answer 1

As someone already wrote in a comment, you don't have to use the cat before readline(). Simply write:

readline(prompt="Press [enter] to continue")

If you don't want to assign it to a variable and don't want a return printed in the console, wrap the readline() in an invisible():

invisible(readline(prompt="Press [enter] to continue"))

Duvetyn

I think this is the best answer here. – Onto

how about adding one more feature to it? press esc keep to exit loop ? – Carbonate

@Duvetyn this does not work if I run a script in rstudio e.g. print("hi") readline("Press a key to continue") print("ho") Its probably because the session is not interactive. How to do this in a non-interactive session? – Isothere

readLine command doesn't work, actually readline does – Superstition

Answer 2

Method 1

Waits until you press [enter] in the console:

cat ("Press [enter] to continue")
line <- readline()

Wrapping into a function:

readkey <- function()
{
    cat ("Press [enter] to continue")
    line <- readline()
}

This function is the best equivalent of Console.ReadKey() in C#.

Method 2

Pause until you type the [enter] keystroke on the keyboard. The disadvantage of this method is that if you type something that is not a number, it will display an error.

print ("Press [enter] to continue")
number <- scan(n=1)

Wrapping into a function:

readkey <- function()
{
    cat("[press [enter] to continue]")
    number <- scan(n=1)
}

Method 3

Imagine you want to wait for a keypress before plotting another point on a graph. In this case, we can use getGraphicsEvent() to wait for a keypress within a graph.

This sample program illustrates the concept:

readkeygraph <- function(prompt)
{
    getGraphicsEvent(prompt = prompt, 
                 onMouseDown = NULL, onMouseMove = NULL,
                 onMouseUp = NULL, onKeybd = onKeybd,
                 consolePrompt = "[click on graph then follow top prompt to continue]")
    Sys.sleep(0.01)
    return(keyPressed)
}

onKeybd <- function(key)
{
    keyPressed <<- key
}

xaxis=c(1:10) # Set up the x-axis.
yaxis=runif(10,min=0,max=1) # Set up the y-axis.
plot(xaxis,yaxis)

for (i in xaxis)
{
    # On each keypress, color the points on the graph in red, one by one.
    points(i,yaxis[i],col="red", pch=19)
    keyPressed = readkeygraph("[press any key to continue]")
}

Here you can see the graph, with half of its points colored, waiting for the next keystroke on the keyboard.

Compatibility: Tested under environments use either win.graph or X11. Works with Windows 7 x64 with Revolution R v6.1. Does not work under RStudio (as it doesn't use win.graph).

Duero

Method 1 could be shortened by using the prompt argument to readline. Method 2 would work with any input (not just numbers) if what="" were added to the call to scan. getGraphicsEvent only works on specific graphics devices on certain platforms (but if you are using one of those devices it works fine). – Pentameter

If you are using this function (Method 1) in a loop and want to stop the loop, include for example : if(line == "Q") stop() – Lise

Answer 3

Here is a little function (using the tcltk package) that will open a small window and wait until you either click on the continue button or press any key (while the small window still has the focus), then it will let your script continue.

library(tcltk)

mywait <- function() {
    tt <- tktoplevel()
    tkpack( tkbutton(tt, text='Continue', command=function()tkdestroy(tt)),
        side='bottom')
    tkbind(tt,'<Key>', function()tkdestroy(tt) )

    tkwait.window(tt)
}

Just put mywait() in your script anywhere that you want the script to pause.

This works on any platform that supports tcltk (which I think is all the common ones), will respond to any key press (not just enter), and even works when the script is run in batch mode (but it still pauses in batch mode, so if you are not there to continue it it will wait forever). A timer could be added to make it continue after a set amount of time if not clicked or has a key pressed.

It does not return which key was pressed (but could be modified to do so).

Pentameter

It's awesome. But just a warning, it won't run on RStudio-Server webclient, for some reason(Error in structure(.External(.C_dotTclObjv, objv), class = "tclObj") : [tcl] invalid command name "toplevel". ) – Hoenir

@milia, that is correct. Code based on tcltk needs to run on the local machine and will not run on RStudio-Server. – Pentameter

This solution works even if R is launched from a batch file in the background. Thanks! – Kilian

Answer 4

R and Rscript both send '' to readline and scan in non-interactive mode (see ? readline). The solution is to force stdin using scan.

cat('Solution to everything? > ')
b <- scan("stdin", character(), n=1)

Example:

$ Rscript t.R 
Solution to everything? > 42
Read 1 item

Tiffanytiffi

Awesome! This nearly solves my problem. Still it would be nice if the console wasn't waiting for text + Return, but rather reacted to the first keypress (as in "Press any key to continue"). – Funches

Answer 5

This answer is similar to that of Simon's, but does not require extra input other than a newline.

cat("Press Enter to continue...")
invisible(scan("stdin", character(), nlines = 1, quiet = TRUE))

Using nlines=1 instead of n=1, the user can simply press enter to continue the Rscript.

Affray

+1 this is the only answer that actually works as desired for me. Inside Rscript: it pauses and only requires to hit Enter to continue. – Cornemuse

this broke R and i had to terminate the session – Longwood

in interactive mode, this breaks R and requires terminating the session. Please add warning on your entry, in which case, I will remove the downvote. – Mattias

Worked for me as expected on Windows!. The accepted solution (above) was skipped over and didn't pause. This one actually paused and waited for me to hit enter. – Scene

Answer 6

The function keypress() from the package keypress reads a single key stroke instantly, without having to hit enter.

However, it only works in the Unix/OSX terminal or Windows command line. It does not work in Rstudio, the Windows R GUI, an emacs shell buffer etc.

Padova

See also the equivalent function in the cli package (cli::keypress()), added in v3.5.0 – Millstream

Answer 7

A way of doing it (kinda, you have to press a button rather than a key, but close enough) is to use shiny:

library(shiny)

ui     <- fluidPage(actionButton("button", "Press the button"))
server <- function(input, output) {observeEvent(input$button, {stopApp()})}

runApp(shinyApp(ui = ui, server = server))

print("He waited for you to press the button in order to print this")

To my experience, this has a unique characteristic: even if you ran a script that had code written following the runApp function, it will not run until you've pressed the button in the app (button that stops the apps from inside using stopApp).

Wernher