Keyboard input/Obtain a Y or N response

Obtain a valid Y or N response from the keyboard. The keyboard should be flushed, so that any outstanding keypresses are removed, preventing any existing Y or N keypress from being evaluated. The response should be obtained as soon as Y or N are pressed, and there should be no need to press an enter key.

BASIC

ZX Spectrum Basic

10 IF INKEY$<>"" THEN GO TO 10
20 PRINT "Press Y or N to continue"
30 LET k$ = INKEY$
40 IF k$ = "y" OR k$ = "Y" OR k$ = "n" OR k$ = "N" THEN GO TO 60
50 GO TO 30
60 PRINT "The response was "; k$

Inform 7

Keyboard input goes through a virtual machine that's only required to provide blocking input operations, so flushing the buffer isn't possible.

Inform 7 has a built-in function to ask the user for yes-or-no input, but it requires them to press enter afterward: <lang inform7>Qwantz is a room.

When play begins: say "A wizard has turned you into a whale. Is this awesome (Y/N)? "; if the player consents, say "Awesome!"; end the story.</lang>

To read a single key without waiting for enter, we can redefine the function by including a snippet of Inform 6 code: <lang inform7>To decide whether player consents: (- (YesOrNoKey()) -).

Include (- [ YesOrNoKey ch;

   do { ch = VM_KeyChar(); } until (ch == 'y' or 'Y' or 'n' or 'N');
   return ch == 'y' or 'Y';

]; -).</lang>

PicoLisp

<lang PicoLisp>(de yesno ()

  (loop
     (NIL (uppc (key)))
     (T (= "Y" @) T)
     (T (= "N" @)) ) )</lang>

PureBasic

Inkey() returns the character string of the key which is being pressed at the time. <lang PureBasic>PrintN("Press Y or N to continue")

Repeat

 ; Get the key being pressed, or a empty string.
 Key$=UCase(Inkey())
 ;
 ; To Reduce the problems with an active loop
 ; a Delay(1) will release the CPU for the rest
 ; of this quanta if no key where pressed.
 Delay(1)

Until Key$="Y" Or Key$="N" PrintN("The response was "+Key$)</lang>

REXX

REXX (in general) requires the user to press ENTER when entering text.

Some REXX interpretors have a keyboard read subroutine so that the program can read keyboard keys as they are pressed. <lang rexx>

 do queued()      /*flush the stack if any lines queued.   */
 pull
 end

prompt='Press Y or N for some reason.' /*prompt message*/ ok='Y N' /*acceptable answers (will be uppercase)*/

 do forever
 say              /*write a blank line for visual fidelity.*/
 say prompt       /*prompt the user for an input.          */
 pull ans         /*get the answer(s), also, uppercase it. */
 ans=strip(ans)   /*get rid of leading/trailing blanks.    */
 if ans= then iterate              /*if blank, try again.*/
 if wordpos(ans,ok)\==0 then leave   /*if ans is OK, leave.*/
 end

                  /*as this point, ANS holds a  Y  or  N.  */

</lang>

Tcl

<lang tcl>proc yesno Template:Message "Press Y or N to continue" {

   fconfigure stdin -blocking 0
   exec stty raw
   read stdin ; # flush
   puts -nonewline "${message}: "
   flush stdout
   while {![eof stdin]} {
       set c [string tolower [read stdin 1]]
       if {$c eq "y" || $c eq "n"} break
   }
   puts [string toupper $c]
   exec stty -raw
   fconfigure stdin -blocking 1
   return [expr {$c eq "y"}]

}

set yn [yesno "Do you like programming (Y/N)"]</lang>