read-line

read-line Function

Syntax:

read-line &optional input-stream eof-error-p eof-value recursive-p

→ line, missing-newline-p

Arguments and Values:

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

eof-error-p—a generalized boolean. The default is true.

eof-value—an object. The default is nil.

recursive-p—a generalized boolean. The default is false.

line—a string or the eof-value.

missing-newline-p—a generalized boolean.

Description:

Reads from input-stream a line of text that is terminated by a newline or end of file.

If recursive-p is true, this call is expected to be embedded in a higher-level call to read or a similar function used by the Lisp reader .

The primary value, line, is the line that is read, represented as a string (without the trailing newline, if any). If eof-error-p is false and the end of file for input-stream is reached before any characters are read, eof-value is returned as the line.

The secondary value, missing-newline-p, is a generalized boolean that is false if the line was terminated by a newline, or true if the line was terminated by the end of file for input-stream (or if the line is the eof-value).

Examples:

(setq a "line 1 
line2") 
→ "line 1 
line2" 
(read-line (setq input-stream (make-string-input-stream a))) 
→ "line 1", *false* 
(read-line input-stream) 
→ "line2", *true* 
(read-line input-stream nil nil) 
→ NIL, *true* 

Affected By:

*standard-input*, *terminal-io*.

Exceptional Situations:

If an end of file₂ occurs before any characters are read in the line, an error is signaled if eof-error-p is true.

See Also:

read

Notes:

The corresponding output function is write-line.

Expanded Reference: read-line

Basic Usage

read-line reads a line of text from an input stream, returning the line as a string (without the trailing newline) and a secondary boolean value indicating whether the line was terminated by end of file rather than a newline.

(with-input-from-string (s (format nil "hello~%world"))
  (multiple-value-list (read-line s)))
=> ("hello" NIL)

Second Return Value

The secondary value is NIL when the line ends with a newline, and T when terminated by end of file.

(with-input-from-string (s (format nil "line1~%line2"))
  (list (multiple-value-list (read-line s))
        (multiple-value-list (read-line s))))
=> (("line1" NIL) ("line2" T))

Handling End of File

With eof-error-p set to nil, returns the eof-value instead of signaling an error.

(with-input-from-string (s "only line")
  (list (read-line s)
        (read-line s nil :done)))
=> ("only line" :DONE)

Reading All Lines from a String

(with-input-from-string (s (format nil "alpha~%beta~%gamma"))
  (loop for line = (read-line s nil nil)
        while line
        collect line))
=> ("alpha" "beta" "gamma")

Empty Lines

Empty lines produce empty strings.

(with-input-from-string (s (format nil "~%~%data~%"))
  (list (copy-seq (read-line s))
        (copy-seq (read-line s))
        (read-line s)
        (read-line s nil :eof)))
=> ("" "" "data" :EOF)