LISP-输入和输出 - 芒果文档

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📜 LISP-输入和输出

📅 最后修改于: 2020-11-03 07:19:16 🧑 作者: Mango

常见的LISP提供许多输入输出功能。我们已经使用了format函数和print函数进行输出。在本节中，我们将研究LISP中提供的一些最常用的输入输出功能。

输入功能

下表提供了LISP最常用的输入功能-

Sr.No.	Function & Description
1	read & optional input-stream eof-error-p eof-value recursive-p It reads in the printed representation of a Lisp object from input-stream, builds a corresponding Lisp object, and returns the object.
2	read-preserving-whitespace & optional in-stream eof-error-p eof-value recursive-p It is used in some specialized situations where it is desirable to determine precisely what character terminated the extended token.
3	read-line & optional input-stream eof-error-p eof-value recursive-p It reads in a line of text terminated by a newline.
4	read-char & optional input-stream eof-error-p eof-value recursive-p It takes one character from input-stream and returns it as a character object.
5	unread-char character & optional input-stream It puts the character most recently read from the input-stream, onto the front of input-stream.
6	peek-char & optional peek-type input-stream eof-error-p eof-value recursive-p It returns the next character to be read from input-stream, without actually removing it from the input stream.
7	listen & optional input-stream The predicate listen is true if there is a character immediately available from input-stream, and is false if not.
8	read-char-no-hang & optional input-stream eof-error-p eof-value recursive-p It is similar to read-char, but if it does not get a character, it does not wait for a character, but returns nil immediately.
9	clear-input & optional input-stream It clears any buffered input associated with input-stream.
10	read-from-string string & optional eof-error-p eof-value & key :start :end :preserve-whitespace It takes the characters of the string successively and builds a LISP object and returns the object. It also returns the index of the first character in the string not read, or the length of the string (or, length +1), as the case may be.
11	parse-integer string & key :start :end :radix :junk-allowed It examines the substring of string delimited by :start and :end (default to the beginning and end of the string). It skips over whitespace characters and then attempts to parse an integer.
12	read-byte binary-input-stream & optional eof-error-p eof-value It reads one byte from the binary-input-stream and returns it in the form of an integer.

从键盘读取输入

读取函数用于从键盘获取输入。可能没有任何论据。

例如，考虑代码片段-

(write ( + 15.0 (read)))

假设用户从STDIN输入中输入10.2，它将返回，

25.2

read函数从输入流中读取字符，并通过解析为Lisp对象的表示来解释它们。

例

创建一个名为main.lisp的新源代码文件，并在其中键入以下代码-

; the function AreaOfCircle
; calculates area of a circle
; when the radius is input from keyboard

(defun AreaOfCircle()
(terpri)
(princ "Enter Radius: ")
(setq radius (read))
(setq area (* 3.1416 radius radius))
(princ "Area: ")
(write area))
(AreaOfCircle)

当您执行代码时，它返回以下结果-

Enter Radius: 5 (STDIN Input)
Area: 78.53999

例

创建一个名为main.lisp的新源代码文件，然后在其中键入以下代码。

现场演示

(with-input-from-string (stream "Welcome to Tutorials Point!")
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (read-char stream))
   (print (peek-char nil stream nil 'the-end))
   (values)
)

当您执行代码时，它返回以下结果-

#\W 
#\e 
#\l 
#\c 
#\o 
#\m 
#\e 
#\Space 
#\t 
#\o 
#\Space

输出功能

LISP中的所有输出函数都使用一个称为output-stream的可选参数，将输出发送到该参数。如果未提及或为nil，则output-stream默认为变量* standard-output *的值。

下表提供了LISP最常用的输出功能-

Sr.No.	Function and Description
1	write object & key :stream :escape :radix :base :circle :pretty :level :length :case :gensym :array write object & key :stream :escape :radix :base :circle :pretty :level :length :case :gensym :array :readably :right-margin :miser-width :lines :pprint-dispatch Both write the object to the output stream specified by :stream, which defaults to the value of standard-output. Other values default to the corresponding global variables set for printing.
2	prin1 object & optional output-stream print object & optional output-stream pprint object & optional output-stream princ object & optional output-stream All these functions outputs the printed representation of object to output-stream. However, the following differences are there − prin1 returns the object as its value. print prints the object with a preceding newline and followed by a space. It returns object. pprint is just like print except that the trailing space is omitted. princ is just like prin1 except that the output has no escape character
3	write-to-string object & key :escape :radix :base :circle :pretty :level :length :case :gensym :array write-to-string object & key :escape :radix :base :circle :pretty :level :length :case :gensym :array :readably :right-margin :miser-width :lines :pprint-dispatch prin1-to-string object princ-to-string object The object is effectively printed and the output characters are made into a string, which is returned.
4	write-char character & optional output-stream It outputs the character to output-stream, and returns character.
5	write-string string & optional output-stream & key :start :end It writes the characters of the specified substring of string to the output-stream.
6	write-line string & optional output-stream & key :start :end It works the same way as write-string, but outputs a newline afterwards.
7	terpri & optional output-stream It outputs a newline to output-stream.
8	fresh-line & optional output-stream it outputs a newline only if the stream is not already at the start of a line.
9	finish-output & optional output-stream force-output & optional output-stream clear-output & optional output-stream The function finish-output attempts to ensure that all output sent to output-stream has reached its destination, and only then returns nil. The function force-output initiates the emptying of any internal buffers but returns nil without waiting for completion or acknowledgment. The function clear-output attempts to abort any outstanding output operation in progress in order to allow as little output as possible to continue to the destination.
10	write-byte integer binary-output-stream It writes one byte, the value of the integer.

例

创建一个名为main.lisp的新源代码文件，然后在其中键入以下代码。

; this program inputs a numbers and doubles it
(defun DoubleNumber()
   (terpri)
   (princ "Enter Number : ")
   (setq n1 (read))
   (setq doubled (* 2.0 n1))
   (princ "The Number: ")
   (write n1)
   (terpri)
   (princ "The Number Doubled: ")
   (write doubled)
)
(DoubleNumber)

当您执行代码时，它返回以下结果-

Enter Number : 3456.78 (STDIN Input)
The Number: 3456.78
The Number Doubled: 6913.56

格式化输出

函数格式用于生成格式正确的文本。它具有以下语法-

format destination control-string &rest arguments

哪里，

目标是标准输出
控制字符串保存要输出的字符和打印指令。

格式指令由波浪号(〜)，用逗号分隔的可选前缀参数，可选冒号(:)和符号(@)修饰符以及指示此指令类型的单个字符。

前缀参数通常是整数，用可选的带符号十进制数表示。

下表简要介绍了常用指令-

Sr.No.	Directive & Description
1	~A Is followed by ASCII arguments.
2	~S Is followed by S-expressions.
3	~D For decimal arguments.
4	~B For binary arguments.
5	~O For octal arguments.
6	~X For hexadecimal arguments.
7	~C For character arguments.
8	~F For Fixed-format floating-point arguments.
9	~E Exponential floating-point arguments.
10	~$ Dollar and floating point arguments.
11	~% A new line is printed.
12	~* Next argument is ignored.
13	~? Indirection. The next argument must be a string, and the one after it a list.

例

让我们重写计算圆面积的程序-

创建一个名为main.lisp的新源代码文件，然后在其中键入以下代码。

(defun AreaOfCircle()
   (terpri)
   (princ "Enter Radius: ")
   (setq radius (read))
   (setq area (* 3.1416 radius radius))
   (format t "Radius: = ~F~% Area = ~F" radius area)
)
(AreaOfCircle)

当您执行代码时，它返回以下结果-

Enter Radius: 10.234 (STDIN Input)
Radius: = 10.234
Area = 329.03473