How to wait for a keypress in R?

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"))

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).