9 Input/Output
9.2 string
string在lisp中用双引号括起来，它的值也是自己本身，和数字一样，string不是symbols
stringp判断是否是一个string
> "hello" "hello" > (stringp "hello") t
9.3 format
format用来输出到显示器或者文件中，函数总是返回nil.输出到显示器的时候，第一个参数是t,第二个参数是string
> (format t "hello") hello nil
string中以下几个符号有特殊含义：
~%: 输出换行~&: 输出换行，除非刚好在新的一行开始的位置。意思就是及时有几个~&符号也只会输出一个换行~S: 用后面的字符替换，如(format t "from ~S to ~S" 'guangzhou 'zhuhai) -> from guangzhou to zhuhai~A: 与~S类似，但是不使用escape characters,eg(defun test (x) (format t "~&with escape chars: ~S" x) (format t "~&without escape chars: ~A" x)) > (test "hello") with escape chars: "hello" without escape chars: hello nil
ex 9.2
(defun draw-line (n)
(cond ((equal n 1) (format t "*~&"))
(t (format t "*") (draw-line (- n 1)))))
ex 9.3
(defun draw-box (x y)
(cond ((equal y 1) (draw-line x))
(t (draw-line x) (draw-box x (- y 1)))))
ex 9.5
(defun print-board (b)
(labels ((print-line (line) (format t " ~A | ~A | ~A ~%" (first line) (second line) (third line))))
(let ((b1 (sublis '((x . "X")
(o . "O")
(nil . " ")) b)))
(format t "~&")
(print-line b1)
(format t "-----------~%")
(print-line (nthcdr 3 b1))
(format t "-----------~%")
(print-line (nthcdr 6 b1)))))
9.4 read
read函数从键盘读取一个对象然后返回对象的值，对象前面不需要加'，因为它不会被evaluated
(defun my-square ()
(format t "enter a number:")
(let ((x (read)))
(format t "the number ~S squared is ~S.~%" x (* x x))))
ex 9.7
(defun cookie-monster ()
(format t "give me cookie!!!~%cookie?")
(let ((response (read)))
(cond ((equal response 'cookie)
(format t "~&thank you!...munch munch munch ...burp~%"))
(t (format t "~&no want ~S....~%" response)
(cookie-monster)))))
9.5 yes-or-no-p
(defun riddle ()
(if (yes-or-no-p "do you seek zen enlightenment?")
(format t "then do not ask for it!")
(format t "you have found it.")))
> (riddle)
do you ...?yes
then do not ask for it!
yes-or-no-p函数输出后面的string内容，然后要求用户输入yes或者no，代表函数返回t或者nil
9.6 with-open-file
(with-open-file (var pathname)
body)
打开路径下文件名的文件，并创建变量名var的stream object用来操作
eg,
(defun get-tree-data ()
(with-open-file (stream "~/hello.dat")
(let* ((tree-loc (read stream))
(tree-table (read stream))
(num-trees (read stream)))
(format t "~&there are ~S trees on ~S." num-trees tree-loc)
(format t "~&they are: ~S." tree-table))))
> (get-tree-data)
there are 100 trees on "the north slope"
they are : ((45 redwood) (12 oak) (43 maple))
nil
read stream每次从文件中读取一个lisp对象
9.7 writing files with with-open-file
format函数的第二个参数是t代表输出到显示器，改成文件的stream object则输出到文件中
我们可以输入任意内容到文件中，内容可以不是lisp的对象，比如不成对的小括号等，但是后面就不能使用read stream的形式来进行读取
ex 9.10
(defun test (n)
(format t "~%>>>")
(space-over n)
(format t "<<<"))
(defun space-over (n)
(cond ((< n 0) (format t "error"))
((zerop n) nil)
(t (format t " ") (space-over (- n 1)))))
(defun plot-one-point (plotting-string y-val)
(space-over y-val)
(format t "~A~&" plotting-string))
(defun plot-points (plotting-string y-vals)
(mapcar #'(lambda (b) (plot-one-point plotting-string b)) y-vals))
(defun generate (m n)
(cond ((= m n) (list m))
((< m n) (cons m (generate (+ 1 m) n)))))
9.8 parameters to format directives
~S输出替换字符，~10S则可以设置输出宽度为10个字符 ~D输出十进制整数 ~F输出浮点数，可以设定总的位数和小数位数，如~7,5F输出5位小数，总共占7个字符
9.9 file EOF
当程序读到文件的结尾时，会产生end-of-file error。给read增加两个参数可以不产生报错，第一个是nil，表示不产生错误，第二个是eof indicator，表示读到这个对象就代表结尾
(defun read-my-file ()
(with-open-file (stream "~/hello.dat")
(let ((contents (read-all-objects stream (list '$eof$))))
(format t "~&read ~S objects from the file." (length contents))
contents)))
(defun read-all-objects (stream eof-indicator)
(let ((result (read stream nil eof-indicator)))
(if (eq result eof-indicator) nil (cons result (read-all-objects stream eof-indicator)))))
例子中使用(list '(eof))作为结尾标识，这是一个新建的cons，后面进行比较的时候用eq而不是equal是为了对地址进行比较，确认就是结尾标识。
ex 9.11
(defun dot-prinl (x)
(cond ((null x) (format t "nil"))
((atom x) (format t "~S" x))
(t (format t "(")
(dot-prinl (car x))
(format t " . ")
(dot-prinl (cdr x))
(format t ")"))))