LISP 简单的数据库 3.6.1 依艺术家字段值进行查询

增加select-by-artist方法

;使用全局变量记录数据
(defvar *db* nil)

;数据记录格式
(defun make-cd (title artist rating ripped)
  (list :title title :artist artist :rating rating :ripped ripped)
)

;添加记录
(defun add-record (cd)
 ( push cd *db*)
)

;查看数据库内容
(defun dump-db()
  (dolist (cd *db*)
    (format t "~{~a: ~10t~a~%~}~%" cd))
)

;提示用户输入CD信息
(defun prompt-read (prompt)
  (format *query-io* "~a: " prompt)
  (force-output *query-io*)
  (read-line *query-io*)
)

;依次提示用户输入信息
(defun prompt-for-cd()
  (make-cd
   (prompt-read "Title")
   (prompt-read "Artist")
   ;(prompt-read "Rating")
   ;(prompt-read "Ripped [y/n]")

   ;快餐式验证数据有效性
   (or (parse-integer (prompt-read "Rating") :junk-allowed t) 0)
   (y-or-n-p "Ripped [y/n]: ")

  )
)

;批量添加数据，使用回车退出
(defun add-cds()
  (loop (add-record (prompt-for-cd))
     (if (not (y-or-n-p "Another? [y/n]:"))
	 (return)
      )
   )
)

;保存数据
(defun save-db (filename)
  (with-open-file(out filename :direction :output :if-exists :supersede)
    (with-standard-io-syntax(print *db* out))
   )
)

;载入数据
(defun load-db(filename)
  (with-open-file(in filename)
    (with-standard-io-syntax(setf *db* (read in)))
   )
)

;通过艺术家查找记录
(defun select-by-artist(artist)
  (remove-if-not #'(lambda (cd) (equal (getf cd :artist) artist)) *db*)
)

DB中的数据

CL-USER> *DB*
((:TITLE "Title3" :ARTIST "Artist3" :RATING 0 :RIPPED NIL)
 (:TITLE "Title2" :ARTIST "Artist2" :RATING 7 :RIPPED T) 
(:TITLE "Title1" :ARTIST "Artist1" :RATING 6 :RIPPED T) 
(:TITLE "Kly" :ARTIST "Gy" :RATING 9 :RIPPED T) 
(:TITLE "Lily" :ARTIST "Pgy" :RATING 8 :RIPPED T) 
(:TITLE "Roses" :ARTIST "Kathy" :RATING 7 :RIPPED T))

执行查询

CL-USER> (select-by-artist "Artist3")
((:TITLE "Title3" :ARTIST "Artist3" :RATING 0 :RIPPED NIL))