Keyboard input/Flush the keyboard buffer
You are encouraged to solve this task according to the task description, using any language you may know.
Flush the keyboard buffer. This reads characters from the keyboard input and discards them until there are no more currently buffered, and then allows the program to continue. The program must not wait for users to type anything.
Ada
<lang Ada>with Ada.Text_IO; procedure Flushtest is
use Text_IO;
begin
Put_Line ("Type anything for 2 s"); delay 2.0;
Flush_Input:
declare Ch : Character; More : Boolean; begin loop Get_Immediate (Ch, More); exit when not More; end loop; end Flush_Input; New_Line; Put_Line ("Okay, thanks. Here is some input from you:"); Put_Line (Get_Line);
end Flushtest;</lang>
BASIC
Locomotive Basic
<lang locobasic>10 CLEAR INPUT</lang>
(Only available in BASIC 1.1 though, i.e. not on the CPC 464.)
ZX Spectrum Basic
There is no need to flush keyboard buffer in Spectrum since key presses are not buffered. If a key is currently pressed, the following waits until key is released. <lang basic>10 IF INKEY$ <> "" THEN GO TO 10</lang>
BBC BASIC
<lang bbcbasic> *FX 15,1</lang> Strictly speaking *FX 15,1 is an Operating System command, but it is emulated in BBC BASIC for Windows. Alternatively the keyboard buffer may be flushed as follows: <lang bbcbasic> REPEAT UNTIL INKEY(0)=-1</lang>
C
Code lifted from Keyboard input/Obtain a Y or N response: <lang c>#include <stdio.h>
- include <stdio.h>
- include <termios.h>
- include <unistd.h>
- include <fcntl.h>
- include <ctype.h>
void set_mode(int want_key) { static struct termios old, new; if (!want_key) { tcsetattr(STDIN_FILENO, TCSANOW, &old); return; }
tcgetattr(STDIN_FILENO, &old); new = old; new.c_lflag &= ~(ICANON); tcsetattr(STDIN_FILENO, TCSANOW, &new); }
int get_key() { int c = 0; fd_set fs;
FD_ZERO(&fs); FD_SET(STDIN_FILENO, &fs); select(STDIN_FILENO + 1, &fs, 0, 0, 0);
if (FD_ISSET(STDIN_FILENO, &fs)) { c = getchar(); set_mode(0); } return c; }
int main() { int c = 0; while (c != 'n') { set_mode(1);
/* flush pending input so we won't format the hardrive because user accidentally typed 'y' before we even prompted */ tcflush(STDIN_FILENO, TCIFLUSH);
printf("Show this prompt again [Yes/No/Ignore you]? "); fflush(stdout);
switch(c = tolower(get_key())) { case 'y': putchar('\n'); break;
case 'n': printf("\nDone\n"); break;
case 'i': puts("\nI'll ignore keys for 5 seconds"); sleep(5); putchar('\n'); break; default: puts("\nAssume that was the cat."); } }
return 0; }</lang>
Euphoria
<lang Euphoria>while get_key()!=-1 do end while</lang>
Icon and Unicon
The following solution works in both Icon and Unicon. <lang unicon>procedure flushKB()
while kbhit() do getch() # flush input
end</lang>
PicoLisp
<lang PicoLisp>(while (key 10))</lang>
PowerShell
The following uses the special $Host
variable which points to an instance of the PowerShell host application. Since the host's capabilities may vary this may not work in all PowerShell hosts. In particular, this works in the console host, but not in the PowerShell ISE.
<lang powershell>while ($Host.UI.RawUI.KeyAvailable) {
$Host.UI.RawUI.ReadKey() | Out-Null
}</lang>
PureBasic
<lang PureBasic>While Inkey(): Wend</lang>
Python
<lang Python>def flush_input():
try: import msvcrt while msvcrt.kbhit(): msvcrt.getch() except ImportError: import sys, termios termios.tcflush(sys.stdin, termios.TCIOFLUSH)
</lang>
REXX
This will work for Regina: <lang rexx>call dropbuf</lang> This will work for CMS, PC/REXX, and TSO: <lang rexx>'DROPBUF'</lang>
Ruby
Each terminal device has an input queue for keyboard input. We can either flush this input queue, or read it empty.
Ruby 1.9.3 adds a new library 'io/console', providing IO#iflush to flush and discard the input queue. If its IO object is not a terminal, it raises an error, perhaps Errno::ENODEV.
<lang ruby>require 'io/console' $stdin.iflush</lang>
The other option uses IO#read_nonblock to read the input, without any blocking or waiting. This has a caveat: if the terminal uses the canonical input mode, IO reads only entire lines; and if the input queue contains part of a line, IO#read_nonblock cannot discard this last partial line!
<lang ruby>loop { $stdin.read_nonblock(256) } rescue nil</lang>
The complete solution calls IO#iflush, or turns off canonical input mode and calls IO#read_nonblock.
<lang ruby>class IO
def discard_input icanon = false if tty? begin # With Ruby 1.9.3, simply call IO#iflush. require 'io/console' return iflush rescue LoadError # Try to run stty(1) to check if this terminal uses # canonical input mode. Acts like `stty -a`, but redirects # stdin from tty. Works with Ruby 1.8, no Process#spawn. r, w, pid = nil begin r, w = IO.pipe pid = fork do IO.for_fd(0).reopen(self) # stdin from tty IO.for_fd(1).reopen(w) # stdout to pipe exec 'stty', '-a' end w.close; w = nil icanon = (not r.read.include? "-icanon") rescue # stty(1) only works with Unix clones. ensure pid and Process.wait pid w and w.close r and r.close end end end
if icanon # Turn off canonical input mode. pid = nil begin pid = fork do IO.for_fd(0).reopen(self) # stdin from tty exec 'stty', '-icanon' end ensure pid and Process.wait pid end end
# Discard input. loop { $stdin.read_nonblock(256) } rescue nil
if icanon # Turn on canonical input mode. pid = nil begin pid = fork do IO.for_fd(0).reopen(self) # stdin from tty exec 'stty', 'icanon' end ensure pid and Process.wait pid end end
nil end
end</lang>
<lang ruby># Demonstration: discard input, then input a line from user. puts 'Type anything for 2 seconds.' sleep 2 $stdin.discard_input print 'Enter a line? ' if line = $stdin.gets then print 'Got line. ', line else puts 'No line!' end</lang>
Seed7
The Seed7 library keybd.s7i defines the file KEYBOARD, which provides functions like keypressed and getc.
<lang seed7>while keypressed(KEYBOARD) do
ignore(getc(KEYBOARD));
end while;</lang>
Tcl
<lang tcl># No waiting for input fconfigure stdin -blocking 0
- Drain the data by not saving it anywhere
read stdin
- Flip back into blocking mode (if necessary)
fconfigure stdin -blocking 1</lang>
XPL0
<lang XPL0>code OpenI=13; OpenI(0)</lang>