Input and Output in Scheme CS 480/680 – Comparative Languages

Input/Output2 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.

Input/Output3 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

Input/Output4 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

Input/Output5 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

Input/Output6 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

Input/Output7 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"

Input/Output8 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

Input/Output9 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

Input/Output10 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

Input/Output11 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