Input and Output in Scheme

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

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. Input/Output

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 Input/Output

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 Input/Output

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 Input/Output

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 Input/Output

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" Input/Output

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 Input/Output

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 Input/Output

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 Input/Output

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 Input/Output