clear-input

clear-input Function

Syntax:

clear-input &optional input-stream → nil

Arguments and Values:

input-stream—an input stream designator . The default is standard input.

Description:

Clears any available input from input-stream.

If clear-input does not make sense for input-stream, then clear-input does nothing.

Examples:

;; The exact I/O behavior of this example might vary from implementation 
;; to implementation depending on the kind of interactive buffering that 
;; occurs. (The call to SLEEP here is intended to help even out the 
;; differences in implementations which do not do line-at-a-time buffering.) 
(defun read-sleepily (&optional (clear-p nil) (zzz 0)) 
  (list (progn (print ’>) (read)) 
	;; Note that input typed within the first ZZZ seconds 
	;; will be discarded. 
	(progn (print ’>) 
	       (if zzz (sleep zzz)) 
	       (print ’») 
	       (if clear-p (clear-input)) 
	       (read)))) 
(read-sleepily) 
▷ > 10 
▷ > 
▷ » 20 
→ (10 20) 
(read-sleepily t) 
▷ > 10 
▷ > 
▷ » 20 
→ (10 20) 
(read-sleepily t 10) 
▷ > 10 
▷ > 20 ; Some implementations won’t echo typeahead here. 

▷ » 30 
→ (10 30) 

Side Effects:

The input-stream is modified.

Affected By:

*standard-input*

Exceptional Situations:

Should signal an error of type type-error if input-stream is not a stream designator .

See Also:

clear-output

Expanded Reference: clear-input

Basic Usage

clear-input clears any available input from an input stream, discarding buffered characters. It always returns NIL.

(clear-input)
=> NIL

With a Specific Stream

You can pass a specific input stream. For non-interactive streams such as string streams, clear-input does nothing meaningful.

(with-input-from-string (s "hello")
  (clear-input s))
=> NIL

Typical Use Case

clear-input is most useful with interactive streams (such as *standard-input*) to discard any type-ahead input before prompting the user.

;; Pattern for interactive input:
;; (clear-input *standard-input*)
;; (format t "Enter value: ")
;; (read *standard-input*)

Always Returns NIL

Regardless of the stream or its state, the return value is always NIL.

(let ((s (make-string-input-stream "data")))
  (read-char s)
  (clear-input s))
=> NIL