Input and Output in Scheme

1
Input and Output in Scheme
CS 480/680 Comparative Languages
2
Basic I/O

• (display arg) writes arg to standard out
• The second (optional) argument to display is the
  output-port to write to.
• You get an output port using open-output-file
• You can assign it to a symbol
• (define i (open-output-file myfile))(display
  Hello world i)
• Likewise, open-input-file opens a file for
  reading and returns an input port.

3
Reading

• These functions return a string
• (read-char port) one character
• (read-line port) one line
• (read port) one S-expression
• When at EOF, the read functions return an
  eof-object, which you can test with(eof-object?
  arg)
• Write first-line.scheme

4
Writing

• These functions write to a port
• (write-char char port) one character
• (write expr port) one S-expression
• expr can be a string, a list, or any other
  S-expression
• (display expr port) like write
• Write puts around strings, \ in front of
  characters, etc.
• Display does not
• (newline port) writes a \newline to the port
• Create output.scheme

5
Automatic Opening and Closing

• Just like Ruby allows you to pass a block to
  open(), Scheme has a mechanism to open a file,
  run a subform, and automatically close the file
  at the end

(call-with-input-file "hello.txt" (lambda (i)
(let ((a (read-char i)) (b
(read-char i)) (c (read-char i)))
(list a b c)))) (\h \e \l)
See read-print.scheme
6
String Ports

• You can read/write from a string as though it was
  a file

(define i (open-input-string "hello
world")) (read-char i) \h (read i)
ello (read i) world
7
Output String Ports

• The procedure open-output-string creates an
  output port that will eventually be used to
  create a string

(define o (open-output-string)) (write 'hello
o) (write-char \, o) (display " " o) (display
"world" o)

• The function get-output-string gets the
  accumulated string associated with the port

(get-output-string o) "hello, world"
8
Loading Files

• (load filename) read each S-expression in a
  file and evaluate it as a form.
• Files can load other files
• (load-relative filename)
• Path is relative to the location of the current
  file

9
Eval and Quote

• The special form (eval list) evaluates a list as
  code
• (define a ( 3 5))a 8
• Quote prevents a list from being evaluated
• (define a ( 3 5))a ( 3 5)(eval a) 8

10
Eval-string

• To evaluate a string, you need to use
  (eval-string string)
• Requires the string.ss library
• (require (lib string.ss))
• See eval-string.scheme

11
Exercises

• Write a program that reads in a file of
  S-expressions containing fully parenthesized
  mathematical expressions in prefix notation.
  Your program should print the result of each
  expression.

Input File ( 3 7) ( ( 3 7) 5) ( ( 2 5) ( 2
3))
Standard Out 10 50 15